Body Composition and Physical Fitness [and Comments and Reply]

Abstract

Body composition, changing characteristically not only according to age and sex but also according to the balance of energy input and output and functional state, has become one of the most interesting morphological features of human physique. Cross-sectional studies have revealed remarkable differences in the proportions of lean body mass and fat related to adaptation for intensive physical activity in both adults and children and also in the aged. These differences were significant even where body weight, absolute and relative, was the same. The dependence of the higher proportion of lean body mass to fat on the degree of physical activity has been further demonstrated by dynamic changes in these proportions during periods of changing intensity of physical activity, often even without marked changes in total body weight. Body composition is very closely related to various functional characteristics-functional aerobic capacity (measured as maximal oxygen consumption during graded work loads), metabolic cost of work, heart volume, creatinine excretion, oxygen consumption at rest, etc.-better functional fitness being connected with a higher proportion of lean body mass. All these relationships can also be demonstrated dynamically; i.e., with changing ratios of lean body mass to fat, these functional indicators change accordingly, especially in subjects in which the changes are highly noticeable (e.g., obese subjects after reduction in weight due to increased physical activity). The underlying mechanisms producing both increased proportion of lean body mass to fat and improved physical fitness have not yet been properly and systematically studied. From a metabolic point of view, it has been shown that marked changes in total body fat during periods of varying intensity of physical activity are connected with more profound changes in fat metabolism: adaptation for increased muscular work probably leads to an increased ability to mobilize fat metabolites (and to utilize them in greater proportion as a fuel for work) and increased lipolytic activity in various tissues (e.g., heart muscle), the contrary being the case with decreased physical activity. In this respect, and in its greater fitness, an organism adapted for greater physical activity resembles a younger one, a finding which is interesting also from the point of view of the so-called civilization diseases-obesity, cardiovascular diseases, etc.