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4th Amino Acid Assessment Workshop

The Role of Leucine in the Regulation of Protein Metabolism^1,2

Department of Animal Sciences, University of Illinois, Urbana, IL 61801

³To whom correspondence should be addressed. E-mail: pgarlick{at}uiuc.edu.

Studies both in vivo and in vitro have shown that leucine at a very high dose can stimulate muscle protein synthesis, an effect that is enhanced in vivo by insulin secreted in response to the leucine dose. High leucine can also inhibit protein degradation in skeletal muscle, as well as in liver. In contrast, at normal physiological levels, increasing leucine concentration by infusion stimulates muscle protein synthesis by enhancing its sensitivity to insulin. It is concluded that the role of leucine in vivo is to provide a signal that amino acids are available, which in combination with the signal of energy availability from insulin, stimulates muscle protein synthesis.

KEY WORDS: • leucine • isoleucine • valine • branched-chain amino acids • protein synthesis • insulin • muscle

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