Quantitative Aspects of Glycine and Alanine Nitrogen Metabolism in Postabsorptive Young Men: Effects of Level of Nitrogen and Dispensable Amino Acid Intake

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Abstract
The nutritionally indispensable amino acids (IAA) alone do not maintain body nitrogen (N) balance; a source of “nonspecific” nitrogen from dispensable amino acids (DAA), such as from glycine and alanine or other N compounds, is required. However, the in vivo regulation of the metabolism of these amino acids in humans with varying nutritional states has received little study. Hence, the effects of N intake and the IAA:DAA ratio on kinetic aspects of whole-body alanine and glycine metabolism were examined in eight healthy young adult male subjects. They received an L-amino acid diet supplying N equivalent to about 1.5 g and 0.6 g protein (N × 6.25) per kilogram body weight per day. All were studies at each N level with the IAA:DAA ratio (wt/wt) of 1:1 and 1:0, each for a 7-d diet period. Constant primed, intravenous infusions of L-[l-13C]leucine together with either L-[15N]alanine (four subjects) or [15N]glycine (four subjects) were given to each subject at the end of the diet period, after an overnight fast, to determine rates of de novo whole-body alanine and glycine N synthesis. The rate of alanine synthesis was similar (P > 0.05) for all four diets. Glycine de novo N synthesis declined (P < 0.01) with removal of dietary DAA, especially at the lower intake, where the mean rates [micromoles/(kilogram·hour)] were 59 and 20 for 1:1 and 1:0 ratios, respectively. The possible significance of reduced rates of glycine N synthesis for maintenance of protein nutritional status in the healthy adult is discussed.