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INTEGRATIVE |
1 Department of Integrative Physiology, University of Colorado, Boulder, CO 80309-0354, USA
ß-Adrenergic receptor (ß-AR) modulation of resting and postprandial energy expenditure (EE) is augmented in regularly exercising compared with sedentary adults, but the underlying physiological mechanisms are unknown. Differences in thermogenic responsiveness to ß-AR stimulation, perhaps secondary to reactive oxygen species (ROS) bioactivity, may be involved. To determine habitual exercise-related differences in ß-AR thermogenic responsiveness and the possible influence of ROS, we measured the percentage increase in EE (
EE%; indirect calorimetry, ventilated hood method) above resting EE in response to non-specific ß-AR stimulation (intravenous isoproterenol (isoprenaline): 6, 12 and 24 ng (kg fat-free mass)–1 min–1) in 25 sedentary (11 males; 51 ± 4 years; body mass index 25.0 ± 0.8 kg m–2, maximal oxygen uptake 29 ± 1 ml kg–1 min–1 (mean ±S.E.M.)) and 14 habitually aerobic exercising (9 males, 46 ± 6 years, 23.1 ± 0.7 kg m–2, 44 ± 3 ml kg–1 min–1) healthy adults under normal (control) conditions and during acute intravenous administration of a potent antioxidant, ascorbic acid (vitamin C; 0.04 g (kg fat-free mass)–1). EE% was greater (P= 0.02) in the habitually exercising (8.6 ± 1.2, 12.9 ± 1.2, 20.0 ± 1.4) versus sedentary (6.3 ± 0.7, 10.4 ± 0.8, 16.0 ± 1.0) adults. Ascorbic acid increased (P= 0.01) EE% only in the sedentary adults (to 9.5 ± 0.9, 12.4 ± 0.7, 18.5 ± 0.8), abolishing baseline group differences. EE% was not related to the amount of body fat, sex, or any other baseline characteristic. Thermogenic responsiveness to ß-AR stimulation is augmented in habitually exercising adults. The mechanism is ascorbic acid dependent, suggesting that it may be linked to decreased ROS bioactivity. Our findings advance a novel mechanism by which habitual physical activity may modulate EE in humans, with potential implications for energy balance and body weight control.
(Received 14 September 2005;
accepted after revision 17 November 2005;
first published online 24 November 2005)
Corresponding author C. Bell: Department of Health and Exercise Science, 205E Moby-B Complex/1582 Campus Delivery, Colorado State University, Fort Collins, CO 80521-1582, USA. Email: cbell{at}cahs.colostate.edu
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