Christopher Bell

Thermogenic responsiveness to β‐adrenergic stimulation is augmented in exercising versus sedentary adults: role of oxidative stress

β‐Adrenergic receptor (β‐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 β‐AR stimulation, perhaps secondary to reactive oxygen species (ROS) bioactivity, may be involved. To determine habitual exercise‐related differences in β‐AR thermogenic responsiveness and the possible influence of ROS, we measured the percentage increase in EE (ΔEE%; indirect calorimetry, ventilated hood method) above resting EE in response to non‐specific β‐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). ΔEE% 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) ΔEE% only in the sedentary adults (to 9.5 ± 0.9, 12.4 ± 0.7, 18.5 ± 0.8), abolishing baseline group differences. ΔEE% was not related to the amount of body fat, sex, or any other baseline characteristic. Thermogenic responsiveness to β‐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.