Body composition and metabolic activity of tissues during ontogenesis

Abstract

Comparison of the young and adult organism revealed the tendency to enhanced deposition of body fat with advancing age in mammals. This ensues not only from the altered calorie balance but in particular from the different level of energy turnover during ontogenesis. The young growing organism ingests a substantially larger amount of food per body weight unit than the full grown or old organism. The basal metabolic rate during growth is at least double that of the full grown organism (Richet 1889, Rubner 1916, Brody 1945, Holliday et al. 1967) and it declines still further in old age. In addition to energy needed for growth there are from the aspect of caloric requirements changes in the trend of spontaneous physical activity, i.e. muscular work which determines the energy output above the basal metabolic rate. Spontaneous physical activity is highest in the early stages of ontogenesis and declines in later life. This was assessed fairly accurately in experimental animals (e.g. Smith and Dugal 1965, Collier 1971a, b) as well as in man (Ledovskaya 1972, Gapon 1972).