[Muscle, metabolic paradigm in nutritional recovery]

Skeletal muscle is a tissue that represents the majority of total body mass and proteins in healthy humans. Muscle mass is the results of balance between synthesis and proteolysis, both processes being sensitive to a variety of factors including nutritional status, hormonal balance, physical activity and exercise, and disease. Indeed, muscle mass loss is associated with several infectious, traumatic and chronic pathologies. Likewise, sarcopenia is a progressive and generalized skeletal muscle disorder appearing with ageing associated with increased likelihood of adverse outcomes including falls, fractures, physical disability, and mortality. Hence, recovery of muscle mass and functionality is a key factor to improve undernutrition associated with many pathological conditions. The aim of the present article is to summarize the most important substrates, metabolic and cell signaling pathways involved in the synthesis, degradation and turnover in skeletal muscle. Moreover, the importance of some myokines in the interaction between skeletal muscle and other tissues, and in the maintenance of homeostasis is highlighted. A great number of positive regulators for muscle protein synthesis has been reported. Especially, during energy restriction, branched chain amino acids, e.g. leucine, contribute majorly to muscle protein synthesis as well as to attenuate nitrogen body excretion and muscle proteolysis.