Abstract: Objective:To investigate the effect of leucine supplementation on muscle atrophy induced by ageing and preliminarily analyzes the possible mechanism.Methods:Thirteen-monthold male ICR mice were randomly divided into dietary leucine supplementation(Leu),alanine supplementation(Ala)and normal control group(NC)by body weight.Mice in leucine(Leu)and alanine(Ala)supplementation group were treated with normal chow diet supplemented with 5% leucine and 3.4% alanine respectively for 8weeks,while normal control group were fed with normal diet.Seventeen hours of fasting after the last feeding,mice were sacrificed.The blood glucose was determined with a portable blood glucose meter.The level of BUN was detected by using spectrophotometry.The profile of plasma free amino acids was detected with an amino acids analyzer.The serum insulin and IGF-1 concentration were determined by ELISA.The skeletal muscle morphology was observed by using hematoxylin and eosin staining.The protein expression of Ub,Atrogin-1,MuRF-1as well as MHCⅡ was detected by the method of Western blotting after BCA protein assay.Results:Compared with both NC and Ala group,the results showed that,leucine supplementation(Leu)did not change the level of body weight,food intake,blood glucose,serum insulin and IGF-1,while the level of BUN was significantly elevated;The content of plasma leucine was increased,content of alanine,glutamic acid and glycine were all depressed significantly.Although the wet ratio of wet white gastrocnemius muscle to body weight,cross-sectional area and diameter of fibers were not influenced significantly by leucine supplementation,still showed a tendency to increase.The incidence of rounded and central nucleus fibers was reduced.Moreover,total protein content,the protein expression of MHCⅡ were both elevated,while the expression of Ub,Atrogin-1and MuRF-1in white gastrocnemius muscle were significantly depressed.There weren't statistically significant differences of all parameters between mice in NC and Ala group.Conclusion:Leucine supplementation significantly attenuates skeletal muscle atrophy in pre-senile mice,which may be correlated with inhibiting the activity of ubiquitin-proteasome system.