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High Energy Flux Mediates the Tonically Augmented ß-Adrenergic Support of Resting Metabolic Rate in Habitually Exercising Older Adults

Department of Integrative Physiology (C.B., D.S.D., P.P.J., D.D.C., D.S.P., D.R.S.), University of Colorado at Boulder, Boulder, Colorado 80309; Department of Food Science and Human Nutrition (K.O., C.L.M.), Colorado State University, Fort Collins, Colorado 80523; and Department of Medicine (Geriatric Medicine) (D.R.S.), University of Colorado Health Sciences Center, Denver, Colorado 80262

Address all correspondence and requests for reprints to: Christopher Bell, Ph.D., Department of Integrative Physiology, 354UCB, University of Colorado, Boulder, Colorado 80309-0354. E-mail: christopher.bell{at}colorado.edu.

The sympathetic nervous system contributes to resting metabolic rate (RMR) via ß-adrenergic receptor (ß-AR) stimulation of energy metabolism. RMR and ß-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·d^–1), caloric intake (7808 ± 431 vs. 9433 ± 528 kJ·d^–1), RMR (5192 ± 167 vs. 5401 ± 209 kJ·d^–1), and skeletal muscle sympathetic nervous system activity (36 ± 2 vs. 42 ± 2 bursts·min^–1). Significant ß-AR support of RMR was observed at baseline (167 ± 42 kJ·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 ß-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 ß-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.

This work was supported by Grant 2 M01-RR00051 from the General Clinical Research Center Program of the National Center for Research Resources, National Institutes of Health (NIH), and from NIH Awards AG15897, AG06537, AG-00828, 1 P30 DK48520, and DDK-07685, the Colorado Agricultural Experiment Station (Project 616), and American Heart Association Grant 9920445Z.

Abbreviations: ß-AR, ß-Adrenergic receptor; MSNA, skeletal muscle sympathetic nerve activity; RMR, resting metabolic rate; SNS, sympathetic nervous system; VO_2peak, peak oxygen uptake.

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