Changes in the maximum level of metabolic activity of lean body mass during ontogenesis

Abstract

When evaluating changes of body composition in different age groups, it is essential to consider not only the ontogenetic aspect from the morphological point of view, i.e. the relative and absolute amount of LBM in relation to depot fat but also the aspect of the level of their metabolic activity characterized e.g. by oxygen consumption under different conditions (von Dobeln 1956, Buskirk a. Taylor 1957). Behnke (1956) described changes in the oxygen consumption in relation to LBM under basal conditions at rest: his observations were made in adult and elderly subjects. The basal oxygen consumption in relation to LBM declined significantly during ageing: the author explained this finding by changes in the composition of LBM (relative diminution of internal organs which under basal conditions have the highest oxygen consumption; in that case muscles consume cca 25% of oxygen, although as regards weight they form the largest constituent of LBM, and also by changes in the internal structure of tissues the sum of which is LBM (sclerotization, increased ratio of fibrous tissue, enhanced deposition of minerals, lipofuscin, etc.). Another aspect in this connection is the maximum level of aerobic processes during ontogenesis related to LBM. Aerobic capacity declines with ageing (Robinson 1936, Astrand and Rodahl 1970, Hollman 1963, Davies 1972 etc.). Here in particular an important part is played by muscular tissue where the intensity of metabolic processes increases most markedly during a maximal load. This maximal metabolic level in relation to LBM has not been compared so far for a wide age range.