High energy flux mediates the tonically augmented β-adrenergic support of resting metabolic rate in habitually exercising older adults

The sympathetic nervous system contributes to resting metabolic rate (RMR) via beta-adrenergic receptor (beta-AR) stimulation of energy metabolism. RMR and beta-AR support of RMR are greater in habitually exercising compared with sedentary older adults possibly due to greater energy flux ( magnitude of energy intake and energy expenditure during energy balance). In 10 older adults regularly performing aerobic endurance exercise ( mean +/- SE, 66 +/- 1 yr) compared with baseline, a reduction in energy flux ( via abstention of exercise and proportional reduction in dietary intake) decreased (P< 0.05) energy expenditure (7746 +/- 440 vs. 9630 +/- 662 kJ center dot d(-1)), caloric intake ( 7808 +/- 431 vs. 9433 +/- 528 kJ center dot d(-1)), RMR ( 5192 +/- 167 vs. 5401 +/- 209 kJ center dot d(-1)), and skeletal muscle sympathetic nervous system activity ( 36 +/- 2 vs. 42 +/- 2 bursts center dot min(-1)). Significant beta-AR support of RMR was observed at baseline ( 167 +/- 42 kJ center dot d(-1)) but not during reduced energy flux. The change in RMR from baseline to reduced energy flux was related to the corresponding change in beta-AR support of RMR ( r = 0.77, P = 0.009). No changes were observed in seven time controls ( 69 +/- 3 yr) who maintained energy flux. High energy flux is a key mechanism contributing to the elevated RMR and beta-AR support of RMR in habitually exercising older adults. Maintenance of high energy flux via regular exercise may be an effective strategy for maintaining energy expenditure and preventing age-associated obesity.