REDUCED SYMPATHETIC NERVOUS ACTIVITY - A POTENTIAL MECHANISM PREDISPOSING TO BODY-WEIGHT GAIN

The sympathetic nervous system is recognized to play a role in the eti ology of animal and possibly human obesity through its impact on energ y expenditure and / or food intake. We, therefore, measured fasting mu scle sympathetic nerve activity (MSNA) in the peroneal nerve and its r elationship with energy expenditure and body composition in 25 relativ ely lean Pima Indian males (means+/-SD; 26+/-6 yr, 82+/-19 kg, 28+/-10 % body fat) and 19 Caucasian males (29+/-5 yr, 81+/-13 kg, 24+/-9% bod y fat). 24-h energy expenditure, sleeping metabolic rate, and resting metabolic rate were measured in a respiratory chamber, whereas body co mposition was estimated by hydrodensitometry. Pima Indians had lower M SNA than Caucasians (23+/-6 vs 33+/-10 bursts/min, P = 0.0007). MSNA w as significantly related to percent body fat in Caucasians (r = 0.55, P = 0.01) but not in Pimas. MSNA also correlated with energy expenditu re adjusted for fat-free mass, fat mass, and age in Caucasians (r = 0. 51, P = 0.03; r = 0.54, P = 0.02; and r = 0.53, P = 0.02 for adjusted 24-h energy expenditure, sleeping metabolic rate, and resting metaboli c rate, respectively) but not in Pima Indians. In conclusion, the acti vity of the sympathetic nervous system is a determinant of energy expe nditure in Caucasians. Individuals with low resting MSNA may be at ris k for body weight gain resulting from a lower metabolic rate. A low re sting MSNA and the lack of impact of MSNA on metabolic rate might play a role in the etiology of obesity in Pima Indians.