EFFECTS OF INCREASED ENERGY-INTAKE AND/OR PHYSICAL-ACTIVITY ON ENERGY-EXPENDITURE IN YOUNG HEALTHY-MEN

This study was designed to examine effects of alterations in energy balance on adaptive changes in components of total energy expenditure (TEE). Nineteen young healthy males were studied during a 10-day sedentary energy balance baseline period and then randomly assigned to one of four 10-day treatment groups: 1) no change in energy intake (EI) or physical activity (PA; energy balance at low energy flux), 2) EI increased by 50% with no change in PA (positive energy balance), 3) TEE increased by 50% by increasing PA, matched by a 50% increase in EI (energy balance at high energy flux), and 4) TEE increased by 50% by increasing PA with no change in EI (negative energy balance). TEE was measured with doubly labeled water, resting metabolic rate (RMR) by indirect calorimetry, and thermic response to feeding (TEF) by indirect calorimetry; energy expenditure of physical activity (EEPA) was estimated by subtracting RMR, TEF, and prescribed PA from TEE. TEE was significantly increased by PA (by design) but not EI. There was a significant main effect of intake and a significant intake-by-activity interaction for changes in RMR. In post hoc analysis, RMR was significantly increased during positive energy balance and energy balance at high energy flux relative to change in RMR when energy balance was maintained at low energy flux. A significant increase in RMR was also noted during negative energy balance after adjustment for change in fat-free mass. There was no significant difference in change in RMR among the three treatment groups. There were no significant main effects of intake or activity on EEPA or thermic effect of a meal. These results suggest that in young healthy males living under tightly controlled conditions 1) adaptive changes in TEE in response to shortterm alterations in energy balance are mediated primarily through changes in RMR, 2) RMR can be elevated during a state of energy balance when energy flux is increased, and 3) magnitude of adaptive change in RMR is similar in response to increased EI and/or PA; therefore simultaneous increases in energy intake and physical activity do not act synergistically to raise RMR.