Description
Availability and consumption
Physiology
Carbohydrates are polyhydroxy aldehydes, ketones, alcohols, acids, their simple derivatives and their polymers having linkages of the acetal type. They may be classified according to their degree of polymerization and may be divided initially into three principal groups, namely sugars, oligosaccharides and polysaccharides (see Figure 1).
Figure 1
The major dietary carbohdrates
|
Class (DP*) |
Sub-Group |
Components |
|
Sugars (1-2) |
Monosaccharides |
Glucose, galactose, fructose |
|
Disaccharides |
Sucrose, lactose, trehalose |
|
|
Polyols |
Sorbitol, mannitol |
|
|
Oligosaccharides (3-9) |
Malto-oligosaccharides |
Maltodextrins |
|
Other oligosaccharides |
Raffinose, stachyose, fructo-oligosaccharides |
|
|
Polisaccharides (>9) |
Starch |
Amylose, amylopectin, modified starches |
|
Non-starch polisaccharides |
Cellulose, hemicellulose, pectins, hydrocolloids |
DP * = Degree of polymerization
Each of these three groups may be subdivided on the basis of the monosaccharide composition of the individual carbohydrates. Sugars comprise monosaccharides, disaccharides and polyols (sugar alcohols); oligosaccharides include malto-oligosaccharides, principally those occurring from the hydrolysis of starch, and other oligosaccharides, e.g. a -galactosides (raffinose, stachyose etc.) and fructo-oligosaccharides; the final group are the polysaccharides which may be divided into starch (a -glucans) and non-starch polysaccharides of which the major components are the polysaccharides of the plant cell wall such as cellulose, hemicellulose and pectin (2,3,4).
Total carbohydrate
Although the individual components of dietary carbohydrate are readily identifiable, there is some confusion as to what comprises total carbohydrate as reported in food tables. Two principal measures of total carbohydrate are used, firstly, that derived by "difference" and secondly the direct measurement of the individual components which are then combined to give a total. Calculating carbohydrates by "difference" has been used since the turn of the century. The protein, fat, ash and moisture content of a food are determined, subtracted from the total weight of the food and the remainder, or "difference", is considered to be carbohydrate. There are, however, a number of problems with this approach to total carbohydrate analysis in that the "by difference" figure includes a number of non-carbohydrate components such as lignin, organic acids, tannins, waxes, and some Maillard products. In addition to this error, it combines all of the analytical errors from the other analyses. Finally, a single global figure for carbohydrates in food is uninformative because it fails to identify the many types of carbohydrates in a food and thus to allow some understanding of the potential physiological properties of those carbohydrates (5,6).
Terminology
In deciding how to classify dietary carbohydrate the principal problem is to reconcile the various chemical divisions of carbohydrate with that which reflects physiology and health. A classification based purely on chemistry does not allow a ready translation into nutritional terms since each of the major classes of carbohydrate have a variety of physiological effects. However, a classification based on physiological properties also creates a number of problems in that it requires a single effect to be considered as overridingly important and to be used as the basis of the classification. This dichotomy has led to the introduction of a number of terms to describe various fractions and sub-fractions of carbohydrate (4,7).
Sugars
The term "sugars" is conventionally used to describe the mono and disaccharides. "Sugar", by contrast, is used to describe purified sucrose as are the terms "refined sugar" and "added sugar"
Extrinsic and intrinsic sugars
These terms had their origin in a United Kingdom (UK) Department of Health committee in 1989 (8), which was looking at the question of sugars in the diet. The terms were developed to help the consumer choose between what were considered to be healthy sugars and those which were not. Intrinsic sugars were defined as sugars occurring within the cell walls of plants, i.e. naturally occurring, while extrinsic sugars were those which were usually added to foods. Because lactose in milk is also an extrinsic sugar, an additional phrase "non-milk extrinsic sugars" was developed. These terms have not gained wide acceptance either in the UK or other countries in the world. There are no current plans to measure these sugars separately in the diet nor to incorporate their use into food tables.
Complex carbohydrates
This term was first used in the McGovern report, "Dietary Goals for the United States" in 1977 (9). The term was coined largely to distinguish sugars from other carbohydrates and in the report denotes "fruit, vegetables and whole-grains". The term has since come to be used to describe either starch alone, or the combination of all polysaccharides. It was used to encourage consumption of what were considered to be healthy foods such as whole-grain cereals, etc., but becomes meaningless when used to describe fruit and vegetables which are low in starch. Furthermore, it is now realized that starch, which is by any definition a complex carbohydrate, is variable metabolically with some forms being rapidly absorbed and having a high glycemic index and some being resistant to digestion. The term "complex carbohydrate" has encompassed, at various times, starch, dietary fibre and non-digestible oligosaccharides. As a substitute term for starch, however, it would seem to have little merit and, in principle, it is better to discuss carbohydrate components by using their common chemical names.
Available and unavailable carbohydrate
A major step forward conceptually in our understanding of carbohydrates was made by McCance and Lawrence in 1929 (10) with the division of dietary carbohydrate into available and unavailable. In an attempt to prepare food tables for diabetic diets they realised that not all carbohydrates could be "utilized and metabolized", i.e. provide the body with "carbohydrates for metabolism". Available carbohydrate was defined as "starch and soluble sugars" and unavailable as "mainly hemicellulose and fibre (cellulose)". This concept proved useful, not the least because it drew attention to the fact that some carbohydrate is not digested and absorbed in the small intestine but rather reaches the large bowel where it is fermented. It suggests that the site of digestion or fermentation in the gut of carbohydrate is of overriding importance. However, it is misleading to talk of carbohydrate as "unavailable" because some indigestible carbohydrate is able to provide the body with energy through fermentation. There are many properties of carbohydrate of which digestibility and fermentability are only two. A more appropriate substitute for the terms "available" and "unavailable" today would be to describe carbohydrates as either as glycemic (i.e. providing carbohydrate for metabolism) or non-glycemic, which is closer to the original concept of McCance and Lawrence.
Resistant starch
One of the major developments in our understanding of the importance of carbohydrates for health in the past twenty years has been the discovery of resistant starch. Resistant starch is defined as "starch and starch degradation products not absorbed in the small intestine of healthy humans" (11). The main forms of resistant starch are physically enclosed starch, e.g. within intact cell structures (RS1), some raw starch granules (RS2) and retrograded amylose (RS3) (11,12).
Modified starch
The proportions of amylose and amylopectin in a starchy food is variable and can be altered by plant breeding. Techniques using genetic engineering are rapidly emerging, enabling starches to be produced for specific purposes by genetically modifying the crop used for their production. High amylose corn starch and high amylopectin (waxy) corn starch have been available for a long time, and display quite different functional as well as nutritional properties. High amylose starches require higher temperatures for gelatinization and are more prone to retrograde and to form amylose-lipid complexes. Such properties can be utilized in the formulation of foods with low glycemic index and/or high resistant starch content.
Physical modifications of starches include pregelatinization and partial hydrolysis (dextrinization). Chemical modification is mainly the introduction of side groups and cross-linking or oxidation. These modifications may be used to decrease viscosity and to improve gel stability, mouthfeel, appearance and texture, and resistance for heat treatment (13). The application of modified starches as fat replacers is another important area. Some modified starches may be partly resistant to digestion in the small intestine, thereby adding to resistant starch (14).
Dietary fibre
The original description of dietary fibre by Trowell in 1972 (15) was "that portion of food which is derived from cellular walls of plants which is digested very poorly by human beings". This is not an exact description of any carbohydrate in the diet but is more a physiological concept. It was linked by Burkitt and Trowell to the etiology of a number of "Western diseases" (16) and on the basis of this a hypothesis relating fibre to health was developed. The use of the term has, however, caused many difficulties over the years because of controversies regarding definition. Moreover, the proposal that there are a number of dietary fibre deficiency disorders is an over-simplification and needs to be modified now in the light of new knowledge of diet and disease.
The main components of dietary fibre are derived from the cell walls of plant material in the diet and comprise cellulose, hemicellulose and pectin (the non-starch polysaccharides). Lignin, a non-carbohydrate component of the cell wall is also often included. Dietary fibre is a term which is felt to be valuable for the consumer who looks upon this as a healthy component of the diet. At the present time there is no consensus as to which components of carbohydrate should be included as dietary fibre and different authors have variously included non-starch polysaccharides and resistant starch. More recently it has been suggested that non-digestible oligosaccharides should also be included. Dietary fibre has also been defined by method. While there is general agreement that the non-starch polysaccharides are the principal part of dietary fibre there is currently no consensus as to whether other components should be included in this term. It has been suggested that the use of the term dietary fibre be gradually phased out (1,17). Its widespread use and popularity with the consumer has made this difficult in practice and the term has been useful in nutrition education and product development.
Soluble and insoluble fibre
These terms developed out of the early chemistry of non-starch polysaccharides which showed that the fractional extraction of these polysaccharides could be controlled by changing the pH of solutions. They proved very useful in the initial understanding of the physiological properties of dietary fibre, allowing a simple division into those which principally had effects on glucose and lipid absorption from the small intestine (soluble) and those which were slowly and incompletely fermented and had more pronounced effects on bowel habit (insoluble). However, the separation of soluble and insoluble fractions is not chemically very distinct being dependent on the conditions of extraction (18). Moreover, the physiological differences are not, in fact, so distinct with much insoluble fibre being rapidly and completely fermented while not all soluble fibre has effects on glucose and lipid absorption.
Methodology for dietary carbohydrate analysis
Mono- and disaccharides
They can be analyzed specifically by enzymatic, gas-liquid chromatography (GLC) or high performance liquid chromatography (HPLC) methods. Depending on the food matrix to be analyzed, extraction of the low molecular weight carbohydrates in aqueous ethanol, usually 80% (v/v), may be advisable before analysis (5,6,7).
The enzymatic procedures are based on specific, highly purified enzymes and have been instrumental in providing means of specific and precise analysis of individual carbohydrates in mixtures without a large investment in instrumentation. Enzymatic methods are still preferable when one single carbohydrate is to be analyzed, e.g. glucose, as the end point of starch analysis.
When several different monosaccharides are to be determined simultaneously, HPLC or GLC methods are preferable. HPLC systems using sensitive amperometric detectors are gaining in popularity over GLC, in that the derivatization necessary before the GLC determination is avoided.
Polyols
Polyols are usually determined by GLC using alditol acetate derivatives. HPLC methods are also available.
Oligosaccharides
Oligosaccharides can also be determined by GLC or HPLC methods. These methods work well for purified preparations, but in complex foods or diets, enzymatic hydrolysis and determination of liberated monosaccharides is an alternative for specific determination. Malto-oligosaccharides are recovered as "starch" if not extracted before starch analysis.
Separation of oligosaccharides from polysaccharides
By definition, polysaccharides have 10 or more monomeric units, and Oligosaccharides less than 10. Analytically, separation is based on solubility in aqueous ethanol, usually around 80% (v/v). The alcohol solubility of carbohydrates, however, is dependent not only on the degree of polymerization (DP), but also on the molecular structure. For instance, highly branched carbohydrates may be soluble in 80% ethanol in spite of a DP considerably higher than 10. In practice, therefore, the separation of Oligosaccharides from polysaccharides is empirical and does not provide an exact division based on DP (18).
Starch
Quantitative analysis of starch in foods by most current methods is based on enzymatic degradation and specific determination of liberated glucose. Nutritionally, starch can be divided into glucogenic ("available") and resistant starch, which is not absorbed in the small intestine. Resistant starch is poorly soluble in water and methods aiming at a total starch analysis employ an initial 2M potassium hydroxide (KOH) or dimethylsulfoxide solvent (DMSO) treatment to disperse crystalline starch fractions that would otherwise remain unhydrolyzed. Methods for measuring resistant starch are still in their infancy and have not yet been tested in formal collaborative studies. They aim at simulating normal starch digestion in the small intestine. A key step is to mimic the normal disintegration of the food which occurs during chewing. One method uses a standardized milling/homogenization technique (12), whereas others employ standardized chewing by volunteers (19,20). Both approaches have been evaluated against human ileostomy experiments with a limited number of food matrices (21).
Non-starch polysaccharides (NSP)
The determination of NSP is based on the following steps: (a) degradation of starch by enzymatic hydrolysis after solublization, (b) removal of low molecular weight carbohydrates, including starch hydrolysis products, (c) hydrolysis of the NSP to their constituent monomers, and (d) quantitative determination of those monomers. The acid hydrolysis step is a critical one, and it has to be designed as an optimal balance between complete hydrolysis and destruction of the liberated monomers (22,23).
The most widely-used method today for specific determination of the liberated monomers is GLC with alditol acetate derivatives. HPLC detection is an alternative gaining in popularity. Colourimetric determination is still preferred for uronic acids, which are derived mainly from pectic substances. A colourimetric method is also available for total NSP.
Fractions of NSP, such as cellulose and non-cellulosic polysaccharides, can be separated by using sequential extraction and hydrolysis methods. For instance, cellulose is not hydrolysed by dilute (1-2M) sulphuric acid, unless it has first been dispersed in concentrated acid.
Dietary fibre
Three methods for dietary fibre analysis have undergone extensive testing in recent years, including collaborative studies satisfactory enough for official approval of bodies such as the AOAC International (Association of Official Analytical Chemists) and the Bureau Communautaire de Reference (BCR) of the European Community (24):
1. The enzymatic, gravimetric AOAC methods of Prosky and co-workers, and subsequently Lee and co-workers.2. The enzymatic-chemical methods of Englyst and co-workers.
3. The enzymatic-chemical method of Theander and co-workers (the Uppsala method).
The enzymatic-gravimetric AOAC methods are derived from methods aiming at simulating the digestion in the human small intestine to isolate an undigested residue as a measure of dietary fibre. This residue is corrected for associated ash and protein. Since no DMSO or KOH dispersion is used, starch that resists the amylases used in the assay will remain as a fibre component. Since the sample has to be milled, and since a heat-stable amylase (termamyl) is used at a temperature close to 100°C, physically enclosed starch (RS1) and resistant starch granules (RS2) will not be included. Retrograded amylose (RS3) that is included is the main form of resistant starch (RS) in processed foods. Lignin, a non-carbohydrate component of the dietary fibre complex is also included, as well as some tannins. These components are a very small proportion of most foods but can be substantial in some unconventional raw materials or special "fibre" preparations (25).
The Englyst method measures the NSP specifically, either as individual monomeric components by GLC (or HPLC) or colourimetrically as reducing substances (total NSP). Accordingly, DMSO is used initially to ensure a complete removal of starch, and lignin is not determined. The difference between estimates with the gravimetric methods and the Englyst method is mainly due to resistant starch and lignin (24).
The Uppsala method employs hydrolysis conditions and GLC determination of monomers in a similar way as in the Englyst method. However, DMSO is not employed for starch dispersion, and a gravimetric estimate of lignin (Klason lignin) is added to obtain the dietary fibre. The Uppsala method and the gravimetric AOAC methods give very concordant results (24).
Labelling
Food labelling has two main aims: to inform the consumer of the composition of the food and to assist them in the selection of a healthy diet. These two aims are not always easy to reconcile because the health benefit of different carbohydrate-containing foods cannot readily be communicated simply from a description of their composition.
Labelling should be based on the chemical classification used in Figure 1. Analytical methods should be clearly defined and validated. The principal information should be total carbohydrate, measured as the sum of the individual components. Further information on carbohydrate composition, based on the classification in Figure 1, could include terms such as sugars, starch and non-starch polysaccharides. Other terms, such as non-digestible oligosaccharides (NDO), polyols, resistant starch and dietary fibre may be used, provided the components included in these terms are clearly defined.
Trends in the supply and intake of carbohydrates can be studied by four principal approaches:
1. Production
2. Food balance sheets
3. Household surveys
4. Individual assessments
Food production statistics, which are available from FAO for every country in the world and for every crop, are useful for examining trends in consumption (26). From these data it can be seen that the major sources of carbohydrate in the human diet are:
1. Cereals
2. Root crops
3. Sugar crops
4. Pulses
5. Vegetables
6. Fruit
7. Milk products
Sustainability
Trends over the last 20-30 years indicate growth in world production of cereals, sugar cane, vegetables and fruit. On the other hand, production of root crops, pulses and sugar beet has changed little on a world basis. Marked decreases have actually been seen in pulse production in some countries in Asia, and in root crop production in Europe. This suggests a change in food preference away from roots and pulses and towards cereals. Examination of eating habits in a number of countries indicates that this is the case (27-29). Since root crops are an excellent source of carbohydrate, there is concern about this downward trend in production.
Populations continue to grow in most parts of the world and, overall, food production would seem to be keeping pace with population growth. Increased production is due to improved agricultural practices rather than increased crop area, the major reason for increases being greater use of fertilizer (30-32). There are, however, specific countries where this is not happening. For the entire continent of Africa, cereal production is inadequate.
A major question is how much more improvement and efficiency in production can be achieved, and whether the amount of carbohydrate will be sufficient for the world's population in the future. Projections for future growth suggest problems ahead, particularly in Africa (33).
Changing patterns of consumption
Both food balance information and results from individual assessments are used to determine carbohydrate intakes. Food balance data is intended to describe food available for consumption. It is unlikely to do so because it does not include home production, which is variable from country to country, and may be considerable in some developing countries (34,35). As a reflection of food consumed, food balance data is questionable, since it does not include food wasted or spoiled, or used for purposes other than human food, the proportion of which may change from year to year. As a result, food balance data for individual countries has failed to demonstrate the changes in consumption of carbohydrates which are seen using individual surveys (36-38).
Data from individual surveys also have limitations. Surveys are carried out by a variety of methodologies. While each has advantages and disadvantages, all suffer from a degree of underreporting. This can be intentional or involuntary, most likely due to individuals forgetting food items or not describing foods thought to be undesirable (35). There is also the failure to record data or the altering of actual diets. The difference, then, between food balance data and individual assessments, for energy and nutrient intakes, is not only the form of wastage and spoilage on the food balance side of the equation, but also the underreporting on the individual intake side. True food intakes therefore lie somewhere between food balance and individual intake estimates (35). Another major problem is the varied carbohydrate terminology used in different countries. Many countries express total carbohydrate 'by difference', rather than as carbohydrate analyzed directly, and this results in overestimates of the percent energy derived from carbohydrate. There is also a great variety in terms used to describe simple sugars, such as "sugars", "sugar", "refined sugar", "added sugar", "sucrose", and "sugars minus lactose". Often there is no description of what is being reported (39). There is a need to standardize the terminology for carbohydrate and its components in individual surveys and a need for consistency in both reporting and the description of the terms used.
In spite of terminology difficulties, it is possible to gain a picture of carbohydrate intakes and trends. Annex 1 gives the intakes of carbohydrate and components where available, from a number of surveys since 1980. As a percent of energy, total carbohydrate ranges from about 40% to over 80%, with the developed countries, such as those in North America, Western Europe and Australia at the low end of the range, and developing countries in Asia and Africa at the high end. Starch accounts for 20%-50% or more of energy where the total carbohydrate intake is in the high range. Sugars account for 9%-27% of energy intake; where total carbohydrate is high, sugar intake is generally low. Where data are available, intake of carbohydrate as a percent of energy is higher for children than for adults.
Trends in consumption indicate a falling carbohydrate intake in developed countries until the last two decades (39). During that time some increase has been noted as fat intakes fall. The major sources of carbohydrate are cereals, representing over 50% of all carbohydrate consumed in both developed and developing countries, with sugar crops the next major source, followed by root crops, fruits, vegetables, pulses and milk products. In some of the developing countries much of the carbohydrate is derived from a single food source such as rice, cassava or maize. Carbohydrate foods are an important vehicle for protein, micronutrients and other food components, like phytochemicals, which have important benefits for health. Individual food sources vary, however, in the provision of these components. A single food source of carbohydrate is therefore undesirable and populations whose diets are primarily based on a single food can suffer from micronutrient deficiencies due to lack of variety. It is important, therefore, that a number of different carbohydrate sources be consumed and efforts should be made to encourage a wide variety of carbohydrate foods.
Data on intake of sources of sugars is only available for developed countries. These data show similar proportions of sugars are derived from cereal products, milk products and beverages, among these countries. There is some variation in the proportions derived from fruit and confectionery, with the UK consuming less fruit and higher amounts of confectionery than countries such as the United States and Australia (40-44).
Intakes of non-starch polysaccharides (45) range from about 19g/day in some countries in Europe and North America, to nearly 30g/day in rural Africa (46-48). Cereals are again the major source of this component. Data on intake of dietary fibre, determined by methods such as that of the AOAC (Association of Official Analytical Chemists) (49) and the older Southgate method (50), are about 15-20 g/day for North America, Europe and Australia, to 25-40 g/day for countries in Asia and Africa (47, 51-53).
Carbohydrates have a wide range of physiological effects which may be important to health, such as:
· Provision of energy· Effects on satiety/gastric emptying
· Control of blood glucose and insulin metabolism
· Protein glycosylation
· Cholesterol and triglyceride metabolism
· Bile acid dehydroxylation
· Fermentation
Hydrogen/methane production
Short-chain fatty acids production
Control of colonic epithelial cell function· Bowel habit/laxation/motor activity
· Effects on large bowel microflora
Carbohydrate as an energy source
Dietary carbohydrates have by convention been given an energy value of 4 kcal/g (17 kJ/g), although where carbohydrates are expressed as monosaccharides, the value of 3.75 kcal/g (15.7 kJ/g) is used. It is now clear, however, that a number of carbohydrates are only partly or not at all digested in the small intestine and are fermented in the large bowel to short chain fatty acids. These include the non-digestible oligosaccharides, resistant starch and non-starch polysaccharides. The process of fermentation is metabolically less efficient than absorption in the small intestine and these carbohydrates provide the body with less energy.
In light of a new understanding of the digestion and metabolism of carbohydrate and developments in methodology, the energy value of all carbohydrates in the diet should be reassessed and more accurate energy factors assigned to each group or sub-group. There are a number of potential approaches to accomplish this. These include the classic calorimetry experiments similar to those first undertaken by Atwater, as well as human balance studies and ileostomy recovery experiments. Knowledge of the chemistry of individual carbohydrates allows a prediction to be made regarding their digestion or fermentation, and an energy value to be assigned. In vitro models of fermentation can be constructed and from these the fermentation stoichiometry can be deduced. Studies using stable isotope tracer techniques may also be of value.
While the energy yield of carbohydrate delivered to the colon will vary according to the extent of colonic fermentation (or the assumptions made in the model used), there may be an argument for assigning a single energy value to all such carbohydrate. Published studies suggest that a caloric value of about 2 kcal/g (8 kJ/g) (54,55) would be a reasonable average figure for carbohydrate which reaches the colon. While individual carbohydrates will have different values, in the range of 1-2 kcal/g, these differences are unlikely to be of importance to health.
Satiety
The possibility of controlling hunger, satiety and food intake by altering the type of carbohydrate in food has intrigued a number of investigators (56). At present the variability of the findings and the lack of understanding of a clear relationship to physiologic parameters thought to be involved in the regulation of food intake limit practical application of this approach. It is unlikely that controlling a single dietary component, such as the type of sugar or starch, will lead to significant changes in the amount of food consumed. Also, compensation for small dietary changes made in one meal may often be seen at a subsequent meal. A better approach to controlling hunger and increasing satiety is likely to be associated with changes in the composition of the total diet (57).
Glucose and insulin
The digestion of dietary carbohydrates starts in the mouth, where salivary a-amylase initiates starch degradation. The starch fragments thus formed include maltose, some glucose and dextrins containing the 1,6-a -glycosidic branching points of amylopectin. The a -amylase degradation of starch is completed by the pancreatic amylase active in the small intestine.
Dietary disaccharides, as well as degradation products of starch, need to be broken down to monosaccharides in order to be absorbed. This final hydrolysis is accomplished by hydrolases attached to the intestinal brush-border membrane, referred to as "disaccharidases". Disaccharidase deficiencies occur as rare genetic defects, causing malabsorption and intolerance of the corresponding disaccharide.
Glucose and galactose are transported actively against a concentration gradient into the intestinal mucosal cells by a sodium dependent transporter (SGLT 1). Fructose undergoes facilitated transport by another mechanism (GLUT 5). Fructose taken together with other sugars (as in naturally fructose-containing foods) is better absorbed than fructose alone (58).
When delivered to the circulation, the absorbed carbohydrates cause an elevation of the blood glucose concentration. Fructose and galactose have to be converted to glucose mainly in the liver and therefore produce less pronounced blood glucose elevation. The extent and duration of the blood glucose rise after a meal is dependent upon the rate of absorption, which in turn depends upon factors such as gastric emptying as well as the rate of hydrolysis and diffusion of hydrolysis products in the small intestine.
Insulin is secreted as a response to blood glucose elevation but is modified by many neural and endocrine stimuli. Insulin secretion is also influenced by food related factors, especially by the amount and the amino acid composition of dietary proteins. Insulin has important regulatory functions in both carbohydrate and lipid metabolism and is necessary for glucose uptake by most body cells.
Lactose
Lactose, a b -linked disaccharide of glucose and galactose, is the principal sugar in milk. At birth, lactase activity is high in the brush-border of the small bowel of infants, but declines after weaning so that most populations of the world have low activity in adult life. The exceptions are Caucasian peoples and some other population groups in whom the majority retain a high lactase activity throughout life (59,60).
During the years since 1980, there has been a major change in the way lactose absorption is viewed and a resultant shift away from the concept that lactose "malabsorption" is a pathological state. Low mucosal lactase activity in adults is the norm throughout most of the world. However, such a state usually allows the drinking of modest quantities of milk spaced throughout the day without adverse symptoms. Milk consumption is therefore now being encouraged in many areas of the world because of its value as a source of protein, calcium and riboflavin. Fermented milk products, which have lower lactose content and contain enzymes and microorganisms that can assist in lactose digestion, are better tolerated than milk. Technology exists to reduce the lactose level in foods and this should be taken into consideration when milk is included as food aid. Cheese, however, has almost no lactose.
Lactose which is not digested, passes into the colon where it is fermented. In some individuals this causes lactose intolerance, the term used to describe the clinical symptoms of abdominal discomfort, flatulence and diarrhoea, associated with the ingestion of lactose containing foods by persons with low lactase activity. It also occurs as a transient phenomenon when the intestinal mucosa is injured following acute infection in children and in protein-energy malnutrition. It is also found in adults, particularly in association with coeliac disease and tropical sprue. In these conditions, lactose malabsorption is said to be "secondary" to intestinal mucosal disease. A small proportion of the Caucasian population also exhibits low lactase activity and lactose intolerance.
Protein glycosylation
The non-enzymatic glycation of proteins is dependent on the concentration of glucose and fructose in blood and the half-life of the protein. The initial reaction is between the monosaccharide and the amino group of an amino acid, usually lysine, to form a Shiff base which undergoes rearrangement and formation of Amadori products. As the reaction progresses, increasingly complex Maillard products are formed with the eventual production of Advanced Glycation End-products or AGEs which are associated with irreversible loss of protein function. The extent of glycation of specific proteins, such as Haemoglobin Ale in diabetics serves as an indication of medium term control of blood glucose. Examples of functional changes induced by glycation include lens proteins in the eye with resultant cataract formation, increased microvascular complications, abnormal fibrin network formation and impaired fibrinolysis. These changes are most clearly seen in diabetic patients (61,62).
Lipids and bile acids
There has been concern that a substantial increase in carbohydrate-containing food at the expense of fat, might result in a decrease in high-density lipoprotein and a corresponding increase in very low-density lipoprotein and triglycerides in the blood. However, there is no evidence that this happens when the increase in carbohydrates occurs as a result of increased consumption of vegetables, fruits and appropriately processed cereals over prolonged periods.
Polysaccharides like oat b -glucan, guar gum and those from psyllium have been repeatedly shown to lower serum cholesterol levels in those with elevated levels, with little change if serum levels are normal (63,64). Proposed mechanisms include impaired bile acid and cholesterol reabsorption through physical entrapment in the small intestine, or inhibitory effects on cholesterol synthesis by products of lower bowel fermentation, particularly propionic acid. Not all fermentable polysaccharides are effective, however, and recent studies have indicated that neither oligosaccharides nor resistant starch have a significant effect on serum lipids in young normolipidemic subjects (21,65).
Fermentation
Fermentation is the colonic phase of the digestive process and describes the breakdown in the large intestine of carbohydrates not digested and absorbed in the upper gut. This process involves gut microflora and is unique to the colon of humans because it occurs without the availability of oxygen. It thus results in the formation of the gases hydrogen, methane and carbon dioxide, as well as short chain fatty acids (SCFA) (acetate, propionate and butyrate), and stimulates bacterial growth (biomass). The gases are either absorbed and excreted in breath, or passed out via the rectum. The major products of such fermentation are the SCFA which are rapidly absorbed and metabolized by the body. Acetate passes primarily into the blood and is taken up by liver, muscle and other tissues. Propionate is a major glucose precursor in ruminant animals such as the cow and sheep, but this is not an important pathway in humans. Butyrate is metabolized primarily by colonocytes and has been shown to regulate cell growth, and to induce differentiation and apoptosis (66).
Bowel habit
It has long been known that non-starch polysaccharides are the principal dietary component affecting laxation. This occurs through increases in bowel content bulk and a speeding up of intestinal transit time. The extent of the effect depends on the chemical and physical nature of the polysaccharides and the extent to which they are fermented in the colon. Fermentable polysaccharides stimulate increases in microbial biomass in the colon, resulting in some increase in fecal weight, but not to the extent of non-fermentable polysaccharides. The latter are not significantly degraded in the colon and become consituents of the stool. In so doing, they hold water and produce a marked increase in fecal weight. Similarly, resistant starch can increase fecal weight, but this again depends on the extent of fermentation (67).
Microflora
Carbohydrate which is fermented stimulates the growth of bacteria in the large gut. This is a generalized effect which leads to an increase in the total number of bacteria or biomass. When bacterial growth occurs, the microflora synthesize protein actively from preformed amino acids and peptides as well as some de-novo synthesis using ammonia as the source of nitrogen. The additional biomass is excreted in feces and is one of the mechanisms whereby carbohydrate influences bowel habit. The increased biomass excretion is accompanied by increased nitrogen excretion. The efficiency of conversion of carbohydrate to biomass is determined principally by the type of substrate, the rate of breakdown and the transit time through the large intestine (68).
One of the more significant developments in recent years with regard to the gut microflora has been the demonstration that specific dietary carbohydrates selectively stimulate the growth of individual groups or species of bacteria. An example of this is the effect of fructo-oligosaccharides on the growth of bifidobacteria. The importance of bifidobacteria is that they may be one of the major contributors to colonization resistance in the colon, thereby protecting the host from invasion by pathogenic species. Foods which selectively stimulate the growth of gut bacteria are known as pre-biotics (69).
