Background: The adequacy of indispensable amino acid supplies has received much attention in studies of protein requirements, but the availability of nitrogen for synthesis and maintenance of the supply of dispensable amino acids has been overlooked.

Objective: We aimed to determine whether nitrogen balance and the endogenous supply of the dispensable amino acids glycine and tyrosine can be maintained with a marginal protein intake.

Design: Phenylalanine, glycine, and tyrosine kinetics were measured in young adults (6 men, 6 women) on 4 occasions during a reduction in habitual protein intake (1.13 g x kg(-1) x d(-1)) to a marginal intake (0.75 g x kg(-1) x d(-1)) by using a multiple stable-isotope-infusion protocol.

Results: During the 10-d period of marginal protein intake, nitrogen excretion fell initially, then remained constant such that nitrogen balance was negative for the first 2 d and then positive or zero thereafter. Whole-body protein degradation and synthesis predicted from phenylalanine kinetics declined significantly (P < 0.05) over the period of marginal protein intake. Despite the reduction in the amount of glycine and tyrosine derived from whole-body proteolysis, the fluxes of glycine and tyrosine were maintained.

Conclusions: The results show that adaptation to a marginal intake of dietary protein consisted of an overall reduction in whole-body protein turnover, net protein catabolism, and the rate of nitrogen excretion. The conserved nitrogen was sufficient to maintain the endogenous synthesis and hence the supply of glycine and tyrosine.