How Body size and composition affecting the BMR?
Basal and resting energy expenditures are related to body size, being most closely correlated with the size of the fat-free mass (FFM), which is the weight of the body less the weight of its fat muss. The size of the FFM generally explains about 70 to 80 percent of the variance in RMR. FFM is the metabolically active tissue in the body; what we also call as the lean body mass (LBM) and so most of the variation in BMR between people can be accounted for by the variation in their FFM. For example, athletes with greater muscular development have approximately a 5% higher basal metabolism than non-athletic individuals. Thus, exercise can help maintain a higher lean body mass and hence a higher metabolic rate. Similarly, the lower basal metabolic energy requirement of women is primarily related to their generally lower amount of lean muscle mass (more of fat mass) as compared to men. The decline in BMR with increasing age is also to some extent the consequence of changes in the relative size of organs and tissues. Further larger people (big size) have higher metabolic rates than people of smaller size. In fact, individuals with greater surface area have higher metabolic rate. To illustrate, if two people of different heights weigh the same, the taller individual with the larger body surface area will have a higher metabolic rate. In adults with higher percentages Bf body fat composition, mechanical hindrances can also increase the energy expenditure associated with certain types of activity.