Tonic Sympathetic Support of Metabolic Rate Is Attenuated with Age, Sedentary Lifestyle, and Female Sex in Healthy Adults

We recently demonstrated in young adult humans that the sympathetic nervous system contributes to the control of resting metabolic rate via tonic β-adrenergic receptor stimulation. In the present follow-up study we determined the respective effects of age, habitual exercise status, and sex on this regulatory mechanism. Resting metabolic rate (ventilated hood, indirect calorimetry) was determined in 55 healthy sedentary or endurance exercise-trained adults, aged 18–35 or 60–75 yr (29 men and 26 women), before (baseline) and during the infusion of either a nonselective β-adrenergic receptor antagonist (propranolol) or saline (control). Relative to baseline values, during β-adrenergic receptor antagonism resting metabolic rate adjusted for fat-free mass was reduced to a lesser extent in older (mean ± se,− 130 ± 46 kJ/d) compared with young (−297 ± 46) adults, sedentary (−151 ± 50) compared with endurance exercise-trained (−268 ± 46) adults, and women (−105 ± 33) compared with men (−318 ± 50; all P < 0.01). Reductions in resting metabolic rate during β-adrenergic receptor antagonism were positively related to higher baseline resting metabolic rate and plasma catecholamine concentrations and negatively related to adiposity (all P < 0.05). Resting metabolic rate was unchanged in response to saline control in all groups. These results provide experimental support for the hypothesis that aging, sedentary living, and female sex are associated with attenuated sympathetic nervous system support of resting metabolic rate in healthy adult humans.