Muscle mass and protein metabolism.

Muscle protein content is regulated acutely (minutes to hours) by modulation of the rates of protein synthesis and degradation. In physiological conditions, a small fraction of muscle protein content is lost in the postabsorptive state, whereas it is immediately regained with the following meal. Thus, the efficiency of the mechanisms responsible for the regulation of protein synthesis and degradation in the postabsorptive and fed states appears to be crucial for maintaining skeletal muscle mass throughout the day, thereby avoiding protein wasting. The hydrolysis protein to their constituent amino acids is a highly regulated process. The adenosine triphosphate (ATP)independent lysosomal proteases, Ca2 -dependent proteases and an ATPdependent pathway involving the ubiquitin-proteasome complex have been identified in skeletal muscle. Under normal physiological conditions, the lysosomes are predominantly involved in the degradation of extracellular and membrane-associated proteins. In contrast, the ubiquitin-dependent system is quantitatively the most important degradative system of myofibrillar proteins in skeletal muscle. Anabolic and catabolic hormones, such as insulin and cortisol, may inhibit or stimulate the activity of the ubiquitin-proteasome system. The synthesis of myofibrillar proteins in skeletal muscle requires the presence of physiological levels of insulin [1]. Nonetheless the importance of amino acid availability in the postprandial stimulation of muscle protein synthesis should be emphasized. Following amino acid administration, muscle protein synthesis increases proportionally to the amount of amino acids administered, up to 3 times the basal postabsorptive value. Amino acids are more efficiently utilized when given in divided doses (as occurs with meal feeding) rather than with continuous administration (as often occurs in artificial nutrition). The Allison SP, Go VLW (eds): Metabolic Issues of Clinical Nutrition. Nestlé Nutrition Workshop Series Clinical & Performance Program, vol 9, pp 111–124, Nestec Ltd., Vevey/S. Karger AG, Basel, © 2004.