Protein Requirements of Man: Variations in Obligatory Urinary and Fecal Nitrogen Losses in Young Men

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Abstract
For 14 days 83 healthy Caucasian male university students, 18 to 26 years of age and 58 to 104 kg body weight consumed daily either an essentially protein-free diet (44 subjects) or one supplying 0.1 g egg protein per kilogram body weight (39 subjects). Body cell mass (BCM) was calculated from whole body 40K in 37 randomly chosen subjects. Urine was analyzed daily for nitrogen (N) and creatinine; fecal N was measured on pooled samples. Urinary N output reached a steady state between days 3 and 8 for all subjects; the average for days 10 to 14 was taken as the measure of obligatory urinary N loss. Mean urine and fecal N losses were not significantly different for the two diet groups and the data were combined for overall analysis. Obligatory urinary N was normally distributed, averaging 37.2 ± 5.5 mg N/kilogram body weight, 76.8 ± 12.5 mg N/kilogram BCM, 1.8 ± 0.30 mg N/basal kilocalorie. Obligatory fecal N was 9 ± 2 mg N/kg body weight, amounting to 20% of the total obligatory N loss. Although statistically significant correlations were found between obligatory urinary N and body weight, BCM, basal metabolic rate, and creatinine, they accounted for little of the variation in daily loss among individuals. Four subjects were restudied after a 3-year interval; obligatory urinary N loss per kilogram body weight did not differ significantly between the two periods.