ABSTRACT
Skeletal muscle represents an important component and a key determinant of energy and substrate metabolism in the body. Because of its relatively low resting energy metabolism, skeletal muscle has often been neglected in explaining interindividual differences in metabolic rate, while it is the largest tissue mass and can account for 30% of the total resting energy expenditure. If the metabolic rate of other organs, such as the brain, heart, liver, or kidney, is constantly sustained and varies very little during the course of the day, skeletal muscle metabolism can change dramatically from resting to maximal physical activity during; this muscle oxygen consumption can account for up to 90% of the whole-body oxygen uptake. It is thus suggested that a major part of the variability between subjects in whole body metabolic rate and energy balance is related to differences in skeletal muscle metabolism. In addition to its contribution to energy metabolism, skeletal muscle is the main site of lipid oxidation in the body through mitochondrial oxidative systems in the fasting state as well as during exercise. Hence, disturbances in fat oxidation may promote the development of increased adipose-tissue stores and obesity but also increased triacylglycerol storage in muscle and skeletal-muscle insulin resistance. Beyond muscle involvement in energy expenditure, reduced muscle fat oxidation may be a primary factor leading to obesity and/or insulin resistance. However, the precise sequence of events leading to increased adiposity and insulin resistance in obesity and type 2 diabetes is yet to be determined. Several factors may influence the variation in muscle energy expenditure and substrate metabolism, such as thyroid hormone, insulin, muscle fiber types, muscle tone, sympathetic innervation, and catecholamines levels. More knowledge on skeletal muscle metabolism is required to understand how the muscle contributes to the development of obesity and fat deposition and to know how skeletal muscle utilization may help prevent obesity and its related metabolic disorders across all human ages. This chapter focuses on the involvement of physical activity (i.e., muscle functioning-related energy expenditure) in the etiology of obesity. Thereafter, the involvement of muscle “genetics” is examined.
