Muscle in adults is constantly being built and broken down. As young adults we keep the two processes in balance, but when we age breakdown starts to win. However, adding the amino acid leucine to the diet of old individuals can set things straight again. This is the finding of research performed by Lydie Combaret, Dominique Dardevet and colleagues at the Human Nutrition Research Centre of Auvergne, INRA, Clermont-Ferrand, France.

After the age of 40, humans start loosing muscle at around 0.5-2% per year. Immediately after a meal degradation of protein slows down and synthesis doubles. This process is triggered by the arrival of a plentiful supply of amino acids. In older animals this stimulus is less effective; synthesis slows down, and previous work also suggests that breakdown may be affected. While adding leucine to the diet restores protein building there was no knowledge about this supplement's effect on breakdown.

To address this, researchers compared protein breakdown in young (8-month) and old (22-month) rats. They discovered that the slow down in degradation that normally follows a meal does not occur in old animals, so there is excessive breakdown. But adding leucine to the diet restored a balanced metabolism.

The team of researchers believe that the age-related problem results from defective inhibition of ubiquitin-proteasome dependent proteoloysis, a complex degradative machinery that breaks down contractile muscle protein, and that leucine supplementation can fully restore correct function.

"Preventing muscle wasting is a major socio-economic and public health issue, that we may be able to combat with a leucine-rich diet," says senior co-author Didier Attaix.

Commenting on the work Michael Rennie from the University of Nottingham Medical School at Derby says: "This is exciting because it strengthens the idea of a co-ordinated linkage between the meal-related stimulation of protein synthesis and the inhibition of breakdown."

Carol Huxley
chuxleyphysoc
Blackwell Publishing Ltd.
www.blackwellpublishing

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