Nitrogen Balance of Men with Marginal Intakes of Protein and Energy

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Abstract
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 ± 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 -0.26 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 ± 18 mg/kg body weight or 3.76 ± 0.61 mg/basal kcal. N balance fell to -0.61 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.