Supplementation with Dietary Leucine to a Protein-Deficient Diet Suppresses Myofibrillar Protein Degradation in Rats

Muscle mass is regulated by the synthesis and degradation of muscle protein, which in turn are affected by aging, several catabolic diseases, and malnutrition. Amino acids, particularly leucine, are known to stimulate muscle protein synthesis and suppress muscle protein degradation, although their long-term effects are unclear. The objective of our research was to elucidate whether long-term feeding of a protein-free or low-protein diet supplemented with leucine suppresses myofibrillar protein degradation. The rate of myofibrillar protein degradation was measured by the rate of release of 3-methylhistidine (MeHis) from isolated extensor digitorum longus (EDL) muscle. The weight of gastrocnemius muscle decreased in rats fed a protein-free diet for 7 d; however, a leucine-supplemented (1.5%) diet tended to suppress this decrease. The release of MeHis from EDL muscle was increased by the protein-free diet and decreased by the feeding of a diet supplemented with leucine to the level of a 20% casein diet. When rats were fed a 5% casein diet, the gastrocnemius muscle weight decreased and MeHis release from EDL muscle increased compared to those fed a 20% casein diet. However, feeding of a 5% casein diet supplemented with leucine (1.15%) reduced muscle weight loss and MeHis release. These results suggest that long-term feeding of leucine suppresses the rate of myofibrillar protein degradation and muscle weight loss in rats fed a protein-deficient diet.