Effect of infused branched-chain amino acids on muscle and whole-body amino acid metabolism in man

Clin Sci (Lond). 1990 Nov;79(5):457-66. doi: 10.1042/cs0790457.


1. Using the forearm balance method, together with systemic infusions of L-[ring-2,6-3H]phenylalanine and L-[1-14C]leucine, we examined the effects of infused branched-chain amino acids on whole-body and skeletal muscle amino acid kinetics in 10 postabsorptive normal subjects; 10 control subjects received only saline. 2. Infusion of branched-chain amino acids caused a four-fold rise in arterial branched-chain amino acid levels and a two-fold rise in branched-chain keto acids; significant declines were observed in circulating levels of most other amino acids, including phenylalanine, which fell by 34%. Plasma insulin levels were unchanged from basal levels (8 +/- 1 mu-units/ml). 3. Whole-body phenylalanine flux, an index of proteolysis, was significantly suppressed by branched-chain amino acid infusion (P less than 0.002), and forearm phenylalanine production was also inhibited (P less than 0.03). With branched-chain amino acid infusion total leucine flux rose, with marked increments in both oxidative and non-oxidative leucine disposal (P less than 0.001). Proteolysis, as measured by endogenous leucine production, showed a modest 12% decrease, although this was not significant when compared with saline controls. The net forearm balance of leucine and other branched-chain amino acids changed from a basal net output to a marked net uptake (P less than 0.001) during branched-chain amino acid infusion, with significant stimulation of local leucine disposal. Despite the rise in whole-body non-oxidative leucine disposal, and in forearm leucine uptake and disposal, forearm phenylalanine disposal, an index of muscle protein synthesis, was not stimulated by infusion of branched-chain amino acids. 4. The results suggest that in normal man branched-chain amino acid infusion suppresses skeletal muscle proteolysis independently of any rise of plasma insulin. Muscle branched-chain amino acid uptake rose dramatically in the absence of any apparent increase in muscle protein synthesis, as measured by phenylalanine disposal, or in branched-chain keto acid release. Thus, an increase in muscle branched-chain amino acid concentrations and/or local branched-chain amino acid oxidation must account for the increased disposal of branched-chain amino acids.

Publication types

  • Clinical Trial
  • Controlled Clinical Trial
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adolescent
  • Adult
  • Amino Acids, Branched-Chain / administration & dosage*
  • Amino Acids, Branched-Chain / pharmacokinetics
  • Female
  • Forearm
  • Humans
  • Infusions, Intravenous
  • Insulin / blood
  • Kinetics
  • Leucine / administration & dosage
  • Leucine / metabolism
  • Male
  • Muscle Proteins / metabolism*
  • Muscles / metabolism
  • Phenylalanine / administration & dosage
  • Phenylalanine / metabolism


  • Amino Acids, Branched-Chain
  • Insulin
  • Muscle Proteins
  • Phenylalanine
  • Leucine