Influence of age on caloric expenditure during exercise

Young (147 +/- 2 g) and adult (322 +/- 7 g) male Sprague Dawley rats were subjected to an incremental treadmill protocol and assessed via the Oxyscan Metabolic System in order to determine if differences exist relative to oxygen consumption (VO2, expressed in ml/kg/min), respiratory exchange ratio (RER) and caloric expenditure of two different age groups. The young rats attained a higher VO2 (81.55 +/- 1.22) compared to adult rats (68.97 +/- 2.05) at a maximal level of exercise. The adult rats became dependent upon carbohydrates as the primary source of energy until reaching 52% VO2 max, whereas the young rats did not resort to carbohydrate utilization as a primary fuel source until reaching 87% VO2max. At peak exercise, the adult animals burned only 15% less kcal/kg/h than the young rats, but the adult rats burned 37% more carbohydrate than the young animals. These data suggest that as exercise intensity increases, younger rats can more readily maintain a higher level of oxygen consumption accompanied by a more efficient use of fat as an energy source compared to adult rats. The clinical implication of this finding suggests that during the growth and development years of life, fat plays a significant role in energy expenditure needs and may therefore be more important than currently believed in young children's nutritional intake.