Human Protein Requirements: Obligatory Urinary and Fecal Nitrogen Losses and the Factorial Estimation of Protein Needs in Elderly Males

Abstract

Eight healthy elderly men, ages 68 to 72 years, were given an essentially protein-free diet for 10 days to determine obligatory urinary and fecal nitrogen losses. Body cell mass (BCM) was estimated from whole body ⁴⁰K, and basal metabolic rate was determined by indirect calorimetry. Urine was analyzed daily for total N and creatinine, and fecal N was measured in pooled samples. The pattern of change in urinary N output during adaptation to the experimental diet was analyzed according to a single exponential equation. Five days were required for urinary N excretion to reach a new equilibrium. After stabilization, daily N excretion was 34.5 ± 9.9 mg N/kg body weight, 98.1 ± 20 mg N/kg BCM, 1.49 ± 0.3 mg N/basal kcal, and 2.2 ± 0.49 mg N/mg creatinine. Obligatory fecal N was 12.2 ± 5.2 mg N/kg body weight/day. Comparison of these results with published data for young men and elderly females indicates that, per unit of body weight, obligatory urinary N losses in elderly men are higher than those for elderly women, but similar to those for young men. Obligatory urinary N output was negatively correlated with the level of energy intake in excess of basal requirements. A protein allowance for elderly men was estimated from these data using the 1973 FAO/WHO factorial method and the practical significance of the estimate is discussed.