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 beta -adrenergic receptor stimulation. In the present follow-up study we determ ined the respective effects of age, habitual exercise status, and sex on th is regulatory mechanism. Resting metabolic rate (ventilated hood, indirect calorimetry) was determined in 55 healthy sedentary or endurance exercise-t rained adults, aged 18-35 or 60-75 yr (29 men and 26 women), before (baseli ne) and during the infusion of either a nonselective beta -adrenergic recep tor antagonist (propranolol) or saline (control). Relative to baseline valu es, during beta -adrenergic receptor antagonism resting metabolic rate adju sted 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 (-1 51 +/- 50) compared with endurance exercise-trained (-268 +/- 46) adults, a nd women (-105 +/- 33) compared with men (-318 +/- 50; all P < 0.01). Reduc tions in resting metabolic rate during <beta>-adrenergic receptor antagonis m were positively related to higher baseline resting metabolic rate and pla sma catecholamine concentrations and negatively related to adiposity (all P < 0.05). Resting metabolic rate was unchanged in response to saline contro l in all groups. These results provide experimental support for the hypothe sis that aging, sedentary living, and female sex are associated with attenu ated sympathetic nervous system support of resting metabolic rate in health y adult humans.