Role of sympathetic neural activation in age- and habitual exercise-related differences in the thermic effect of food.

The thermic effect of food (TEF) declines with advancing age in adult humans but is enhanced in the habitually exercising state. The responsiveness of the sympathetic nervous system (SNS) has been implicated in these differences in TEF. We tested the hypotheses that 1) the reduction in TEF with aging is associated with an attenuated SNS response to acute energy intake; and 2) the greater TEF observed in endurance exercise-trained adults is associated with an augmented SNS response. Four groups of healthy men were studied: 16 young and 11 older sedentary men and nine young and 10 older habitually exercising men. Metabolic rate (indirect calorimetry, ventilated hood), skeletal muscle sympathetic nerve activity (MSNA; peroneal microneurography), and plasma norepinephrine and plasma epinephrine concentrations were measured before and for up to 4 h after ingestion of a carbohydrate drink (2.5 g/kg fat-free mass). TEF was approximately 50% greater in young compared with older men (P < 0.05) and approximately 25% greater in exercising compared with sedentary men (P < 0.05). In contrast, the MSNA, plasma norepinephrine, and plasma epinephrine responses were not different among the four groups. Covarying for MSNA did not significantly alter the observed differences in TEF. Habitual exercise status did not affect the age-associated decline in TEF. These findings demonstrate that altered postprandial whole-body and skeletal muscle SNS activation is not an important mechanism mediating either the reduction in TEF with aging or the augmented TEF associated with the exercise-trained state in healthy men. Differences in beta-adrenergic responsiveness to postprandial sympathoadrenal stimulation and/or nonsympathetic adrenergic influences likely explain the age- and habitual exercise-related differences in TEF.