Human protein requirements: interrelationships between energy intake and nitrogen balance in young men consuming the 1973 FAO/WHO safe level of egg protein, with added non-essential amino acids.

Our recent studies have shown that the 1973 FAO/WHO "safe level of intake" of egg protein (0.57 g/kg/day) is inadequate for maintaining protein nutritional status in young men receiving generous energy intakes. Therefore, an experiment was conducted to determine nitrogen (N) balance and the energy intake needed to support it when supplementary N, equivalent to 0.23 g of protein (N X 6.25)/kg/day from a nonessential amino acid mixture, was added to a diet containing 0.57 g of egg protein/kg/day. Four young men, 20 to 21 years old, participated in the 58- to 79-day metabolic N balance study. This group required significantly lower energy intakes to maintain N balance than a previously studied group fed only 0.57 g of egg protein under identical conditions. The energy intakes predicted to maintain N balance were approximately 10 to 15% less than the requirements estimate from body weight and N balance data. Present results, although based on a limited number of subjects, suggest that total N may be the limiting factor in short-term N balance at the 1973 FAO/WHO egg protein intake level for a significant proportion of young adult male populations. Long-term metabolic studies will be necessary before the practical significance of the observations can be determined.