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Protein Requirements of Man: Variations in Obligatory Urinary and Fecal Nitrogen Losses in Young Men^{1, 2,}

Department of Nutrition and Food Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

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.

KEY WORDS: • obligatory nitrogen excretion • protein requirements • man

¹ Financed by a contract, no. AF, 41-(609)3151, with the United States Air Force and utilizing the facilities of the MIT Clinícal Research Center supported by a grant (RR-88) from the General Clinical Research Centers Program of the Division of Research Resources, National Institutes of Health.

² Contribution no. 1893 from the Department of Nutrition and Food Science, MIT, Cambridge, Massachusetts. A preliminary report of this study was presented at the 52nd Annual Meeting of the Federation of American Societies for Experimental Biology, Atlantic City, April 1968. Federation Proc. 27: 485 (abstr.).

³ Present address: Nutrition Institute, Kasr, El-Airi Street, Kasr El-Airi Post Office, Cairo, U. A. R.

Manuscript received 13 December 1971.

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