An interrelationship exists between utilization of dietary protein and dietary energy. In particular, although some studies suggest otherwise, higher energy intakes usually increase nitrogen retention when nitrogen intake levels are marginal or deficient. However, protein-energy interactions are not necessarily reflected simply in terms of protein or energy per se. Alterations in the relationship between protein and energy may be expressed indirectly as changes in body weight or composition and level of activity. Changes in the interrelationship may also occur as a result of differences in the composition of the energy source, the level of dietary fiber, and the nitrogen or energy digestibility. Moreover, for an accurate assessment of studies on the adequacy of protein or energy intakes, other factors must be considered, such as the activity level, the degree of sweating (temperature), the presence of disease, infection or parasites, the absence of other nutrients (minerals, vitamins), and the physiological status of the individuals on which the data were obtained (e.g., with children, normal vs. “catch-up” growth).
Research on the interactions between protein and energy centered on the following two questions:
Is the present “safe level” of protein intake adequate:
at habitual levels of energy intake?
at levels of energy intake that correspond with present estimates of energy requirements?
Would habitual protein intakes be adequate if energy intakes corresponded to present estimates of energy requirements?
This question was specifically addressed by studies in Thailand (paper 22) and Guatemala (paper 23) with both children and adults. The Thai studies with children, 1–3 years of age, consisted of both a short- (11 days) and a long-term (40 days) study period. The Guatemala study was long-term (40 days) and included children 2–4 years of age. These studies showed that if the present FAO/WHO recommended energy level for children (101 Kcal/kg/day) is met using the habitual diet, the “safe level” of protein is also met, N-balance is positive, and growth (including catch-up growth) occurs. However, at the habitual levels of intake, only 75 to 80% of the current FAO/WHO requirement for energy is satisfied while more than the current safe level for dietary protein is supplied.
With respect to adults, it was reported that subjects in Thailand, given their habitual diets with ad libitum intake of rice, maintained N-balance when protein and energy intakes ranged from 0.82 to 1.08 gm protein/kg/day and 50 to 66 kcal/kg/day, respectively. Both levels are significantly higher than present recommended levels. When protein was fed at the current safe level, nitrogen balance was achieved at about 50 kcal/kg/day, but body weight was lost.
This suggests that habitual levels of protein intake may not be adequate for some populations when fed with energy intakes below 50 kcal/kg. It must be stressed that in none of these studies was the current safe level of protein fed together with adequate energy to maintain body weight. Therefore, the question of the adequacy of the current safe protein level at habitual energy intakes for adults has not been answered, since the test diets provided more protein than this. No studies were reported in which the current safe level has actually been tested with the energy intake at habitual levels. In the previously published study of Garza et al. (11) the safe level was tested at habitual energy intakes and proved inadequate.
Adults
Several long-term studies were conducted to test the adequacy of estimates of protein-energy needs derived from short-term N balance studies. In Korea (paper 12) and Chile (paper 15), groups of healthy men were fed predominantly mixed vegetable proteins (1.0 to 1.36 g/kg/day), primarily from rice and soy, or wheat, beans, rice, and skim milk. In general, subjects maintained nitrogen balance for long periods (52–90 days) when their energy intakes were determined by ad libitum staple intakes of 37 to 47 kcal/kg/day. Women in the Philippines (paper 14) maintained approximately zero nitrogen balance on 1.16 g protein/kg/day from rice and fish for 84 days, with a mean energy intake of 40 kcal/kg/day.
A group of 12 village men from Thailand (paper 13), who were allowed to eat rice ad libitum with a fixed intake of fish, required between 51 and 63 kcal/kg to maintain nitrogen balance; this energy value is probably above that generally considered to meet FAO/WHO requirements for men with similar activity levels. However, as the requirement for energy is highly dependent upon the level of activity, energy needs for a group cannot be evaluated unless direct values for energy expenditure are known.
Children
In the case of children, in two long-term studies in Guatemala, children consumed their habitual diets of corn and beans in amounts to supply 1.85 to 2.1 g protein/kg/day. The diets were supplemented with oil and sugar to provide energy intakes of 92 to 98 kcal/kg/day. These studies were confounded because the children were gaining weight at greater than normal rates, probably indicative of “catch-up” growth and energy intakes in excess of their habitual ones.
It would appear that intake of protein at requirement levels estimated from short-term N balance studies is adequate to maintain nitrogen equilibrium over the long-term in adults, and to promote lean tissue accumulation in children at the recommended level of energy intake. However, if adequate energy intake is defined as that required to maintain constant body weight, the energy intakes needed to support the protein intakes tested may be greater than the currently recommended energy intake level.
As noted by Waterlow (Appendix 2), adequate intakes for children of energy and protein should allow for a) adequate nitrogen retention, b) normal weight gain, and c) normal levels of activity. Similarly, adequate intakes for adults of energy and protein should allow for a) nitrogen balance, b) stable body weights, and c) normal levels of activity.
Adults
Adult studies in Guatemala, Chile, Japan, Korea, the Philippines, Thailand, and the U.S. demonstrate similar phenomena, but are more difficult to evaluate since the energy intakes required for energy balance, as indicated by steady body weights, were more variable. All of these studies have shown that at energy intakes sufficient to maintain body weight, nitrogen balance is maintained at the protein intakes studied (all of them higher than the current recommendation). However, in general, as protein intake falls to a lower level, increased energy is needed to maintain both constant body weight and nitrogen balance.
Children
Both Thai and INCAP studies in children show that at the current safe level of protein intake, and at 85–90% of the recommended level of energy intake, the diet is adequate as measured by rate of weight gain, nitrogen retention (24–35 mg N/kg/day), and level of activity. Nitrogen balance was maintained even when the energy intake was as low as 75% of the recommended level, but at the expense of a reduced rate of weight gain and/or levels of activity.
Preliminary data from Jamaica (paper 25) show comparable results. At the lower levels of energy intake, nitrogen turnover increased with an increase in the rate of both protein synthesis and breakdown. In two children fed a level of protein intake of 0.7 g/kg/day, nitrogen balance was maintained when the energy intake was greater than 90 kcal/kg/day. However, nitrogen turnover and protein synthesis and breakdown were reduced to 50 percent of that observed at 1.7 gm protein/kg/day. Nitrogen balance could not be maintained at 80 kcal/kg/day.
