Human protein requirements: evaluation of the 1973 FAO/WHO safe level of protein intake for young men at high energy intakes

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Abstract
1. Six Caucasian male Massachusetts Institute of Technology students participated in a 77–87 d metabolic balance study to determine the adequacy of the 1973 FAO/WHO egg protein allowance for men (0.57 g/kg body-weight per d). Each subject was given an initial energy allowance calculated to meet his particular requirements, and these intakes were raised by increments of 10% approximately every 2 weeks until a slightly positive balance was achieved. Each individual's energy intake was maintained at this final level for the remainder of the study.2. At energy intakes sufficient to meet their estimated requirements, five of six subjects were in negative nitrogen balance. In five subjects, N balance improved with increased energy intake until N balance was achieved. The mean change in N balance was 0.335 mg N/additional kJ consumed. All of the subjects gained weight at the higher energy intakes.3. Serum aspartate aminotransferase and alanine aminotransferase activities increased with continued intake of the experimental diet and reached abnormal levels in five of the six subjects. In the two subjects showing the earliest increases in serum transferase activity, the levels returned towards normal when protein intake was raised to 0.73–1.0 g/kg body-weight per d. All subjects showed normal serum transferase values on an ad lib. diet 2–3 weeks after termination of the study.4. The findings indicate that at energy intakes necessary to bring subjects into slightly positive N balance at the level of 0.57 g egg protein/kg per d, a significant proportion of young adults continue to gain weight for as long as 70 d.5. These findings suggest that the 1973 FAO/WHO safe allowance of 0.57 g egg protein/kg per d is not sufficient for most healthy young men receiving dietary energy intakes appropriate for long-term maintenance of body-weight.