Annex three - The scientific basis for diet, nutrition and health relationships
4. Scientific areas relevant to the development of food-based dietary guidelines
4.1.1 Energy intake, macronutrients and food-based dietary guidelines
4.1.4 Non-nutrient food components of biological significance
4.1.5 Dietary goals for vulnerable groups.
4.1.6 Nutritional quality of dietary patterns
4.1.7 Nutrient requirements and recommended nutrient intakes
4.2 Food science and technology
Dietary patterns have varied over time depending on agricultural practices and climatic, ecological, cultural and socioeconomic factors which determine the foods that are available. At present most naturally occurring dietary patterns satisfy or even exceed the nutritional needs of most individuals, except where agricultural and socioeconomic conditions limit food availability and food purchasing capacity or cultural practices restrict the choice of foods.
The quantitative definition of nutrient needs and efforts to express them as RNI have been the focus of attention by international bodies and nutrition scientists in many countries. However, this nutrient-based approach is commonly misapplied and has led to considerable confusion among policy-makers in both the food and health sectors as well as among nutrition educators and consumers.
FBDG can serve as an instrument of nutrition policies and programmes, and should be based directly upon diet and health relationships of particular relevance to the individual country. In this way, priorities in establishing dietary guidelines can address the relevant public health concerns whether they are related to dietary insufficiency or excess.
In this context, the first step in the process of setting dietary guidelines is to define the significant diet-related public health problems in a given community. Once these are defined, the diet component is evaluated, including the assessment of the adequacy of the food supply and consumption pattern. A comprehensive strategy to attack the problem should be defined including the development of dietary guidelines. In this setting, RNI can assist in assessing whether the proposed diet is likely to meet nutritional needs and provide a basis for consumer information about the nutritional quality of foods.
Dietary goals should be specific for a given ecological setting. Their purpose is to promote overall health, control specific nutritional diseases whether they are induced by deficiency or by excess of nutrient intake, and reduce the risk of multifactoral diet-related diseases. Dietary guidelines represent the practical way to reach the nutritional goals for a given population. They take into account the customary dietary pattern and indicate what aspects should be modified. They consider the ecological setting, the socioeconomic and cultural factors, and the biological and physical environment in which the population lives.
No single food—except breast milk in the first 4 to 6 months of life—provides all the required nutrients. A range of nutrients are needed, in amounts that change throughout life, for optimal growth, health and avoidance of disease. There are a great variety of diets and foods consumed in various combinations. Over the ages many national and local dietary patterns have shown themselves capable of providing adequate nutrients and supporting good health.
There is good scientific evidence that dietary patterns, i.e. the daily combination of foods and beverages generally consumed, have specific health or disease outcomes. For example, a diet may be apparently adequate in all ways but still be deficient in, say, vitamin A or iron and this will lead to a specific disease outcome. Conversely, a diet high in saturated fats and energy is known, on a population basis, to lead to an increased likelihood of certain NCD.
Consequently, FBDG need to take into account food/health patterns; the relative comprehensiveness of the food- versus the nutrient based approach; the practicality of suggested goals; the limits of nutrition labelling, which may direct the consumer to an over-simplified view of foods; and shifting paradigms of the nutritional basis of disease and health (the biological effects of food and food patterns can be greater than the sum of their nutrient and non-nutrient parts). Such guidelines should also be based on some of the following generally established principles. Food availability is a first requisite for the application of dietary guidelines which direct food choice. Sufficient food diversity is required for acceptable macronutrient intakes, quite apart from optional or preferred intakes. Measures of food diversity are required in the next generation of dietary guidelines. The cultural context in which foods are produced, prepared and eaten should be taken into account for the satisfactory structuring of dietary guidelines in relation to macronutrients and other nutrients.
Although the main document generally takes a non-traditional approach by looking at foods, the review of current scientific evidence in this annex will be addressed by nutrient: macronutrients, including water and alcohol; micronutrients, including vitamins and minerals; and non-nutrient food components, including the nutritional implications of novel or "functional" foods.
The traditional approach to providing dietary guidance and evaluating the nutritional adequacy of diets has focused on RNI or RDA for specific nutrients. This approach is commonly misapplied and has proved inadequate for developing effective nutrition education programmes. An alternative approach to traditional RNI, and one which can better address issues of "optimal" nutrient intakes, is the use of the nutrient density concept applied to the total diet.
The concept of nutrient density was originally developed to compare the contribution of a food or diet to the intake of essential micronutrients and protein, in relation to the energy which it provides. Thus, a high nutrient density would illustrate those foods that are good sources of micronutrients or protein, because the food would make a greater contribution to the intake of an essential nutrient than to meeting total energy needs. The concept is especially useful when energy intake is low and it is essential that nutrient-dense foods be included in the diet. A low nutrient density could lead to a situation in which excess calories should be consumed to meet the needs for essential nutrients. In the original development, the concept was applied to individual foods or meals on the understanding that no single food or meal will meet all the requirements for essential nutrients.
In developing the concept of nutrient density for use in FBDG, the original nutrient density approach has been modified to include other factors. The concept is being used to express required nutrient intakes (e.g. protein), desirable nutrient intakes (e.g. folic acid) and population goals (fat, sodium) relative to energy intake. Obviously, FBDG need to integrate these various ways to express the nutritional quality of the diet, but it is useful to spell out the distinction. The required nutrients intake is based on the value set in an RDI or RDA which includes the minimum required level, plus a safety margin. The desirable range of intake refers to an amount ranging from the RDI to a higher level that may be protective (e.g. vitamin C to promote iron absorption or folic acid to lower the risk of neural tube defects). Population goals refer to the range of desirable average intakes within a population that may lower the risk of NCD.
The reference of "high" or "low" nutrient density should be considered in the context of the nutrient and the intention of the FBDG. For example, a high nutrient density for vitamin A activity or iron may be important for preventing deficiencies of these nutrients, whereas a relatively low density of fat or sodium and a high density of fibre may be desirable to lower the risk of NCD.
Because of this more comprehensive approach, the concept is most appropriately used by health professionals or policy-makers in developing dietary goals or targets which are the background for devising FBDG. Additionally, care should be taken to apply these standards to the total diet rather than to individual foods or meals.
The RNI values used to derive these nutrient densities have been obtained from the latest international RDA (1-9). The conditions for this model are that a diet which provides for the energy needs of family members will also satisfy the RNI for all essential nutrients.
Nutrients with relevant public health implications have been included in Table 3 and in the accompanying text, expressing their need not as an absolute requirement but as nutrient density per 1000 kcal. This should not be interpreted as a physiological relationship between the specific nutrients and energy requirements but as a way of defining the adequacy of a given diet to meet the needs for specific nutrients if sufficient energy is consumed. This approach permits the simplification of age and sex RNI figures since, if the figures are expressed per 1000 kcal, the values do not differ substantially among the various groups except for calcium and possibly certain trace elements. In addition, for the purpose of establishing dietary guidelines for the general population, precise age- and sex-specific RNI are not needed. The figures presented should be interpreted as a way to assess dietary quality.
The household is the basic unit for food consumption under most settings. Thus if the amount of food is adequate, individual members of the household can consume a diet with the recommended nutrient densities and meet their nutrient needs. The problem of intra-family distribution needs to be considered since children and women may not receive a proportional part of foods with higher nutrient density. This should be taken into account in establishing both general dietary guidelines and those specifically addressing the needs of vulnerable groups in the community.
If intake for adolescents or adults is <2000 kcal per day, the diet will not secure vitamin and mineral needs, so sedentary lifestyles should be discouraged. Children below 2 years of age need a different diet and require specific dietary guidelines.
In using nutrient densities to establish dietary goals and guidelines, the quantitative and qualitative aspects of the food supply should be considered. The quantitative aspects include an estimation of the amount of nutrients and the relative proportion of a nutrient in the food sources that will permit satisfaction of nutritional needs in practice. The qualitative aspects relate to the biological quality of the nutrients in the food source, and to the potential for interaction of nutrients which may enhance or inhibit biological quality of a given source if both are ingested simultaneously.
Dietary goals for some nutrients where there may be risks associated with high as well as low intakes are specifically identified by indicating maximum intake. If the upper range of safe intake is so high that in practice most diets fall well below this limit, no mention of the upper range is made. Where the data in the various available reports provide justifiable differences in figures, densities are also expressed as ranges; in this case the selection of value should be based on the user's assessment of the background information.
The recommended nutrient densities in Table 3 refer to the total diet available and not to individual foods.
Four broad areas of science have been utilized in taking this initial look at FBDG: (a) nutritional science (i.e. food components as nutrients and non-nutrients); (b) food science and technology; (c) educational, social and behavioural sciences; and (d) agricultural and environmental sciences. This Annex then looks at the areas, as related to foods and health, that are not clear, and areas where there may be some scientific uncertainties and limitations. Appropriate references will be cited from recent reviews, and from FAO and WHO documents.
The study of energy balance and metabolism, and of macro- and micronutrient physiology, is well-established. Nutritional science has now also evolved to take account of non-nutrient components of foods having biological significance.
Macronutrients are defined broadly as those food components which are present in quantities of one gram or more in the daily diet, and which generally provide energy. They therefore include protein, fat, carbohydrates, and most dietary fibres and alcohol. Although unable to provide energy, water is also considered a macronutrient.
Table 3. Reference nutrient densities for selected nutrients relevant to developing and evaluating dietary guidelines
(These nutrient densities refer to total diet; If intake is sufficient to meet energy needs, the diet will also meet needs of all members except possibly infants <2 years of age and pregnant lactating women).
Nutrient density per 1000 kcal |
Comments | |
Energy |
see age, sex and activity specific recommendation |
Energy density 2-5 yrs of age: 0.6-0.8 kcal/ml liquid foods 2 kcal/g solid foods |
Protein |
20 - 25 g |
8-10 % of total energy if protein quality is high 10-12% of total energy |
25 - 30 g |
if animal protein intake is low | |
Fats |
16 -39 g (max) |
15-35 % of energy Cholesterol: < 300 mg/d |
Saturated |
<11 g (see text) |
Saturated up to 10 % of total energy intake |
Carbohydrates |
140 -190 g |
55-75 # of energy |
Fibre |
8 -20 g |
Total dietary fibre must be accounted, not only " crude fibre" |
Vitamin A (retinal) |
350-500 µg RE |
1 Retinol Equivalent (RE)=1 µg retinol or 6 µg b- carotene as provitamin A |
b-carotene |
Functions as antioxidant; no RDA/RNI for b-carotene; (see text). | |
Vitamin D |
2.5-5.0 µg |
promotes bone health |
Vitamin E |
3.5-5.0 mg a-TE |
1 mg a-TE = 1 mg a-d-tocopherol; inhibits lipoprotein oxidation |
Vitamin K |
20-40 µg |
|
Vitamin C (ascorbic acid) |
2530 mg |
Functions as an antioxidant. Enhances iron absorption. |
Thiamine |
0.5-0.8 mg |
|
Riboflavin |
0.6-0.9 mg |
|
Niacin (or equivalent) |
6-10 mg |
60 mg trypto-phan equivalent to 1 mg niacin |
Vitamin B6 |
0.6-1.0 mg |
|
Vitamin B12 |
0.5-1.0 µg |
reduces homocysteinaemia |
Folate |
150-200 µg |
Intakes of 400 µg/d associated with reduced risk of neural tube birth defects; reduces hyperhomocysteinaemia |
Iron |
3.5, 5.5, 11 or 20 mg |
For high, intermediate, low and very low bioavailability diets (see text) |
Zinc |
6 or 10 mg |
For high and low bioavailability diets |
Calcium |
250-400 mg |
Ca-rich foods especially for adolescents and lactating pregnant women |
Iodine |
75 µg |
100-200 µg/day in regions free of goitre. Salt fortification usually required |
Fluoride |
0.5-1.0 mg (max) |
If water has >= 1 ppm requirement is met. |
Sodium as NaCl |
<2.5 g |
Total sodium as NaCl < 6 g/day (population mean) |
Infants under 1 year of age should be fed according to age: up to 4-6 months of age exclusively human milk; after this period breast milk should be complemented with appropriate foods to provide additional energy, protein and specific nutrients.
Macronutrients may also have various physiological functions other than the provision of energy which make them more or less essential in their own right; and they may serve to identify foods of biological importance for reasons other than their macronutrient composition, i.e. macronutrients may be "marker nutrients". It is for these reasons that they are referred to in dietary guidelines.
Compounds related to a derivative of macronutrients may also need to be considered in developing dietary guidelines, e.g. "bioactive peptides" as well as protein; "resistant starch", and oligosaccharides as well as digestible starch and di- and mono-saccharides; essential fatty acids of the omega-3 and omega-6 polyunsaturated (PUFA) kinds; etc. (1,9-14). To some extent, RNI for amino acids and essential fatty acids, rather than protein or fat, may determine how FBDG should develop (1,9).
• Energy
Energy intake cannot be allowed to fall below limits which allow for basal energy expenditure, thermic response to food, physical activity and intermittent illness. There are also limits below which food intake ceases to be sufficient to serve as a vehicle for the adequate supply of essential nutrients and other compounds of biological importance. Evidence from prospective studies in developed, as well as developing countries, is that those who eat most are those who live longer, provided they do not become obese and that they choose a diet with an emphasis on plant-derived foods and, in some cases, on fish (e.g. Zutphen study (15,1 6), Swedish Women Study (17), Boston/Ireland study (18)).
It follows that dietary guidelines should not seek to restrict food intake, but should encourage those food choices, like careful use of energy-dense, fatty foods, which enable energy balance to be achieved. Dietary guidelines which encourage energy balance cannot be constructed without reference to physical activity, and to some measure of energy stores, usually body fatness (a weight/height relationship such as body mass index, BMI) and its distribution e.g. abdominal/hip ratio as per a recent WHO recommendation (19). The corollary is to encourage physical activity so that enough food can be eaten to match the associated energy expenditure. In developing FBDG, energy balance and food energy density are more important than setting an upper limit on energy intake (20,21).
Under conditions of energy deficit or excess certain important physiological adaptations come into play which allow individuals to preserve critical functions that depend on energy adequacy. This adapted state includes adjustments in basal energy expenditure, facultative thermogenesis or changes in the efficiency of physical work. On the other hand if the changes in intake are beyond the adaptive range, the subject will accommodate to a new level of energy balance by changing body mass, or slowing growth rate in the case of energy deficit. Individuals may also accommodate by reducing physical activity; this may not be apparent since activity to render economic benefit may be saved, but at the cost of socially desirable activity.
Thus, changed social or family behaviour in adults may be the sole consequence of energy deficit. In small children, altered physical growth and mental development are the usual manifestations of energy deficit. In older children, decreased physical activity rather than altered growth is the hallmark of low energy intake.
In the presence of excess energy intake, the adaptive range is extremely small (<5%), and body energy reserve as adipose tissue increases rapidly. The metabolic and pathological consequences of obesity include hypertension, hyperlipidaemia and type II diabetes; in some studies obesity has also been found to be an independent risk factor for atherosclerosis and myocardial infarction. The prevalence of obesity in urban developing societies is on the rise and efforts should be made to prevent the related adverse consequences. Nutritional goals and guidelines should be set to prevent the social and pathological consequences of both energy deficit and excess.
Most recommendations concerning energy intake are based on the approach described in the 1985 FAO/WHO/UNU report "Energy and Protein Requirements" which defines energy needs based on energy expenditure. Estimating energy needs involves the summation of: (a) basal needs, derived from age- and sex-specific equations and actual or desirable body weight; (b) needs for growth in the case of children, based on usual energy content of tissue gain; and (c) needs for activity calculated as multiples of basal metabolic rate. Economically necessary activity and socially desirable activity should be considered, in addition to sedentary requirements. Pregnant and lactating women need additional energy to meet the requirement of tissue formation and milk secretion, respectively.
The digestibility of energy sources varies depending on the fibre content of the foods. Thus, following FAO/WHO/UNU 1985 recommendations on formulating dietary guidelines for rural populations in developing countries where fibre intake is high, energy RNI should be increased by 15%. For populations where fibre intake is moderate the correction is 5%.
Energy density for small children should be considered in developing energy dietary guidelines, since low energy density may limit total energy intake. For liquid foods, energy density should be 0.60.75 kcal/g whereas for solids it should be 1.5-2.0 kcal/g. For older children and adults, the suggested energy density of the combined diet is 1.5-2.5 kcal/g. In the case of obese subjects, lower energy densities (<1 kcal/g) are desirable since gastric repletion may help to maintain lower energy intakes.
• Protein, peptides and amino acids
Protein requirements and ways of achieving them from foods have been well-covered in previous reports (22,23,24). Increasingly, biologically active peptides like those which affect growth, gut physiology, and blood pressure control mechanisms (ACE inhibitors) and, conditionally essential amino acids, like taurine, arginine and glutamine tend to be reflected in food choices. Thus milk proteins may yield biologically active peptides, nuts are a good source of arginine, and so on (10,13,14,25).
For protein, virtually all RNI are based on the protein needed for nitrogen balance and growth. The 1985 FAO/WHO/UNU report suggested the mean requirement +25% (equivalent to 2SDs) as the RNI for age- and sex-specific groups. In general, recommendations for developing countries take into account a lower average protein quality and digestibility. Digestibility of the mixed diet is considered to be 80-85% and the amino acid score only 90%, therefore RNI are higher. As an example, the RNI of 0.75 g of reference protein (per kg body weight) becomes 1.0 g per kg after adjusting for protein quality in Latin America.
In circumstances where the mixed diet does not contain sufficient legumes and/or animal protein, corrections for amino acid score may also be needed. In the past, correction for amino acid score was emphasized since single sources of vegetable proteins may be limiting in one or more amino acids (Iysine for most cereals, methionine for most legumes). Yet mixed diets composed of cereal/legume mixes with some animal protein (10-20%) are sufficient in essential amino acids to meet RNI. Thus, except under very restrictive diets, amino acid score is not usually a problem.
New approaches to defining amino acid needs based on amino acid kinetic indices may provide a different perspective on the amino acid needs of humans and may require additional adjustments. At present, these indices suggest that existing recommendations greatly underestimate the amino acid needs to sustain optimal protein synthesis rates. The interpretation of optimal in this context is elusive at best, since the physiological consequences of higher or lower levels of protein synthesis have not been established.
In settings where environmental sanitation is inadequate and diarrhoeal disease is prevalent, an increase of protein intake by 10% is recommended. For children recovering from acute infection or malnutrition, protein intake should be increased to meet the demands imposed by rapid tissue synthesis. Depending on the degree of deficit, protein needs may be 2-3 times the normal amount. Even for children, protein needs during convalescence are elevated by 20-40%. Protein intake, especially animal protein, has been linked to a lower prevalence of stunting in developing countries. Although this may not necessarily be linked to essential amino acid supply, since micronutrients such as Zn are also growth promoters, it is suggested that 10-25% of dietary protein should be of animal origin.
At present no upper limit on protein intake has been set, yet animal and human data suggest that excessive protein intake has an adverse effect on kidney function. Glomerular filtration rates increase acutely in response to protein overload, but long-term hyperfiltration may lead to excessive loss of nephrons and thereby, with advancing age, reduced renal function. The latest US recommendation is to limit total protein intake to twice the RDA (26).
Table 4. Safe levels of protein intake: children and adults (summary)
Males |
Females | |||||
Age (years) |
Reference protein (g) |
High biological value diet* |
Low biological value diet* |
Reference protein (g) |
High biological value diet* |
Low biological value diet* |
0.5 |
12.9 |
12 9 |
11.9 |
11.9 |
||
1 |
14.1 |
21.2 |
23.5 |
13.3 |
20.0 |
22.2 |
2 |
15.5 |
23.3 |
25.8 |
15.0 |
22.6 |
25.0 |
3 |
16.9 |
25.4 |
28.2 |
16.5 |
24.8 |
27.5 |
1-3 |
15.5 |
23.3 |
25.8 |
15.0 |
22.6 |
26.6 |
4-6 |
19.7 |
29.6 |
32.8 |
18.6 |
28.0 |
31.0 |
7-9 |
23.0 |
28.0 |
31.5 |
22.9 |
27.9 |
31.4 |
10-12 |
36.8 |
44.9 |
50.4 |
38.4 |
46.8 |
52.2 |
13-15 |
51.6 |
54.8 |
60.7 |
46.9 |
49.4 |
55.4 |
16-19 |
58.3 |
61.3 |
68.6 |
45.3 |
47,7 |
53.3 |
Adults (g/kg body weight) | ||||||
2029 |
0.75 |
0.79 |
0.88 |
0.75 |
0.79 |
0.88 |
30-59 |
0.75 |
0.79 |
0.88 |
0.75 |
0.79 |
0.88 |
60+ |
0.75 |
0.79 |
0.88 |
0.75 |
0.79 |
0.88 |
Pregnant |
add (g) |
6.0 |
6.3 |
7.1 |
||
Lactating |
first 6 mo: |
add (g) |
16.0 |
16.8 |
18.8 |
|
next 6 mo: |
12.0 |
12.6 |
14.1 |
|||
thereafter: |
11.0 |
11.6 |
12.9 |
|||
* See Energy and Protein Requirements. Report of a Joint FAO/WHO/UNU Expert Consultation Geneva, World Health Organization, 1985 WHO Technical Report Series, No. 724
RNI for protein by age and sex according to the 1985 FAO/WHO/UNU (23) are provided in Table 4 for two diets having different biological value. The protein densities recommended have been derived as protein-energy ratios to represent the protein quality of the mixed diet. For high quality proteins, requirements can be met by providing 8-10% of total energy as protein. For predominantly vegetable mixed diets in developing country settings, 10-12% is suggested since protein needs should be corrected for lower digestibility and increased incidence of diarrhoeal disease. Finally, in the case of the elderly where energy intake is low, protein should represent 12-14% of total energy.
• Fat
In general adults should consume at least 15% of their energy intake from dietary fats and oils, and women of childbearing age should consume at least 20%. Active individuals who are not obese may consume up to 35% fat energy as long as saturated fatty acids do not exceed 10% of energy intake. Sedentary individuals should limit fat to not more than 30% of energy intake and also limit saturated fatty acids to less than 10% of intake, while ensuring that needs for all other specific nutrients are met (9). RDI for adults include limiting cholesterol intake to 300 mg/day. infants fed human milk or formula usually receive 50-60% of their total energy intake from fat. The use of vegetable oil in food preparations for infants and children is important to maintain energy density of liquid foods >0.6 kcal/ml. Infants should receive breast milk, but if they do not the fatty acid composition of infant formulas should correspond to the range found in breast milk from omnivorous women. During the complementary feeding period—up to 2 years of age or beyond—diet should provide 30-40% of energy from fat.
Recommendations for fat vary, depending inter alia on the prevalence of protein- energy malnutrition and diet-related NCD. For the former promoting increased consumption of fat is usually desirable, while for the latter decreasing it may be in order. The prevalence of these problems varies widely between countries and may coexist within countries. Urban slums and suburban affluence are not uncommon in many developing and newly industrialized countries, and dietary guidelines should address the problems in both. Thus, for- low socioeconomic groups in some developing countries, reaching 20% of energy from fat is a difficult goal, while among some affluent populations in the same countries there may be a need to reduce fat intake to less than 30% of total energy.
Excessive (>40 percent) intake of fat among adults may be associated with increased prevalence of obesity and associated disorders (27). It may also increase the risk of certain types of cancer (9,27,28). High fat intake reduces the nutrient density per kcal of the diet (with the exception of those nutrients occurring naturally in fats, e.g. vitamin E in vegetable fats)(29). Although it may be easier to reduce and to modify the intake of visible fat (30), hidden fat from foods like high-fat meat, cheese, baked products and some fried food also has to be considered, and included in the fat-intake calculations.
The balance between different classes of fatty acids is important, particularly with respect to the risk of coronary heart disease (9,31). Saturated fatty acids with lauric (C12:0), myristic (C14:0) and palmitic (C16:0) acids elevate serum cholesterol and LDL-cholesterol. Polyunsaturated fatty acids (PUFA) lower serum cholesterol and LDL, while monounsaturated (MUFA) are less effective in lowering cholesterol or LDL levels. However, MUFA and saturated fatty acids raise HDL cholesterol levels. MUFAs are not as easily oxidized and their effects on lipoprotein metabolism are neutral (32-34). There is no reason to set an upper limit for MUFA within accepted total fat and calorie intakes. Cholesterol intake can also increase total cholesterol and LDL-cholesterol levels in the serum of susceptible individuals, but response is less than that induced by changes in the fatty acid composition of the diet (34). Compared with dietary monounsaturated fatty acids (MUFA), trans fatty acids increase serum LDL-cholesterol, and may in fact lower HDL-cholesterol (9,35,36).
Both n-6 and n-3 polyunsaturated fatty acids (PUFA) are essential fatty acids. The balance between n-6 and n-3 fatty acids is of importance in determining the biological effects of dietary PUFAs. Some studies have shown that consumption of marine foods containing long-chain n-3 fatty acids such as eicosapentaenoic acid (EPA 20:5 n-3) and docosahexaenoic acid (DHA 22:6 n-3) is associated with lower cardiovascular mortality. EFA are also important for normal fetal and infant growth as well as for brain development and visual maturation. In vitro experiments indicate a higher susceptibility for oxidative modification of PUFA-containing lipoproteins. High intake of n-6 PUFA may have adverse effects on eicosanoid production (34). In animal studies, consumption of n-6 PUFA has been found to promote the growth of certain forms of carcinogen-induced cancer (28), which may indicate that growing tumours require PUFAs. There is, however, no compelling, direct evidence of harmful health effects of PUFA in man. There are sufficient antioxidants (e.g. vitamin E) in unrefined and properly processed high-PUFA oils to protect lipoproteins. The n-3 fatty acids may have specific beneficial effects on platelet function and insulin sensitivity.
Food sources of n-6 fatty acids are abundant in most diets. Vegetable oils commonly used such as corn oil, sunflower and safflower oil are all good sources of n-6 fatty acids. N-3 fatty acids are present in green leafy vegetables and in some vegetable oils; soy oil and low erucic acid rapeseed oil (canola oil) are good sources of n-3 fatty acids. The long-chain (greater than 18 carbon chanin length) essential fatty acids (AA, and DHA) are present mainly in animal foods, while marine foods are good sources of EPA and DHA. RNI for EFA of the n-6 family are 3-12% of energy and for the n-3 family 0.5-1.0% of energy. The ratio of linoleic (18:2 n-6) to alpha linolenic (18:3 n-3) acids should be between 5:1 and 10:1. No specific recommendations for longchain EFA have been established except for infants but the epidemiological information supports the incorporation of EPA and DHA from foods in the diet. Pregnant women should receive adequate EFA to meet the demands of fetal development.
Both the amount and the type of fats consumed by population groups along with other dietary and lifestyle factors are considered of great importance in determining the prevalence of NCD throughout the world. Dietary guidelines should include a consideration of the type and the amount of dietary fat as this is of great importance in promoting health.
Recent evidence indicates that trans fatty acids produced during hydrogenation of PUFAs may behave more like saturated fatty acids relative to their effects on cholesterol metabolism than like MUFAs. Hydrogenated vegetable fats that are high in trans fatty acids (>10%) should not be promoted as having the same health effects as fat sources containing predominatly PUFAs in the cis conformation and low in saturated fatty acids. As consumers become more informed about trans fatty acids, and as more is known of their effect, it may be useful to consider the sum of saturated and trans fatty acids in evaluating the health effects of fats and oils. Where CHD is a concern it may be appropriate to encourage the use of liquid oils and soft margarines rather than hard fats, which have a higher content of saturated and trans fatty acids.
Fats and oils also provide a vehicle for fat soluble vitamins such as vitamin A and tocopherols, and an adequate intake of dietary fat is essential for the absorption of these vitamins. Red palm oil is an excellent source of carotenoids and tocopherols; it should therefore be considered an especially good source of low-priced oil for populations where vitamin A deficiency is prevalent. Tocopherols are important natural antioxidants and will help to prevent peroxidation of PUFAs. A vitamin E to PUFA ratio of >0.6 mg per gram of PUFA is recommended.
• Carbohydrate
Carbohydrates are the main source of energy in the diet (>50%) for most people. Most dietary carbohydrates come from vegetable foods and include sugars, oligosaccharides, starch and dietary fibre.
Grain products, tubers, roots and some fruits are rich in complex CHO. They generally need cooking before they are fully digestible, especially by children. They are less soluble in water and will form gels when mixed with water, which limits the energy density of the final food product. These considerations are particularly relevant when establishing dietary guidelines for small children.
Sugars (mono- and disaccharides) found in foods or added to the diet usually increase the acceptability of the diet. Sucrose is the most common sugar in most diets and increases the palatability and energy density of diets (since it is readily soluble and well- digested). In developing dietary guidelines, when considering sugar intake it is important to consider all sources of sugars in the diet and the contribution of other nutrients from these foods.
Frequent consumption of sugar and other fermentable carbohydrates throughout the day increases the cariogenic risk potential of the diet, especially in the absence of reasonable oral hygiene practices (37,38). On the other hand, sugar intake plays a minor role in caries prevention where fluoridation and hygienic measures have been taken.
Although some foods are relatively high in fat and sugars, there is no evidence that foods high in sugar contribute significant amounts of fat to the diet (39,40). Furthermore, total sugar intake is commonly inversely related to total fat intake (40). Moderate intakes of sugar are therefore compatible with a varied and nutritious diet. No specific limit for sugar consumption is proposed, since the putative relationship of sugar consumption to obesity is offset by the inverse relationship observed between sugar and fat intake.
The glycaemic effect of starchy foods, often measured as the glycaemic index, depends on rate of digestion (41,42). This is determined to some extent by the fibre content but mainly by the availability of starch to digestion. Leavening and baking increases the glycaemic effect of starch in bread, but starch in pasta, pulses and legumes has a low and retarded glycaemic effect (41,42). The glycaemic effects of simple sugars are mainly comparable to (or less than) those of starch from cooked foods. In hypertriglyceridaemic subjects, long-term consumption of low-glycaemic index foods may reduce the risk of cardiovascular diseases by improving glucose tolerance, reducing insulin secretion and lowering blood lipids (41). A part of starch is resistant to digestion because of its crystalline or other properties. It acts like dietary fibre, passes unabsorbed into the colon, promotes bowel function, increases production of short-chain fatty acids and may reduce the risk of cancer (43).
Only small amounts of CHO are necessary in the human diet in order to prevent ketosis. Ten percent of energy or approximately 50 g per day are sufficient for this purpose. Yet, since CHOs represent on a relative basis over half of the energy intake for most populations and since the relative contribution of fat and CHO may affect the prevalence of chronic NCD, it is important to consider them when setting dietary guidelines.
Lactose is the sugar in milk and the main source of CHO in small children. It is well-digested by small children but most older children and adults in tropical countries have a limited capacity to digest and absorb a large lactose load (50 g). Nevertheless, even with laboratory-determined lactose intolerance it is possible to tolerate a glass of milk, which provides 15 g of lactose in a fat- and protein-containing solution. So this should not be a reason to limit the consumption of milk or the use of milk in food distribution programmes in developing countries. Moreover, milk represents an excellent source of high quality protein, calcium and riboflavin, and is well-tolerated by most children, even those recovering from malnutrition. Children with prolonged diarrhoea may benefit from lactase-containing fermented milk products (yoghurt) (44,45).
Dietary fibre includes non-starch polysaccharides such as cellulose, hemicellulose, gums and pectins with different solubility and viscosity characteristics (46). Fibre components have beneficial effects on glucose and cholesterol metabolism and on bowel function (47). A regular and varied intake of cereals, fruit and vegetables ensures the supply of both soluble and insoluble types of fibre.
Fibre consists primarily of the components of cell walls in vegetable foods that are non-digestible complex CHO, but also includes gums, lignins and polyphenols. An important physiological function of dietary fibre is to provide bulk for laxation and stool formation. Although most food composition tables are based on acid and alkali insoluble vegetable food components (crude fibre), this in fact includes only cellulose and lignin.
Other components of vegetable cell walls such as hemicellulose, pectins and gums have important properties which affect the intestinal micro-environment and may affect the prevalence of chronic disease. Volatile fatty acids derived from colonic fermentation of fibre component may increase metabolizable energy and also serve as fuel for intestinal cell metabolism. Fibres decrease the energy density of the diet and may be beneficial in preventing obesity (48,49). On the other hand, fibres decrease the bioavailability of specific nutrients, especially minerals, thus affecting the biological quality of foods. Most groups recommend intakes ranging from 15-20 g/day for adults (1). That is, 8-10 g per 1000 kcal are recommended; up to 20 g per 1000 kcal can be fed daily without untoward effects. If high fibre diets (>40 g per day) are customary in a given population, protein and mineral RNI should be adjusted accordingly.
• Alcohol
Alcoholic beverages are intrinsic to the dietary patterns of many human cultures. In subsistence-agriculture societies, alcoholic beverages often provided significant essential nutrients (amino acids from fermented cactus for meso-Americans, thiamin in traditional beer in Africa). However, alcoholic beverages are now generally viewed for their adverse social and health effects rather than as part of a spectrum of fermented food beverages. Evidence about the acute effects of alcohol indicates that its adverse effects (high blood alcohol-related impairment of cognitive function, or decreased cardiac output or elevated keto and lactic acids) can be partially reduced by co-ingestion with food. The effects of alcohol on increased energy density and the displacement of more healthful nutrients require particular consideration when food-based dietary goals are being developed.
There is considerable interest in the possible effects of ethanol or associated compounds (like phenols) in alcoholic beverages on the prevalence of CHD. Even though plausible pathways exist by which cardio-protection may occur (increased HDL cholesterol, decreased platelet aggregation, anti-oxidation and stress reduction), potentially adverse pathways exist as well (blood pressure-elevating and myocardial damage). A recent review of alcohol, diet and mortality data from 22 countries concluded that ethanol, particularly wine ethanol, is inversely related to coronary heart disease but not to longevity in populations (50). Light to moderate alcohol intake may improve longevity—but alcohol abuse which sharply reduces it—is correlated with average alcohol consumption in populations. Promoting the use of alcohol would appear to be unwise as a public health cardio-protective measure (50).
Much more research is required to determine both the protective effects of alcoholic beverages on CHD and how protective such a mode of alcohol ingestion may be for more chronic alcohol-related health problems, such as hepatic cirrhosis, cardiomyopathy or Korsakoff's psychosis. In some cultures where wine is regularly consumed with food, cirrhosis still remains a major health problem. Although individual differences in patterns of
Table 5. Promoters and antagonists of the uptake or utilization of inorganic elements from foods
Essential elements |
Promoters |
Factors reducing availability |
Iron |
Vitamin C; iron in the haem of animal tissues |
Excessive consumption of vegetable sources of phytates,* oxalates (e.g. spinach), tannins (e.g. tea), polyphenols and calcium salts (>500 mg Ca) |
Iodine |
Brassicae consumed in excess; Cassava inadequately dried or fermented to destroy CN | |
Calcium |
Lactose-containing foods |
Excessive dietary fibre rich in associated phytate*; plants rich in oxalates, uronic acids. Fat malabsorption |
Unidentified factors in milk |
||
Zinc |
Unidentified factors in animal protein foods |
Phytate rich foods* (e.g. whole grain cereal foods) if accompanied by high calcium |
Magnesium |
Phytate rich foods* if dietary calcium high | |
Potentially Toxic elements (pollutants) |
Factors promoting retention or toxicity |
|
Cadmium |
Low dietary Ca, low iron status, low copper status |
|
Low dietary Ca, low dietary |
||
Lead |
phytate |
|
Fluoride |
Low dietary Ca. |
* Includes maize, brown rice, beans, whole wheat flour, sorghum alcohol consumption could explain this relationship, many factors are still unknown.
Fluid needs are met by a combination of food and beverages. A dependable and chemically and microbiologically safe source of water is essential.
Obtaining a safe source of water has introduced new problems for water recirculation, packaging and storage. At the same time, depending on the location, water may be an important source of such essential elements as iron, iodine, fluoride, calcium and copper (51). Assuming a water intake of 1.5 Iitres a day, 10% of total intake for iodine, cadmium and copper can come from water, 15-20% for lead and chromium, and up to 32% for fluoride (51-55). In an aging population, thirst mechanisms may be compromised (56), so that special attention to water intake in the elderly may be worth including in FBDG.
Micronutrient deficiencies are now recognized as important in the aetiology of communicable disease, classical deficiency disorders and NCD. Therefore, the formulation of FBDG in all cultures with varying health patterns needs to take account of the fact that vitamin and mineral components of foods are essential for growth and development, for the utilization of the macronutrients, for the maintenance of adequate defences against infectious diseases and for many other metabolic and physiological functions. Stress and physical trauma such as that resulting from prolonged physical over-activity also influence micronutrient demands.
Today more than 2000 million people are at risk from micronutrient deficiencies and more than 1000 million are actually ill or disabled by them, causing, for example, nutritional anaemias, mental retardation, learning disabilities, low work capacity, and blindness (57-60). More than 13 million people suffer night blindness or total blindness due to lack of vitamin A (60,61). Low folate status is one of the earliest indicators of a vitamin-deficiency state, and represents a sensitive index of inadequate food supply in addition to its role in the prevention of birth defects (62). Risk of vitamin D deficiency in older adults, especially in those who receive little exposure to sunlight 163), and/or do not consume fortified foods, is common and is associated with increased prevalence of osteoporosis. Osteoporosis is also associated with insufficient absorbable calcium, potassium and vitamin C.
The proposed nutrient densities for the family diet depend on the consumption of adequate amounts of energy for adults and adolescents. If intake for adolescents or adults is <2000 kcal per day, it is unlikely that their vitamin and mineral needs will be met.
Interactions among the micronutrients as well as other food components are of great importance. For example, a recommendation in dietary guidelines to increase intakes of one or more micronutrients may also affect other micronutrients as well, through effects on their absorption, availability or excretion. Some foods contain inhibitors of element absorption that reduce their availability, e.g. the restriction of calcium utilization by the presence of oxalates in dietary spinach or rhubarbs. Other significant examples are the decreased availability of iron from foods rich in tannins or through excess of calcium, and the effects of dietary phytate from whole grain cereals and pulses restricting utilization of iron, zinc and possibly magnesium and calcium. On the other hand, absorption of non-haem iron is enhanced by increasing the vitamin C content of the diet- a factor that is likely to be important when plant sources of iron predominate over animal sources. Some examples are given in Table 5.
Role of vitamins in health promotion and prevention of NCD
Vitamins were discovered early this century, and the evidence of their importance was slowly pieced together, initially from observational studies and then from balance studies that measured the levels at which signs and symptoms of deficiency of specific vitamins would appear. A classical example is thiamin deficiency resulting from diets largely consisting of polished rice. Thiamin deficiency has remained an area of concern in countries such as Thailand, and in populations in developed countries who consume large amounts of alcohol. More recently, there has been an enormous amount of interest in vitamin A which, in addition to xerophthalmia, is generally considered to have an impact of around 20-30% in reducing mortality among young children in areas where vitamin A deficiency is endemic, possibly even among those with subclinical deficiency (64).
The scientific evidence supporting an important role for vitamins in promoting health and preventing NCD, independent of other nutritional constituents, is currently receiving considerable attention (65). Much of the evidence has been derived from epidemiological studies of food intake and clinical trials with vitamin supplements as well as experimental animal studies. While all three of these research approaches have implicit limitations, the overall consistency, strength and biological plausibility of the results from the work to date are sufficient now in many cases to be translated into FBDG.
Still, in many parts of the world, efforts to ensure adequate vitamin status for the primary prevention of deficiency states continue to be of paramount importance. Thus, recommendations can be offered to ensure that attention is focused on consumption of appropriate foods (or fortified foods) that provide adequate vitamin A to promote growth, prevent night blindness and strengthen corneal structure and immune function in children; folic acid to help reduce the risk of neural-tube birth defects as well as anaemia and perinatal mortality in women of childbearing years; vitamin D to promote bone health in children and adolescents, and reduce the risk of osteoporotic fractures in the elderly, especially in older women; and so on.
While currently emerging evidence remains sometimes equivocal in identifying specific vitamins as the critically responsible element providing certain health benefits, the data are sufficiently strong now to encourage the view that all populations should ensure adequate consumption of foods rich in antioxidant nutrients (i.e. vitamin C, E and b-carotene) and B vitamins (i.e. vitamins B6, B12 and folate) to reduce the risk of cardiovascular and cerebrovascular disease; and foods rich in antioxidant nutrients (i.e. vitamins C, E and 13-carotene), vitamin A and folate to reduce the risk of some forms of cancer. Experimental evidence supports a biological plausibility for these healthpromoting effects of vitamins (66). The putative protective role of antioxidants against vascular disease appears based in part upon their ability to inhibit the oxidative modification of low density lipoprotein cholesterol, a critical early step in the atherogenic process (67). The action of vitamins B6, B12 and folate may be beneficial against vascular disease, due in part to their ability to modulate homocysteine metabolism (68).
Vitamins may serve as chemo-preventive agents via their ability to prevent the formation and/ or increase the detoxification of carcinogens as well as via their regulatory role in cell differentiation processes. As the qualitative and quantitative nature of the relationship between these vitamins and the prevention of vascular disease and cancer becomes further established, this information can be more precisely incorporated into FBDG.
It is also important to recognize as part of FBDG the impact of vitamins on specific physiological function and performance, with potentially significant benefits for individual and public health: e.g. antioxidant nutrients (i.e. vitamin C, E and b-Carotene) and vitamin B6 on improving immune function in older adults; and B complex vitamins and vitamin C on improving physical performance in children.
Provocative but preliminary data regarding the relationship between generous intakes of certain vitamins and the reduction of some other diseases are continuing to evolve, but they are premature and/or insufficient to merit consideration as part of a dietary guideline, e.g. antioxidants and reduced risk of degenerative eye diseases such as cataract and age-related macular degeneration; and B vitamins (particularly vitamins B6, B12 and folate) and reduced risk of age-related cognitive impairments and dementias.
What is more important is that the evidence substantiating the disease risk reduction by vitamins indicates that most, and perhaps all, of the benefits can be provided by eating a diversity of foods rich in these vitamins and related phytochemicals (69). Moreover, as noted above, the increased intake of a few vitamins from fruits and vegetables may provide a wide spectrum of benefit in respect of several physiological functions and many diseases. These intakes, while sometimes in excess of current recommended standards for dietary requirements, can nonetheless be met through appropriate food selection. However, groups at particular risk of inadequate intake of these vitamins include children, pregnant and lactating women, and the elderly because of their increased needs or low energy intake.
The elderly represent a growing percentage of the world's population and are at risk for inadequate vitamin consumption due to their decreased energy intake and increased requirement for selected micronutrients (70). Recent research has demonstrated that elderly persons, to maintain their metabolic balance, have a higher dietary requirement than younger adults for vitamins B6, B12, D and folate (71). The physiological and health consequences of these changes in nutrient requirements can be illustrated by the potential adverse impact in older adults of low vitamin B6 status, such as impaired glucose tolerance, decreased immune responsiveness and altered cognitive function (70).
As noted, desirable ranges of vitamin intake for health promotion and prevention of NCD may be similar to, or above, current dietary recommendations (73,74). However, individual vitamin needs—and their relationship to individual predisposition to NCD—appear, in part, dependent upon differences in genetic inheritance and lifestyle factors. Increasing dietary intakes of one or more vitamins from a variety of sources to these desirable ranges does not appear to promote harmful imbalances affecting other nutritional or metabolic conditions in the body. More important still, vitamin-rich foods prepared in a way that maintains their vitamin content should be consumed on a regular and frequent daily basis, as the time frame for the utilization and elimination of some vitamins is relatively short.
Food-based approaches, including fortification, to improve vitamin status, are most likely to prove widely applicable, sustainable and affordable in the long term for avoiding deficiency diseases, improving health and preventing both NCD and many infectious diseases. Nonetheless, further research efforts, including controlled clinical trials with food-based interventions, are still required to fully establish the efficacy of this approach. Foods making important contributions to daily intakes of selected vitamin can be seen in Table 6.
• Vitamin A (retinol). Vitamin A can be preformed in the diet as retinol in animal products or derived from plant carotenoids. The biological vitamin A activity of the diet should be computed in retinol equivalents (RE). A dietary goal of 350 RE per 1000 kcal was recommended in order to meet the needs of all family members. The presence of diarrhoeal disease, infection and intestinal parasites increases vitamin A needs. Vitamin A supplementation in vitamin A-deficient children has been shown to decrease the risk of dying from diarrhoea and sometimes from other infections. Present RNI do not provide for the increased needs of vitamin A in acute infections such as measles, where adequate intakes of vitamin A are required to decrease both complications and mortality. Vitamin A fortification of foods has contributed to decreasing the prevalence of deficiency in some parts of the world (75). Low fat diets and diets poor in animal products decrease vitamin A bioavailability by limiting absorption and utilization.
Table 6. Foods making important contribution to daily intakes of selected vitamins
Vitamin |
Sources |
Vitamin A (as retinol) |
meat, fortified dairy products, liver and other organ meats, egg yolks, fish liver oils |
Vitamin A (as b-carotene) |
red/orange/yellow fruits and vegetable, e.g. papaya, pumpkin (winter squash), carrots, etc. dark-green leafy vegetables, e.g. spinach, kangkong, coriander, etc. |
Thiamin (vitamin B1) |
whole-meal cereals (wheat germ), pork, lamb, fish, poultry, liver |
Riboflavin (vitamin B2) |
soybeans, nuts, milk, cheese, yoghurt, brewer's yeast, whole grain cereals, eggs, green leafy vegetables, organ meats |
Niacin (vitamin B3) |
peanuts, enriched breads and cereals, lean meats, poultry, fish, octopus, taro, bulgar, couscous |
Pyridoxin (vitamin B6) |
green leafy vegetables, avocados, bananas, dried beans, potatoes |
Vitamin B12 |
organ and lean muscle meats, fish, shellfish, dairy products, brewer's yeast |
Folate |
dark-green leafy vegetables, avocados, liver, brewer's yeast, amaranth, oranges, corn peas, chick peas |
Vitamin C |
citrus fruits, tomatoes, peppers, leafy green vegetables, potatoes, papaya, lychee, daikon radish |
Vitamin D |
fatty fish, fortified dairy products, liver, egg yolk |
Vitamin E |
vegetable oils, nuts, wheat-germ, whole-grain cereals, green vegetables, seeds, dried beans |
Vitamin K |
broccoli, cabbage, vegetable oils, seaweeds, leafy green vegetables, yoghurt, egg yolks, liver, soya beans, potatoes, dairy products |
• b-Carotene. b-Carotene is a provitamin A compound but also acts in an independent fashion in several cellular functions, e.g. as an antioxidant trapping free radicals and quenching singlet oxygen. The bioavailability of carotenoids in many foods is less than that of vitamin A, 6 µg b-Carotene being nutritionally equivalent to 1 µg retinol. A dietary intake of 2100-3000 µg b-Carotene as provitamin A per 1000 kcal appears to be a healthy goal for all family members. The absorption and utilization of b-Carotene is enhanced by dietary fat, protein and vitamin E and is depressed by the presence of peroxidized fat and other oxidizing agents in food. It is important to note that, because only a few carotenoids serve as provitamin A compounds and many other yellow/orange pigments are present in plants, the colour intensity of a fruit or vegetable is not a reliable indicator of provitamin A content. Biologically active carotenoids are found in abundance in carrots and dark-green leafy vegetables. Additional knowledge of the carotene content and retinol bioequivalence of plant foods throughout the world is needed.
Carotene-containing foods have been implicated in a decreased risk of some forms of cancer (colon, lung and stomach) and reduced risk of cardiovascular disease in a limited number of clinical trials and several epidemiological studies.
• Ascorbic acid. This essential nutrient not only participates in intermediary and oxidative metabolism but also improves iron absorption. In addition, ascorbic acid has been shown to be necessary for a normal immune response. The proposed nutrient density of 25-30 mg per 1000 kcal may exceed true ascorbic acid needs of most individuals, but it underlines the fact that enhancement of iron absorption from plant foods is very desirable. The range in this case is derived from existing recommendations. Preventing or decreasing the prevalence of iron deficiency, a significant problem worldwide, is an important consideration in setting dietary guidelines.
Ascorbic acid in foods is affected by heat treatment in cooking and in the processing of foods. The general recommendation to avoid overcooking of vegetables and fruits is supported by the effect of heat on ascorbic acid, of which commonly about half is lost when cooking-water is discarded.
• Vitamin E. Vitamin E represents a group of dietary tocopherols and tocotrienols which serve as antioxidants, trapping oxygen free radicals to prevent the propagation of the oxidation of unsaturated fatty acids and other target molecules. Requirements of vitamin E are increased with increasing intakes of unsaturated fats. The biological vitamin E activity of the diet should be computed in milligram a -tocopherol equivalents (mg a-TE). A dietary intake of 3.5-5.0 mg a-TE per 1000 kcal appears to be a healthy goal for all family members. Frank vitamin E deficiency is usually limited to premature infants and patients with diseases characterized by fat malabsorption, but marginal intakes and subclinical deficiencies may be prevalent in many populations.
• Vitamin E has been associated with a reduced risk of a number of chronic diseases, including coronary heart disease, cataract and some forms of cancer in a limited number of clinical trials and several epidemiological studies. Vegetable oils, including those from soybean, safflower and corn, and seed oils, nuts, whole grain and wheat germ represent the principal source of vitamin E. Absorption of vitamin E is dependent upon concomitant digestion and absorption of dietary fat.
• Folate. The suggested nutrient density for the family mixed diet is 150-200 µg per 1000 kcal. This level is sufficient to prevent folate deficiency in all family members including women of childbearing age. It is now well-established that diets providing higher folate intake or supplementation with 400 µg of folate per day before and during pregnancy are associated with a lower prevalence of congenital neural tube defects. In addition, recent evidence on the role of folate in cysteine metabolism indicates that the suggested folate density of the diet is also adequate to lower homocysteine plasma levels and decrease cardiovascular risk. The folate recommendation is higher than that recommended by FAO/WHO but is consistent with the US RDA for pregnant women (26). It is also in line with the views expressed by the Scientific Committee for Food of the Commission of the European Community (76). The consultation felt that the new evidence was sufficiently compelling to use the higher value.
Folates found in animal and plant foods can be easily destroyed by cooking since they are labile to moderate heat. Boiling milk or heat processing it destroys folates, leaving minimal amounts in the powdered milk product. Folate deficiency is prevalent in many developing countries, especially during pregnancy and lactation. The recommendation to avoid overcooking food is supported by the effect of heat on folate.
The potential benefit of folate on the prevention of some types of cancer (lung, stomach) remains speculative and any recommendation based on this effect should await further evidence.
• Thiamin, riboflavin and niacin. Deficiency of these B vitamins is now uncommon except under conditions of extreme social deprivation or among alcoholics. The proposed minimum nutrient densities of 0.5, 0.6 and 6 mg per 1000 kcal for thiamin, riboflavin and niacin, respectively, are sufficient for all family members. Grain products lose part of these vitamins during processing, dehulling and refining. Thiamin is particularly heat-sensitive in an alkali environment; thus it is recommended not to use bicarbonate in water when cooking. Niacin can be taken preformed from the diet or synthesized from dietary tryptophan (60 mg yields 1 mg of niacin); this should be considered in defining the potential niacin content of a food. On the other hand, if the diet is limited in tryptophan, little or no niacin will be formed. Maize is treated with alkali and heat as part of traditional food processing to prepare tortillas in Central America and Mexico, thus freeing up niacin and making it absorbable; the tryptophan too becomes more absorbable. This may explain the low prevalence of pellagra in this region despite the people's predominantly maize diet, which is low in niacin. Fortification of wheat flours or other staple foods has been successful in eradicating B complex deficiency in various parts of the world.
• Vitamin B6. Comprising of pyridoxine, pyridoxal and pyridoxamine, Vitamin B6 is converted in the body to pyridoxal phosphate and pyridoxamine phosphate, which serve primarily as coenzymes in transamination and decarboxylation reactions. The requirement for vitamin B6 increases with increasing intake of protein because of its role in amino acid metabolism. A dietary goal of 0.6-1.0 mg vitamin B6 per 1000 kcal appears satisfactory to meet the needs of all family members. Vitamin B6 deficiency is generally noted only among people deficient in several B-complex vitamins.
In addition to its essential role in the metabolic transformation of amino acids and the metabolism of lipids and nucleic acids, it has been suggested that vitamin B6 modulates immune responsiveness and glucose homeostasis. The most nutrient-dense sources of vitamin B6 include chicken, fish, kidney, liver, pork and eggs, although unmilled rice, soy beans, whole-wheat products and peanuts are also good sources. Substantial amounts of vitamin B6 are lost through food processing. A wide variety of therapeutic drugs have been reported to interfere with the bioavailability and/or metabolism of vitamin B6.
Minerals and trace elements
Minerals and trace elements of public health importance that have received attention include calcium, iron, iodine, zinc, sodium and fluoride but, depending on geographical, environmental and cultural factors, copper, selenium and probably others could also be included. Physiologically the trace element nutrients have an extremely wide range of functions, many of which are still under investigation. Foods making important contributions to daily intakes of essential inorganic elements can be seen in Table 7.
Table 7. Foods making important contributions to daily intakes of essential inorganic elements
Element |
Sources |
Iron |
Meat (ruminant, pig, pigeon), liver, blood, green vegetables, cereals, pulses, teff |
Iodine |
seafood, milk, cheeses, cereals, (all contents influenced greatly by regional iodine contents of soils), iodized salt, some (few) kinds of rock salt |
Calcium |
milk, cheeses, legumes, pulses, green leaves |
Phosphorus |
milk, cheeses, cereals, meat |
Potassium |
root vegetables, green vegetables, bananas/plantain |
Zinc |
red meats, cheeses, milk, pulses, legumes |
Magnesium |
green vegetables, cereals |
Copper |
Liver, green vegetables |
Selenium |
Cereals (reflect selenium contents of soil) fish, meat, eggs |
Chromium |
Red meats, whole cereal products, pulses, spices |
Molybdenum |
Legumes, pulses |
Fluoride |
Tea (content influenced strongly by fluoride in ground water or irrigation water) |
Boron |
Vegetables |
The susceptibility of communities to the effects of heavy metal pollutants is influenced by dietary mineral balance as well as by protein status and the physical form in which pollutants enter the diet. Thus cadmium and lead absorption are potentiated by lower dietary calcium and low iron (51,52). In contrast, foods high in physic acid content are potent inhibitors of lead absorption (52,77) and of iron absorption.
Recognized geochemical variables have a profound influence on the content of some elements such as iodine, selenium, molybdenum and manganese in foods. The soil parent and agricultural conditions in various ecological zones can influence, sometimes more than tenfold, the content of these elements and of fluoride in staple crops. FAO has reported extensively upon the ways in which such regional factors influence mineral element supply (53,78,79). They will have a profound influence on the significance of iodine deficiency as a cause of delayed development in infants and children. Losses of elements during food processing should be taken into account; for example, some forms of selenium and of iodine in foods can be lost during heat treatment. Sodium, potassium and magnesium are all lost to a greater or lesser extent during boiling.
• Calcium. This element is essential as a component of the mineral matrix of bone, and as a regulator of nervous system and muscle membrane function and clotting mechanisms. Virtually all calcium in the body (99%) is deposited in bone. Mineral deposition in bone peaks by 25 years of age.
After the menopause, calcium loss in bone exceeds deposition and this leads to progressive demineralization. Osteoporosis with advancing age increases the risk of fractures. Most recommendations are based on needs for calcium balance and retention in bone, rather than on acquired peak bone mineral density. Thus present RNI may need to be re-examined.
The relatively high calcium requirements needed to confer protection on infants against depletion before weaning and to promote satisfactory bone density during adolescence are now recognized internationally (82). The debate continues about the wisdom of further increases in calcium intakes by other age groups, in an attempt to reduce the high incidence of osteoporosis and bone fracture in some geographical regions as compared to others (83,84)). Genetic variables, differences in physical activity, nutrient and dietary interactions, and ethnic factors influencing the relationships between calcium intake and skeletal density are inadequately understood.
Evidence from non-invasive measurements of bone density indicate that a range of dietary variables influence the beneficial effects of calcium-rich foods upon bone density. These include vitamin C, fruit and vegetable intake, and intakes of potassium and fibre (85). Whether these act directly or indirectly is not known. Calcium loss is promoted by even mild degrees of dietary acidosis (86) or by excessive intake of protein (particularly of animal protein sources) (87).
Sequelae of over-enthusiastic use of calcium supplements—often disregarded include—reduced absorption of iron and of zinc from diets rich in vegetable sources of the physic acid, the antagonistic effects of which are potentiated by high dietary calcium.
Metabolic synergisms involving calcium, magnesium and potassium influence the physiological and functional effectiveness of all three elements in the maintenance of healthy nervous tissue and of skeletal integrity (88). Dietary balance should be maintained between these elements even though their contents in staple foods differ markedly.
In tropical developing countries, higher sun exposure and increased activity levels may be protective for osteoporosis, so RNI are usually set at lower levels. in addition, recent evidence indicates that bone mineral density in China is adequate despite chronic lower Ca intake, suggesting that some populations may have increased absorption of calcium and can therefore have adequate bone mineralization on lower levels of intake (80).
The calcium RNI in the USA are higher than those accepted internationally and extend the increased needs of adolescents into young adults (24 yrs of age), the point at which peak bone mass is reached. The newly acquired results concerning bone density favour calcium intakes beyond the needs for Ca balance and retention for growth. Recent evidence that bone mineral mass can be influenced by calcium intake in prepuberty through early adulthood has resulted in an increase in the RNI proposed for this age grouping.
A nutrient density of 250400 mg Ca per 1000 kcal is proposed in the light of diverse point of views on this subject. The higher level is suggested for industrialized countries; it will meet the needs of all family members except pregnant women. This goal is virtually impossible to meet unless dairy products are consumed in sufficient amounts. Sea foods, sardines with bone, legumes and lime (calcium hydroxide)-treated maize products are important potential sources of calcium where dairy products are not consumed.
• Iron. This essential metal is a constituent of haem and cytochromes, and is a co-factor for redox and other key enzymatic reactions. Relatively minor decreases in iron status associated, for example, with haemoglobin concentrations within the range 120-100 g Hb/litre, are now recognized to delay development of cognitive function (89) and to induce behavioural changes in children (59). Available iron should be sufficient to reduce the current high prevalence of iron deficiency as manifested in more than 2000 million people worldwide by clinical anaemia, behavioural changes and other covert signs of deficiency (59). Voluntary work performance is adversely affected in adults (81). The absorption of heavy metal environmental pollutants, such as cadmium and lead, is enhanced by even a modest fall in iron status (51,52). Absorbed iron needs are defined by replacement of obligatory losses and requirements for growth. Milk is a relatively poor source of iron; thus infants weaned to unfortified cow's milk are at high risk of iron deficiency. Women, because of menstrual iron losses, are at higher risk of deficiency. Pregnancy also imposes additional needs (30-60 mg/day) that can be difficult to meet by regular mixed diets. Due to the variable (1-30%) absorption of iron in the mixed diet, RNI are best defined based on iron bioavailability within the mixed diet.
Animal foods and ascorbate are important enhancers of non-haem iron absorption; phytates, polyphenols, tannins and fibres lower it. Thus, recent RNI (90) are defined in terms of very low (<5%), low (5-10%), intermediate (1118%) and high (>19%) bioavailability. The corresponding densities suggested for the mixed diet at the family level are 20,11,5.5 and 3.5 mg per 1000 kcal. Low bioavailability diets are cereal-based and have low ascorbate content. Intermediate absorption is found in diets based on plant foods with some animal protein and ascorbic acid; similarly, high bioavailability diets have intermediate absorption if consumed with coffee or tea, which contain polyphenols and tannins, respectively. High bioavailability is found in predominantly animal protein-based diets with fruits rich in ascorbic acid. It is evident that high-income groups will have lower needs and higher intake with greater bioavailability than low-income groups. Fortification of staple foods such as wheat or maize flour, sugar, salt or soy sauce have been successfully implemented in various countries to help prevent iron deficiency.
• Iodine. Iodine deficiency disorders, manifest as goitre, cretinism and learning defects in children exposed to deficiency during fetal development, are widespread and have important public health consequences in all regions of the world (57). This element is vital for cell differentiation and thyroid hormone synthesis. Deficiency in early life can affect brain development even in the presence of normal thyroid function. Low iodine content of water is considered a valuable indicator of iodine content of foods in a given ecological setting. Low iodine in foodstuffs is a key determining factor in the prevalence of iodine deficiency goitre. A density of 75 µg 1000 kcal is suggested. Consumption of marine foods is desirable, although iodine fortification of salt is in practice the most effective way of eradicating this deficit.
• Zinc. This essential mineral has gained greater significance in human nutrition since it was demonstrated to be crucial for linear growth (particularly of males) and normal immune function. Susceptibility to zinc deficiency is high at times of maximal rates of protein synthesis. Thus, the substitution of sources of highly available zinc such as milk, meat and fish by vegetable sources of protein, rich in physic acid, is frequently the cause of deficiency during rehabilitation from infection and from any nutritional insult that has restricted growth.
Zinc absorption in the mixed diet can vary from 10-30% and is dependent on interactions with other nutrients such as phytates and fibres. The Zn nutrient densities suggested for high (20%) and low (10%) bioavailability are 6 mg and 10 mg per 1000 kcal, respectively. Recent studies indicate that growth faltering of young children after weaning can sometimes be prevented if Zn supplements are provided. Although the concentration is low, zinc in human milk (but not cow's milk) is well absorbed. Animal proteins other than milk are excellent sources of bioavailable zinc, thus supporting the suggestion that 1025% of dietary protein should be of animal origin.
• Selenium. A low selenium status restricts the efficiency with which iodine is incorporated into the thyroid hormone, triiodothyronine (91) and its deficiency leads to reduced rates of tissue growth and other IDD-related symptoms.
Recently, concurrent deficiencies of selenium and vitamin E have been shown to increase susceptibility to several viral infections inducing enteritis and cardiomyopathy (92-94). Recent studies indicate that selenium has at least two major physiological roles. It is involved, like vitamins E and C, in processes protecting body tissues against the damaging effects of highly reactive oxygen-rich compounds generated after tissue injury or infection, and in the synthesis of the iodine-containing hormone, triiodothyronine. Indications are that selenium intakes exceeding about 10 µg 1000 kcal afford protection against these effects. Concurrent exposure to some viral infections is now believed to potentiate expression of the effects of selenium deficiency, as for example in the cardiomyopathy of infants and young children with Keshan disease associated with selenium deficiency in China.
• Fluorine. This element, absorbed as fluoride, is necessary to strengthen hydroxyapatite crystals in dental enamel and dentin. It also affects bone growth and remodeling, and is considered an essential element. Ingested water is an important contributor to fluoride intake in many areas of the world, while in other areas foods of marine origin and tea represent the main sources. Fluoridation of water supplies where their natural content is low is the most practical way of assuring sufficient intake. The suggested dietary goal for fluoride is 0.7 mg per 1000 kcal. Fortification of salt has also been proposed (and is implemented in Switzerland). Excess fluoride intake (>2.5 mg/day) causes mottling of enamel; extremely high intakes (>8mg/day) cause deformations of the skeleton.
• Sodium. Sodium is an essential component of extracellular fluid, serving as the primary regulator of osmotic pressure in the extracellular space. It also determines trans membrane bioelectric potentials, and changes in membrane permeability trigger depolarization and conduction of electric signals. Sodium is also required for regulation of osmolarity and acid-base balance. Excess sodium is lost through the kidneys. Sodium deficiency resulting from low dietary intakes is rare; it can occur with heavy and persistent sweating or when the body is unable to retain sodium due to trauma, chronic diarrhoea or renal disease.
High sodium intake, as measured by increased urinary sodium excretion, is associated with elevated incidence of cerebrovascular stroke in some populations (95). A specific genetic susceptibility to sodium is likely to be a determining factor in the causation of essential hypertension in human populations. There are data indicating that a high sodium intake in elderly subjects may be associated with elevated blood pressure (96). Some hypertensives benefit from sodium restriction.
It is important to note the interaction between sodium, potassium and calcium with regard to blood pressure. Adequate calcium:sodium and potassium:sodium ratios help to maintain optimal blood pressure.
Dietary guidance to use salt and sodium in moderation should depend on the extent of hypertension as a public health problem and its causes. "Moderation" has been defined differently for different populations, usually depending on the level of sodium currently being consumed. The customary sodium content of natural foods is sufficient to meet requirements, therefore no lower limit for intake has been established except for growing infants. An upper limit of 6 g sodium per day or 2.5 g per 1000 kcal is commonly suggested as a population mean target value by a majority of RNI.
Several compounds of interest have traditionally been investigated as anti-nutrients, including phytates, antitrypsin and other enzyme inhibitors, tannins, phenolic compounds and lecithins. When diets are marginal for nutritional adequacy, these anti-nutritional effects may be of significance. By contrast, in diets that are nutritionally adequate, the physiological activities associated with other compounds may provide some of the health benefits now known to be associated with plant foods, e.g. flavonoids and salicylates.
Most food salicylates (in microgram amounts) come from fruit (97), and their presence may explain, in part, some of the relationships between fruit intake and NCD. Although acetylsalicylic acid (aspirin) is not always biologically equivalent to salicylate, aspirin studies provide some insight into how food salicylate might be important in the prevention of cardiovascular disease and gastrointestinal cancers (98).
There is a problem, however, for those who are salicylate- or aspirin-sensitive, although this is dose-related and less likely when salicylate is ingested as part of the food supply (99).
Non-provitamin A carotenoids. There are over 500 plant carotenoids and many of these compounds appear to have significant biological activity in humans. Lycopene is a main precursor of carotenoids in plants (100). It has no vitamin A activity but is a particularly powerful trapper of singlet oxygen (101). Lycopene is a main colorant of tomatoes and watermelons. It exemplifies how a non-nutrient, by its colour, may help to identify foods in FBDG with biological properties beyond those usually sought from foods. Several carotenoids lacking provitamin A activity are readily absorbed from the diet and distributed to tissues. For example, lutein and zeaxanthin are selectively concentrated in the retina, and their dietary intake may be correlated with a reduced risk of age-related macular degeneration (AMD). In any case, increasing the consumption of foods rich in certain carotenoids, in particular dark-green, leafy vegetables, appears to decrease the risk of developing advanced or exudative AMD, the most visually disabling form of macular degeneration among older people (102). Some carotenoids also possess potent antioxidant activity which may be a basis for their association with reduced risk of some forms of cancer although a recent report of the seven-country study concluded that, though average flavonoid intake may partly contribute to differences in coronary heart disease mortality across populations, it does not seem to be an important determinant of cancer mortality (1 03).
Polyphenols. Plant-derived polyphenols include a wide variety of chemical compounds, including the bioflavonoids. Many bioflavonoids possess antioxidant activity and have been shown to reduce oxidative stress status by quenching free radicals and/or binding to minerals like iron which catalyse free radical reactions. A limited number of experimental and epidemiological studies have associated increased intake of bioflavonoids with a reduced risk of cardiovascular disease and cancer (104-106).
Phyto-estrogens. These fall into three families of compounds, flavonoids, coumestans and lignans, which are found in a wide range of plant foods and—following their ingestion in— animalderived foods (e.g. milk from cows feeding on certain clovers). They have been shown in human studies to moderate the expression of the menopause (107) and are likely to have wide-ranging biological effects on bone, cardiovascular disease and the risk of certain cancers (e.g breast and prostate), although they are generally weakly oestrogenic or even anti-oestrogenic. Some, like genistein, may be immuno-modulatory and anti-angiogenic, with potential importance in metastatic cancer and diabetic retinopathy (108).
Consequently, in terms of FBDG, these non-nutrient food components will become increasingly important as more is discovered about them, and constitute an important rationale for FBDG.
For the purpose of dietary guidelines the family should be considered the unit of consumption. This approach assumes that all members of the family consume the same mixed diet except for children under 2 years of age, elderly people who present difficulties in eating enough food, and persons who are ill and have special requirements.
In most countries there are some particularly vulnerable age/sex groups (some of which are dealt with below) and other groups which are vulnerable because of geographic or socioeconomic conditions, including groups at risk of specific deficiencies, e.g. rickets, beriberi or scurvy, for which specific guidelines may be necessary.
The "menu" of specific dietary guidelines will depend on the circumstances of a country, and so will their content. Only some general principles are illustrated below. In these groups, examples are given of guidelines appropriate for some commonly occurring deficiencies or imbalances; but the content of these guidelines should be specifically tailored to the particular nutritional and dietary situation in the country.
In most of the groups listed below there are certain health, food and nutritional beliefs whose nutritional impact may be beneficial, neutral or harmful. Those which are beneficial should be encouraged, while those which are harmful should as far as possible be identified and circumvented through culturally acceptable alternative dietary or health practices.
• Infants and young children
The general guidelines are designed to cover children above 2 years of age. Infants and children to 2 years are an especially vulnerable group in almost every country. Special attention is needed to ensure achieving full growth and development potential. This development should be followed by growth monitoring and promotion, preferably on an individual basis or community-based.
Nutritional requirements are normally fully covered by breast milk (if the mother is in good health and well-nourished) for the first 4-6 months of life. During this period exclusive breast-feeding is recommended. Too early introduction of any other liquid or solid feeding has been shown, in a wide variety of socioeconomic conditions, to reduce breast-milk production and increase risk of infection. Several measures to promote breast-feeding are already widely implemented (109,1 10).
After 46 months breast milk alone usually cannot provide all the nutritional requirements, and thus complementary foods are needed. These should be introduced following a country-specific infant feeding guideline which indicates at least in a broad way the quantity, quality and timing of foods to be introduced with proper hygienic care, in culturally acceptable and socioeconomically feasible ways (therefore, to be elaborated in partnership with the communities concerned). A clear national guideline and even adaptations to different dietary patterns are needed. These guidelines should be made available at all levels of the health system, including agricultural extension, community development and education services. WHO recommends that children should continue to be breast-fed up to two years of age or beyond, while receiving nutritionally adequate and safe complementary foods (111-113).
Major nutritional deficiencies such as protein-energy malnutrition, iron deficiency anaemia, vitamin A deficiency and so on, often become manifest during the second year of life but may begin to develop even during early infancy (due to intrauterine growth retardation, low birth weight, prematurity, inadequate lactation because of poor maternal health, etc.) or during the second six months of life. Laying a good nutritional foundation during the second semester of life is often the key to subsequent good growth and nutrition. Ensuring frequent feeding (4-6 times daily) during this period is of great importance, and also ensuring adequate amounts of food and adequate nutrient density of complementary foods. For this purpose, it is recommended to provide relatively thick porridge and add some oil or fat (non-saturated) where possible to achieve this, e.g. one teaspoon of oil with each serving of porridge.
Adequate feeding during the phase of mixed feeding, and after weaning, needs to be accompanied by appropriate public health measures to minimize infections (environmental health, food safety, immunizations, etc.) to ensure adequate nutrition. All this can best be done in the context of primary health care programmes and of community-based feeding, as well as through optimal family feeding and intra-family food distribution.
• Pregnancy
Dietary intakes, especially in food-deficit, low-income populations, are commonly not increased much or at all during pregnancy, and can thus be less than is recommended to increase maternal fat stores to support lactation It is usually important to recommend moderately increased dietary energy intake (approximately 10%) during the second and third trimesters. Body weight should be monitored during pregnancy. Particularly in the presence of excessive thinness, low BMI, no weight gain or low weight gain (<1 kg per month), energy intake should be increased. This is not necessary, however, if the gain is consistently >1 kg per month.
Iron deficiency anaemia is very common (>50% in developing countries; about 17% in industrialized ones) and is often associated with folate deficiency, particularly where malaria occurs. Foods relatively rich in iron, vitamin C, folates, and also vitamin A in vitamin-A-deficient areas, should be particularly encouraged as well as some foods of animal origin, while avoiding consumption of inhibitors of iron absorption, e.g. tea/coffee with meals.
Calcium intakes can be increased by ensuring daily intake of moderate amounts of dairy products, small fish (eaten whole) and soft bones (e.g. chicken). Tubers usually provide more calcium than cereals. Excessive calcium inhibits iron absorption.
Fruits and vegetables and good food-protein sources should, where possible, be increased in the daily diet.
Salt and energy intake should be restricted in case of oedema.
• Lactation
Lactating women are at some risk of nutritional depletion, particularly in low-income, food-deficit populations, where energy and other nutrient intakes may not be increased adequately to cover their increased requirements.
Energy and protein dietary intakes should be increased by about 20% based on the existing dietary pattern, preferably with additional protein foods. Body weight should be monitored regularly. Those women losing weight steadily should be encouraged to eat more and, if necessary, provided supplements, but not those who are overweight or gaining weight normally.
Fruits and vegetables should also be increased.
• The elderly
Most reports of nutrient recommendations currently in use do not specify the particular needs of the older age groups.
With advancing age, particularly beyond the age of 70, among the most important changes affecting appetite strength and energy needs are a decline in lean body mass and BMI. In the diet of the elderly it is therefore important to maintain adequate nutrient density to ensure adequate nutrient intake, to compensate for these changes. Thus, if nutrient sufficiency is to be maintained, the values of nutrient densities in Table 3 may need to be increased when dealing with the needs of the elderly because of the lower appetites, lower BMIs and lower total calorie intakes. As with other age groups, the importance of continued physical activity among the elderly—to stimulate appetite and maintain muscle mass and strength—should be a priority.
In countries where restricting fat intake is a priority, the detrimental effects of fat and energy restriction for those already at risk of losing weight and lean body mass should be considered. Restricting energy intakes in this group is not good nutritional practice.
Consideration should also be given to vitamin D intake for those among the elderly without adequate sun exposure and activity, and to vitamin B12 in the presence of lower efficiency in absorbing food-bound vitamin B12. Folate and pyridoxine may also be problem nutrients in the elderly. Among the minerals, intakes of calcium and zinc may be low with the problem compounded by decreased absorption.
Given the variety of foods that can be combined to provide a healthy diet, it is difficult to define the ranges of intakes for specific foods that can be combined to provide a nutritionally adequate diet. Although a large set of food combinations that are compatible with nutritional adequacy could be defined, these can hardly be extrapolated to different ecological settings. Another approach to defining nutritional adequacy of diets based on a scientific understanding of the biochemical and physiological basis of human nutritional requirements in health and disease has evolved over the past two centuries. This has permitted the definition of essential nutrients and the establishment of RNI.
FBDG as an instrument and expression of food and nutrition policy can be based directly upon diet and disease relationships of particular relevance to the individual country. In this way priorities in establishing dietary guidelines can address the relevant public health concerns, whether they are related to dietary insufficiency or excess. In this scenario, meeting the nutritional needs of the population takes its place as one of the components of food and nutrition policy goals, along with the priorities embodied in the FBDG for improved health and nutrition for a given population.
Three possible approaches are useful in assessing nutritional quality in the development and evaluation of FBDG:
• Food pattern
Adherence to a reference food pattern with an apparent favourable health relationship is one way of evaluating the nutritional quality of an envisaged dietary guidelines approach.
This is most likely to be a traditional food pattern of people with longevity, low morbidity and low perinatal and infant mortality rates (e.g. Scandinavian, Japanese, Mediterranean) whether the diet was established by tradition or acculturation. In most of these populations, other factors such as health care, educational system, safe water and socioeconomic development play important roles in relation to favourable health indicators.
Tracking health indices in populations in relationship to their intake patterns has, so far, been the most readily available evidence on which to base FBDG. Some key studies are:
• The Zutphen (Netherlands) study by Kromhout et al (1 14,1 15) shows that those men in the highest quintile of consumption of plant-derived foods (using dietary fibre as a marker) lived longest with lowest morbidity. Even one meal of fish a week also increased cardiovascular survival.
• Studies of US nurses (women) and health workers (men) by Willet and co-workers show that certain food sources of micronutrients (and possibly supplements) may reduce the risk of CVD, while alcohol may increase the risk of breast cancer in women (116-120). The complete data set clearly provides an opportunity to look at overall health/food patterns in prospective fashion, but this analysis awaits final compilation.
• Singapore's Food and Cancer studies by Lee et al (1 21) demonstrate that deviations from traditional Chinese and South-East Asian food patterns (e.g. those based around soya products) are associated with increased rates of certain cancers (e.g. breast).
• The Boston-lreland cardiovascular disease study of men by Kushi et al (122) study shows that plant-food intake is linearly and inversely related to increasing coronary mortality over 20 years, whereas fat-cholesterol indices (Keys and Hegsted scores) had a more J-shaped relationship. This suggests that food scores may be more useful than nutrient scores in the development of dietary guidelines.
• Data from the Women in Western Sweden study by Lapidus et al (123) show that higher levels of energy intake and, with it, more food were predictive of lower coronary mortality rates in women controlling for body weight. This is helpful in developing dietary guidelines for women that are not based on life-long dietary restriction.
• In US NHANES (124), a food diversity score, based on food groups, had predictive power for mortality and allowed sex differences in this prediction to be recognized.
• Food variety indices
While the value of increased food variety in either ensuring essential nutrient adequacy or decreasing the risk of food toxicity has been understood for sometime (125), food variety as a predictor of health outcome is relatively recent (124,126). But enough evidence is available to justify this technique's inclusion in the development of FBDG as a technique to reduce morbidity and mortality while awaiting further scientific studies on how it operates.
In deriving indices of food variety, decisions are required about the numerator (categorization of food) and denominator (time base over which variety is achieved).
These have been the subject of technical reports by both FAO and WHO, which are currently being updated, and many countries have established their own recommended nutrient intakes.
Food science and technology are creating a new framework for FBDG, principally in the areas of food physico-chemistry; methods of food storage and preservation; changes in food preparation; and opportunities for use of formula foods where energy intake is low or usual foods cannot be eaten.
Physico-chemistry: Factors like viscosity and particle size of food with the same chemistry can influence food digestion and absorption (e.g. protein, bioavailability from peanuts, glycaemic index) and the colonic microenvironment (e.g. microflora, substrates for the colonocyte) (127129).
Preservation: Reduced use of salt and curing, and increased canning, dehydration, freezing and irradiation, are a result of innovations in food technology. New packaging techniques are also reducing food wastage and prolonging the shelf-life of fruits and vegetables. At the same time, new microbiological problems, e.g. Iisteriosis, may arise where the full impact of newer storage technologies is not appreciated. Some of these have implications for particular groups, e.g. the avoidance of soft cheeses, which may contain listeria, by pregnant women who could be at risk of abortion. Such factors may create the need for specific FBDG for special groups like pregnant women.
Preparation: Food-based dietary goals can encourage or discourage the use of certain food preparation techniques, e.g. boiling because of nutrient loss, frying with fat, microwaving for flavour retention, cooking with herbs and spices rather than with sodium chloride, soya sauce or monosodium glutamate.
Formula Foods: Where energy needs are low or usual foods cannot be eaten, nutrient (and non-nutrient) needs may be met by formula foods, and guidelines for their use may be required.
Functional foods: Foods are emerging—referred to as functional foods (130)— which do not resemble, or are surrogates for, traditional foods, or […] address some newly understood biological or health need. Some basic food commodities such as cereals and fruits, are being used more as nutrient delivery systems than as a basic commodity. The ability to distinguish between traditional foods and analogue foods is limited; hence consumers may be unaware of differences in nutrient content or bioavailability, and how these differences influence the adoption of FBDG (131).
Both the practicality of FBDG and the consequences of following them require analysis through the behavioural and social sciences. For example, urging a reduction in the traditional use of salt may lead to an unintended reduction in the use of certain foods, e.g. potatoes or salted fish. Likewise, if energy restriction is inferred in FBDG, this may further exacerbate the current epidemic of eating disorders in young women in Western countries. An understanding of motivation and behaviour can anticipate and help to avoid unintended consequences of introducing FBDG.
Several research studies in nutrition education have attempted to document the difficulties that the general public has when interpreting dietary guidelines presented in terms of nutrients, even if the public is well-educated (132,133). For example, "cholesterol" is often equated with blood lipids, "saturated fat" as a food full of fat, and "polyunsaturated fat" as a food empty of fat (134). More complicated nutrient-based statements, including mention of specific fatty acids, may be dismissed altogether due to unfamiliarity. It is recommended, therefore, that dietary guideline advice be presented in food-based terminology whenever possible. Nutrients should be mentioned only in an instructional context, where the public has had an opportunity to learn new word meanings, rather than relying on their own word meanings for interpretive purposes.
Cognitive science has produced several findings relevant to the formation of dietary guidelines. First, it has been shown that people have the ability to code information in at least two different ways, either verbally or visually. Verbal coding includes words or text that are heard or seen. Visual coding refers to pictures or graphics that have been seen or observed. When both coding systems are used, the information is much more easily retained (135-137). In addition, the amount of information presented at one time will also influence its retention. Humans have an information-processing capacity of 7(±2) concepts, or not more than 5-9 concepts in their working memory at one time. Thus, if people are expected to recognize the information as a set and use it all together (138), any information presented as a list or set should never exceed 5-9 concepts. In addition to being limited to 5-9 statements (the lower limits of information-processing capacity) they should be illustrated to maximize their effectiveness.
Considerations of agricultural and environmental science are important in developing and implementing dietary guidelines. After all, it is activities within these domains that largely determine whether the food supply can support and sustain adherence to dietary guidelines. Dietary advice is only appropriate if it can be put into practice. One implication of this is that, for dietary guidelines to be followed, the range of foods available to the target population should be nutritionally adequate, of good quality and able to support the recommendations in the guidelines. Most importantly, the recommended foods should be accessible to and affordable by the target population.
This does not mean that dietary guidelines should be based solely on currently available food supplies. In fact, they can be useful in assessing the adequacy of food supplies and, especially where food supplies are obviously inadequate, in assisting producers and planners to improve the quantity, quality and variety of available foods. For example, the inadequacy of food supplies in low-income, food-deficit countries does not preclude dietary guidelines from recommending adequate intakes. On the contrary, the need to assure the availability of nutritionally adequate food supplies should be a driving force behind food, agriculture and trade policies in those countries.
However, stressing that dietary guidelines should be feasible does imply that any changes in the food supply that might be required to implement the guidelines are not only possible but probable. In a country with an efficient system of food production, processing, and marketing and trade, changes in consumer demand for food products will alter their availability and price. To the extent that dietary guidelines can alter food demand and consumption patterns, supplies can also be altered. The greatly increased supply of low-fat foods in many countries, in response to increased consumer demand for these products, is a good example. But if dietary guidelines attempt to promote unrealistic, unattainable or undesirable (from a food consumer's point of view) dietary intakes, they will not be followed. To promote higher intakes of olive oil in countries where it is unavailable, prohibitively expensive or simply undesirable to consumers, is not an effective strategy to decrease intakes of saturated fats, particularly in the near- to medium-term.
In all cases the role of environmental and agricultural science in developing and implementing FBDG needs to be recognized. This includes a range of issues related to land and water use and to the production and processing of crops, livestock, fishery and forestry products (139). Both basic and applied research can be especially important in either expanding the production of particular foods or in modifying their nutrient content and that of non-nutrient components to satisfy dietary guidelines. The concept of sustainable agriculture development is particularly important and needs to underpin efforts to meet the food needs of present and future generations.
Because many factors are still not understood in the relationships between food constituents, disease and health, there are numerous residual scientific concerns and areas of potential interest in those relationships. Much of the current research isolates and attributes the observed effects to a specific nutrient or non-nutrient component. However, it is well-recognized that the effect may be due to, or modified by, as yet unidentified food components. Research in this area will bring together work by food scientists, epidemiologists, biochemists and nutritional scientists.
Among the issues that have been identified are the following:
• The physico-chemical properties of foods and how they are processed, refined, prepared and stored affect their biological activity (140). This is important in diabetes, as the glycaemic index of a specific food is altered by particle size, viscosity etc. (141). There may be a further effect on the microflora of the large gut, and even on nutrient uptake, as less-digested food proceeds into the large bowel (142). To cite one example, cereal as an extruded product has different nutritional effects depending on the base from which it is made e.g. rice, wheat, or whole-meal flour, and with which it is chemically identical. Peanuts as peanuts or as peanut paste will affect differently protein bioavailability and fat content (143). Differences in the granularity of protein clots produced in the young stomach are believed to be related to differences in the availability of zinc, copper, iron and calcium from analogous or heterologous units.
• Food patterns and food preparation can also be important, e.g. in levels of vitamin A which vary depending on whether foods are steamed, fried or boiled (144). Food science will explore such relationships in greater detail as foods become the reference targets of guidelines. A dietetic component here is the consumer's ability to associate nutrient content (e.g. vitamins, fats or sugars) with various foods or food patterns.
• A possible future area of concern, based on experimental animal evidence, is the increasing refinement of the food supply which, in extreme cases, may lead to a risk of reduced availability of micronutrients such as copper, iron, selenium or zinc. Geochemical and agricultural variables can influence the content of iodine, selenium, fluoride and molybdenum in staple foods.
• Dietary guidelines will have to be developed concerning new foods which become available to populations in a particular cultural context, e.g. white bread and extruded snacks coming into traditional societies or instant noodles into Asian societies. Issues of dietary and plant genetic diversity retain their full importance.
• The increasing impact of "designer" or "functional" foods (130) is an extreme example. These simulated foods may lead to difficulties in food choice, in that they will not deliver what consumers expect based on existing knowledge. Also relevant are issues such as appropriate labelling, food legislation, and nutrition education. In formulating such foods, other non-nutrient components and even nutrients may not be included, e.g. breast-milk substitutes and various components such as long-chain fatty acids or immunological factors. Some foods may merely become vehicles for added nutrients, such as breakfast cereals.
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