Nitrogen balance of men with marginal intakes of protein and energy.

Present protein allowances are based on amounts of nitrogen (N) that maintain balance in adults in laboratory tests. In most tests of minimum N need, energy intakes were higher than present allowances and generally the participants maintained body weight or gained. To evaluate the relative importance of energy and protein intakes in the near-adequate range on the N equilibrium, healthy men were given two levels of protein with energy constant and three levels of energy with protein constant. In the first two 12-day periods, diets provided 5 and 7% of energy (E) from egg white protein with enough E to maintain weight essentially constant (39.6 plus or minus 4.4 kcal/kg). N balance data with these diets were used to select an individual protein intake level nearest to need (5, 6, or 7%), and that level was fed for the next three periods with the same E intake as before (100 E) and 85 or 115% of it. Crude N balance (dietary-fecal-urinary N) was minus0.26 g/day with 5% diet and 0.33 g/day with 7%. Balance was improved by 280 mg/g N fed between these levels. Predicted minimum N need to maintain crude N balance at 100 E is 89 plus or minus 18 mg/kg body weight or 3.76 plus or minus 0.61 mg/basal kcal. N balance fell to minus0.61 g/day with 85 E and increased to 0.59 g/day with 115 E. N balance changed by 174 mg/100 kcal between 85 and 100 E and 112 mg/100 kcal between 100 and 115 E. Energy intake appears to have a much greater effect on N balance than does protein intake in the marginally adequate ranges of intake.