Thermic effect of food and β-adrenergic thermogenic responsiveness in habitually exercising and sedentary healthy adult humans

Abstract

The thermic effect of food (TEF) is an important physiological determinant of total daily energy expenditure (EE) and energy balance. TEF is believed to be mediated in part by sympathetic nervous system activation and consequent β-adrenergic receptor (β-AR) stimulation of metabolism. TEF is greater in habitually exercising than in sedentary adults, despite similar postprandial sympathetic nervous system activation. We determined whether augmented TEF in habitually exercising adults is associated with enhanced peripheral thermogenic responsiveness to β-AR stimulation. In separate experiments in 22 sedentary and 29 habitually exercising adults, we measured the increase in EE (indirect calorimetry, ventilated hood) during β-AR stimulation (intravenous isoproterenol: 6, 12, and 24 ng·kg fat-free mass−1·min−1) and EE before and after a liquid meal (40% of resting EE; 53% carbohydrate, 32% fat, 15% protein). The increase in EE during incremental isoproterenol administration was greater ( P = 0.01) in habitual exercisers (0.34 ± 0.03, 0.54 ± 0.04, 0.81 ± 0.05 kJ/min; means ± SE) than in sedentary adults (0.26 ± 0.03, 0.40 ± 0.03, 0.64 ± 0.04 kJ/min). The area under the TEF response curve was also greater ( P = 0.04) in habitual exercisers (160 ± 9 kJ) than in sedentary adults (130 ± 11 kJ) and was positively related to β-AR thermogenic responsiveness ( r = 0.32, P = 0.02). We conclude that TEF is related to β-AR thermogenic responsiveness and that the greater TEF in habitual exercisers is attributable in part to their augmented β-AR thermogenic responsiveness. Our results also suggest that peripheral thermogenic responsiveness to β-AR stimulation is a physiological determinant of TEF and hence energy balance in healthy adult humans.