Energy and protein intakes as determinants of nitrogen balance.

Changes in caloric intake below or above energy needs affect nitrogen balance, so that any change in caloric intake results in a corresponding alteration in nitrogen balance. The improvement in nitrogen balance caused by an increase in energy intake, however, can be frustrated if intake of protein is inadequate; conversely, the beneficial effects of an increase in protein intake can be inhibited by an inadequate energy intake. These observations mean that, in experimental studies, nitrogen balance is the result of levels of both energy and protein; in consequence, protein requirements can be interpreted only from such studies, where energy intake is also defined under the experimental conditions. In addition to the effect of dietary energy supply, dietary carbohydrate has a short-term specific effect on protein metabolism not shared by fat, in which plasma amino acids are diverted into muscle protein through the action of insulin released by the dietary carbohydrate. This metabolic interaction also occurs between carbohydrate and amino acids absorbed from the same meal.