FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS	ESN:FAO/WHO/UNU/ EPR/81/34 August 1981
	WORLD HEALTH ORGANIZATION
	THE UNITED NATIONS UNIVERSITY

Provisional Agenda Item 3.2.6

Joint FAO/WHO/UNU Expert Consultation on
Energy and Protein Requirements

Rome, 5 to 17 October 1981

ENERGY INTAKE IN RELATION TO NITROGEN UTILIZATION

by

Vernon R. Young
Massachusetts Institute of Technology
Cambridge
USA

1. Introduction

The intimate relationships between the metabolism of amino acids and that of the major fuels (carbohydrate, fat) are now well recognized. It is not surprising, therefore, that body N balance is affected by the status of body energy balance and level of dietary energy intake (1–3). These associations were also recognized in the report of the 1971 FAO/WHO committee which related obligatory urinary N losses to basal energy expenditure, as a basis for estimating the magnitude of these losses in subjects of differing age, weight and sex. Since the report of the 1971 committee, a number of additional observations on the relationships between energy intake and nitrogen utilization have been made. These observations and their relevance to the assessment of human protein and amino acid requirements consitute the basis of the brief discussion given here.

2. Responses of N Balance to Alterations in Energy Intake.

The fact that N balance is affected by the level of energy intake has been accepted for many years and the earlier data have been extensively summarized by Munro (1,2) and Calloway and Spector (3). From these reviews it is clear that N balance is highly sensitive to altered energy intakes and that the level of N balance is determined by both the protein and energy contents of the diet. As discussed in more detail below, changes that occur in N balance with alterations in energy intake may complicate estimation of requirements for dietary protein and essential amino acids when these are derived from N balance criteria. A summary (Table 1) of the more recent findings in healthy young men and in depleted patients maintained with intravenous nutrition (9) confirms that changes in energy intake have a profound effect on N balance, especially when the diet supplies generous protein of high quality or where the intital energy intake is low.

From these and the earlier observations it appears that in both well-nourished and depleted adult subjects changes in energy intake bring about alterations in N balance over a wide range in both energy and protein intakes. The relationship is approximated by a change in N retention of about 1.5–2mgN per kcal of change in energy intake. High energy intakes of low N intakes are associated with a smaller impact of changes of energy on N retention (2,3). However, it is the former dietary conditions that have particular significance in reference to the estimation of amino acid and nitrogen requirements, as discussed below.

In addition to the influence of level of energy intake per se, the source of the energy yielding nutrients may also have specific effects on dietary N utilization and retention (2). Thus, carbohydrates and lipids serve as the major sources of dietary energy for support of body protein metabolism but carbohydrate has specific actions on protein metabolism not shared with fat; for example, administration of carbohydrate lowers body N output in the fasting subject, whereas fat does not. Also, isocaloric substitution of fat for carbohydrate results in a transient increase in urinary N output and the separation of meals containing dietary carbohydrate from those containing protein also causes a transient rise in N output. From these findings, Murno (2) concluded that carbohydrate exerts a specific action on the utilization of amino acids during their absorption from protein-containing meals, in addition to an effect shared by fat and carbohydrate on the metabolism of amino acids during the post-absorptive period.

The possible practical nutritional significance of differences in energy source on N metabolism and utilization has not been extensively studied because most previous studies have involved intakes of protein that greatly exceed those considered to be sufficient for long-term maintenance of N balance. However, Richardson et al. (10) examined the relationship between N balance and energy intake in young men given diets high or low in carbohydrate and a protein intake that approximated their mean requirements (0.6g egg protein/kg/day). As shown in Figure 1 a particularly strong association was observed between N balance and energy intake, and the results also indicated that the nitrogen-sparing effects of carbohydrate, relative to fat, were more pronounced for subjects whose energy intakes were the least.

In summary, N balance in adults is affected by the relative proportion of non-protein energy sources when the protein intake level meets the 1973 FAO/WHO recommendations for young adults and where changes in the proportion of dietary carbohydrate and fat are in the range found in usual diets.

Precisely how dietary carbohydrate exerts this specific effect on the sparing of body protein is not fully understood. Munro (2) proposed a scheme for protein-sparing action of dietary carobhydrate that is dependent on the secretion of insulin and he cites evidence to support this. To explore this problem, Fuller et al. (11) used a continuous infusion of physiological amounts of exogenous insulin together with glucose in well-nourished pigs. Glucose was infused at a rate sufficient to maintain plasma glucose concentrations within the normal physiological range, and to avoid the normal hypoglycemic and counter-regulatory response in body metabolism. The response to this treatment over a period of 3 to 7 days was a 2- to 7-fold increase in plasma insulin, a 50% decrease in plasma glucose, a 40% decrease in plasma urea concentration, and a 30% fall in urinary excretion of urea N. After infusion, plasma urea levels rapidly returned to those of the control period. These authors concluded that a major component of the protein-sparing effect, achieved by a surfeit feeding of carbohydrate, is mediated by insulin.

3. Implications of Effects of E Intake for Estimation of Protein Requirements.

The more recent studies concerned with the relationships between energy intake and body N balance (and dietary N utilization) have largely confirmed the quantitative associations between alterations in energy intake and N retention that were accepted by the 1971 Committee. The significant developments in this area, however, concern (a) study of the mechanisms responsible for the changes in N retention and (b) recognition of the possible confounding influence of this association on the estimation of human protein and amino acid requirements. The following examples and observations serve to emphasize this latter aspect of the energy intake-N utilization axis.

First, in the determination of requirements for essential amino acids in adult, Rose (12) utilized diets providing generous energy intakes (~55 kcal kg^-1 day^-1). Although this investigator considered these high intakes troublesome, he was of the opinion that this was irrelevant to the precision of the determination of the amino acid requirements of adults. I consider this view to be unacceptable. For example, in the study by Rose and Wixom (13) the minimum requirement for nitrogen was determined with a diet providing all of the dispensable amino acids at the “safe” level. For two adult subjects participating in this study, significant body weight gains occurred during the month-long experiment; as shown in Table 2 one subject gained 5 kg and the other about 2 kg. Clearly, excess energy intakes were responsible for these consistent body weight changes and this generous level of energy would undoubtedly lead to an underestimation of the requirement for N (or essential amino acids). The adequacy of the N balance data obtained in these studies for purposes of establishing the requirements for nitrogen or specific amino acids is, therefore, highly questionable. The same criticisms apply to the studies of Leverton and co-workers and of later investigations, the results of which have been summarized by Irwin and Hegsted (14).

Second, the studies by Inoue and co-workers (4) as summarized in Table 3, dramatically emphasize the serious confounding effect of energy intakes, in excess of needs, for estimation of total protein requirements. Thus, at maintenance energy intakes (45 kcal/kg) the mean requirements for egg or rice proteins were 90 and 119 mg N/kg/day but at excess energy intakes (57 kcal/kg) these requirements were reduced to 67 and 89 mg/kg/day respectively. Clearly these differences are marked and they emphasize the significant problems introduced into the interpretation of N balance data in adults, especially when energy intakes may be ±20% of actual needs over short periods of time without a recognition of this problem.

For these reasons the effects of changes in energy intake will have a profound influence on the estimation of minimum protein requirements. This poses significant problems for the interpretation of N balance data because of the difficulties in judging precisely the energy requirements of adult subjects. These associations between energy intake and N retention may also explain much of the variation in N balances and estimations of protein requirements in adults that has been observed both within and among laboratories.

4. Some Concluding Thoughts.

The foregoing represents only a brief introduction to a broad and highly complex problem concerning the effects of energy intake on the estimation of adult human protein requirements. Although the quantitative effect of excess energy intake on N balance in adults appears to be reasonably well established, there is little if any information available on the quantitative aspects of this relationship for other age groups, including infants, children and the elderly. This relationship may not necessarily be constant if, for example, the mobilization and/or deposition or expenditure of energy occurs with varying efficiency under differing physiological states. Thus, it is highly important to explore this problem further in order to establish the associations between energy intakes and protein requirements for the different age groups.

The effects of energy intakes on N utilization clearly deserve detailed further attention by the 1981 Expert Committee. Indeed it might be questioned whether additional N balance measurements concerned with the quantitative definition of protein requirements and evaluation of dietary protein quality are meaningful without concurrent information of the quantitative effects of excess energy on dietary N retention.

Finally, in view of the marked effects of energy intake on N utilization, it has been suggested that protein requirements can only be reasonably stated in reference to the energy intakes of individuals. This idea has arisen, in part, from consideration of animal or farm-livestock nutrition where protein requirements have been related to dietary energy intake. In the case of animals fed ad libitum (or under differing conditions of total food (energy) intake) the proposition of relating the protein need to dietary energy concent or intake seems to be of practical value. However, it is unclear as to how this approach could be applied or usefully improve the definition of the human protein requirement since the human energy requirement is established as a single value, depending of course, upon age, sex and physical activity.

REFERENCES

1. Munro, H.N., 1951. Carbohydrate and fat as factors in protein utilization and metabolism. Physiol. REv. 31:449-

2. Munro, H.N., 1964. General aspects of the regulation of protein metabolism by diet and by hormones. In: Mammalian Protein Metabolism (eds. H.N. Munro, and J.B. Allison), Vol. 1., Chpt. 10, p. 381–481, Academic Press, New York.

3. Calloway, D.H., and Spector, H., 1954. Nitrogen galance as related to caloric and protein intake in active young men. Am. J. Clin. Nutr. 2:405–412.

4. Inoue, G., Fujita, Y., and Niiyama, Y., 1973. Studies on protein requirements of young men fed egg protein and rice protein with excess and maintenance energy intakes. J. Nutr. 103:; 673–1687.

5. Calloway, D.H., 1975. Nitrogen balance of men with marginal intakes of protein and energy. J. Nutr. 105:914.

6. Garza, C., Scrimshaw, N.S., and Young, V.R., 1976.Human protein requirements: the effect of variations in energy intake within the maintenance range. Am. J. Clin. Nutr. 29:280–287.

7. Garza, C., Scrimshaw, N.S., and Young, V.R., 1977. Human protein requirements: evaluation of the 1973 FAO/WHO safe level of protein intake for young men at high energy intakes. Brit. J. Nutr. 37:403–420.

8. Kishi, Y., Miyatani, S., and Inoue, G., 1978. Requirement and utilization of egg protein by Japanese young men with marginal intakes of energy. J. Nutr. 108:658–669.

9. Elwyn, D.H., Gump, F.R., lles, M., Long, C.L., and Kinney, J.M., 1978. Protein and energy sparing of glucose added in hypocaloric amounts to peripheral infusions of amino acids. Metabolism 27:325–331.

10. Richardson, D.P., Wayler, A.H., Scrimshaw, N.S., and Young, V.R., 1979. Quantitative effect of an isoenergetic exchange of fat for carbohydrate on dietary protein utilization in healthy young men. Am. J. Clin. Nutr. 32:2217–2226.

11. Fuller, M.F., Weekes, T.E.C., Codenhead, A., and Bruce, J.B., 1977. The protein-sparing action of carbohydrate. 2. The role of insulin. Brit. J. Nutr. 38:489-

12. Rose, W.C., 1957. The amino acid requirements of adult man. Nutr. Abstr. Rev. 27:631–647.

13. Rose, W.C., and Wixom, R.L., 1955. The amino acid requirements of man. XVI. The role of the nitrogen intake. J. Biol. Chem. 217: 997–1004.

14. Irwin, M.I., and Hegsted, D.M., 1971. A conspectus of research on amino acid requirements of man. J. Nutr. 101:539–566.

¹ Rose and Wixom (13).Diet provided the safeintake of eight amino acids.

¹ Excess energy = 57 ± 2 kcal/kg; maintenance energy = 45 ± 2 kcal/kg.

² Value obtained from slope of regression of N balance on N intake. Source: Inoue et al. (4)

FIG. 1. Relationship between N balance (mg N/kg body weight per 24 hr) and energy intake (kcal/kg body weight per 24 hr) during diet periods A (•) and B (o). Diet A provides carbohydrate and fat in a ratio of 1:1. Diet B provides carbohydrate and fat in a ratio of 2:1. The regression equations are: Diet A: N balance = 2.576 × energy intake - 119.49. s = 5.586; s_b = 0.393; r = 0.918. Diet B: N balance = 1.297 × energy intake - 55.832: s = 5.767: s_b = 0.425: r = 0.733. Statistical difference between regression slopes = P < 0.05.