Variation in Endogenous Nitrogen Excretion and Dietary Nitrogen Utilization as Determinants of Human Protein Requirement

Abstract

Dietary protein requirement is usually conceptualized as consisting of an amount needed to replace the individually variable endogenous nitrogen losses corrected for the biological quality of the protein eaten. This concept was reexamined by measuring the endogenous nitrogen excretion of healthy men, observing their nitrogen balances when fed an amount of high quality protein (egg white and whole egg) equal to the endogenous output, and determining the dietary level needed to attain balance. Caloric intakes were adjusted to maintain body weight constant. Daily endogenous nitrogen loss of 13 men was 2.41 g in the urine, 0.96 g in the feces, and 0.129 g from the body surface. Nine men failed to achieve nitrogen balance with diets containing 3.5 or 4.0 g of nitrogen, all but 0.5 g of which was derived from egg proteins. True minimum egg nitrogen requirement, predicted by regression analysis of balances at several levels of intake, was 4.8 g per day. Minimum requirement was not correlated with weight, lean body mass, urinary creatinine, basal metabolic expenditure or total caloric requirement. The need was affected by the magnitude of endogenous losses but more dominantly by the efficiency with which fed nitrogen was utilized. Net utilization of both egg white and whole egg was about 65% but ranged from 46 to 81%. Digestibility of the proteins was nearly complete, the difference in utilization being apparent only in the urine.