Intravenous glutamine does not stimulate mixed muscle protein synthesis inhealthy young men and women

We investigated the effects of a glutamine-supplemented amino acid mixture on vastus lateralis muscle protein synthesis rate in healthy young men and women. Three men and 3 women (27.8 +/- 2.0 yr, 22.2 +/- 1.0 body mass index [BMI], 56.1 +/- 4.5 kg lean body mass [LBM]) received a 14-hour primed, co nstant intravenous infusion of L[1-C-13]leucine to evaluate the fractional rate of mixed muscle protein synthesis. In addition to tracer administratio n, a clinically relevant amino acid mixture supplemented with either glutam ine or glycine in amounts isonitrogenous to glutamine, was infused. Amino a cid mixtures were infused on separate occasions in random order at a rate o f 0.04 g/kg/h (glutamine at similar to0.01 g/kg/h) with at least 2 weeks be tween treatment. For 2 days before and on the day of an infusion, dietary i ntake was controlled so that each subject received 1.5 g protein/kg/d. Com pared with our previous report in the postabsorptive state, amino acid infu sion increased the fractional rate of mixed muscle protein synthesis by 48% (P <.05); however, the addition of glutamine to the amino acid mixture did not further elevate muscle protein synthesis rate (ie, 0.071% +/- 0.008%/h for amino acids + glutamine v 0.060% a 0.008%/h for amino acids + glycine; P =.316). Plasma glutamine concentrations were higher (P <.05) during the glutamine-supplemented infusion, but free intramuscular glutamine levels we re not increased (P =.363). Both plasma and free intramuscular glycine leve ls were increased when extra glycine was included in the infused amino acid mixture (both P <.0001). We conclude that intravenous infusion of amino ac ids increases the fractional rate of mixed muscle protein synthesis, but ad dition of glutamine to the amino acid mixture does not further stimulate mu scle protein synthesis rate in healthy young men and women. Copyright (C) 2 000 by W.B. Saunders Company.