Submitted to: American Journal of Clinical Nutrition
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/19/2001
Publication Date: 3/20/2002
Citation: GIBSON, N.R., JAHOOR, F., WARE, L., JACKSON, A.A. ENDOGENOUS GLYCINE AND TYROSINE PRODUCTION IS MAINTAINED IN ADULTS CONSUMING A MARGINAL-PROTEIN DIET. AMERICAN JOURNAL OF CLINICAL NUTRITION. 2002. v. 75(3). p. 511-518. Interpretive Summary: Proteins in the body are made of 20 amino acids which we get from the protein in our meals. Ten of these amino acids are, however, required in large quantities because they are used to make numerous other compounds that the various organs need to function properly. These 10 amino acids are usually made from other amino acids that are left over after the body has made full use of the 20 it received from the diet. Although a protein intake of 3/4 grams per kilogram body weight per day is recommended for most populations in the world and considered adequate for maintenance of good health, it has been argued that this protein intake may not be enough to maintain adequate supplies of these 10 amino acids. In this study we wanted to see whether a protein intake of 3/4 grams per kilogram body weight per day was sufficient to maintain adequate supplies of two of these 10 amino acids, glycine and tyrosine. We fed 12 young adults (6 men, 6 women) a diet that had sufficient calories but only 3/4 grams per kilogram body weight per day of protein and found that they were still able to make adequate quantities of glycine and tyrosine because they reduced the amount of protein that they were breaking down and excreting in the urine.
Technical Abstract: 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.