Protein and Amino Acid Requirements of Adults: Current Controversies

Abstract

Protein intakes vary widely but costs and benefits of such variation is a long standing unresolved issue. The wide range of reported values for the minimum protein intakefor N equilibrium in adults, i.e. 0.39 to 1. 09 g/kg is best explained by an Adaptive Metabolic Demands model in which metabolic demands include amino acid oxidation at a rate varying with habitual protein intake and which changes slowly with dietary change. Thus within the reported data the true minimum requirement intake, the lowest values in the range at intakes approaching the Obligatory Nitrogen Loss, allows onlyfully adaptedsubjects to achieve N equilibrium. The higher values reflect incomplete adaptation. 13C-1 leucine tracer balance studies of this model show (a) a fall with age in apparent protein requirements, (b) better than predicted efficiency of wheat protein utilization, and (c) controversially, lower lysine requirements than other workers, consistent with new evidence ofde novo synthesis of lysine from urea salvaged by large bowel microflora. The main implication of the requirements model for athletes on high protein diets is increased exercise induced amino acid oxidation and risk of loss ofbody N when such high intakes are not maintained.