[Metabolic fitness: physical activity and health]

Physical inactivity is strongly associated with an increased risk of premature disease and death, and the falling level of physical activity in Denmark (as in many other countries) makes physical inactivity a major life-style risk factor in many western countries today. Both aerobic fitness (maximum oxygen uptake) and metabolic capacity of the muscles are important in this matter. The present paper focuses on the role of the metabolic capacity/fitness of muscle, because this appears to be especially critical for the development of metabolic-related diseases and thus for the health of the individual. A definition of metabolic fitness is proposed as the ratio between mitochondrial capacity for substrate utilisation and maximum oxygen uptake of the muscle. Indirect means of determining this parameter are discussed. Skeletal muscle is an extraordinarily plastic tissue and metabolic capacity/fitness changes quickly when the level of physical activity is altered. High metabolic fitness includes an elevated use of fat at rest and during exercise. The capacity for glucose metabolism is also enhanced in trained muscle. Some of these adaptations to physical activity are explained. Exercise-induced activation of genes coding for proteins involved in metabolism is described as an underlying mechanism for some of these adaptations. The increased gene expression is of relatively short duration, which implies that a certain regularity of physical activity is required to maintain high metabolic fitness. Thus, metabolic fitness is directly related to how much the muscle is used, but even low levels of physical activity have a beneficial effect on metabolic fitness and the overall health of the individual.