Ethnic differences in sympathetic nervous system-mediated energy expenditure

The impact of sympathetic nervous system (SNS) activity on energy expenditure (EE) was evaluated in nondiabetic Caucasian and Pima Indian men while on a weight-maintenance diet using two approaches as follows. 1) The relationship between 24-h EE, measured in a respiratory chamber, and 24-h urinary norepinephrine was studied in 36 Caucasians [32 ± 8 (SD) yr, 95 ± 41 kg, 22 ± 13% fat] and 33 Pimas (29 ± 6 yr, 103 ± 28 kg, 30 ± 9% fat). There was no difference between the two groups in 24-h EE (2,422 vs. 2,523 kcal/24 h) and in urinary norepinephrine (28 vs. 31 μg/24 h), even after adjusting for body size and composition. Twenty-four-hour EE correlated significantly with 24-h urinary norepinephrine in Caucasians (r = 0.78, P < 0.001) but not in Pimas (r = 0.03), independent of fat-free mass (FFM), fat mass, and age. 2) The effect of β-adrenoceptor blockade with propranolol (120 μg/kg FFM bolus and 1.2 μg·kg FFM^-1·min^-1 for 45 min) on the resting metabolic rate (RMR) was evaluated in 36 Caucasians (30 ± 6 yr, 103 ± 36 kg, 25 ± 11% fat) and 32 Pimas (28 ± 6 yr, 100 ± 34 kg, 27 ± 10% fat). The RMR was similar in the two groups (2,052 vs. 1,973 kcal/24 h) even after adjustment for FFM, fat mass, and age and dropped significantly after propranolol infusion in Caucasians (-3.9%, P < 0.001) but not in Pimas (-0.8%, P = 0.07). The decrease in RMR correlated significantly with the RMR (adjusted for FFM, fat mass, and age) before propranolol infusion in Caucasians (r = -0.57, P < 0.001) but not in Pimas (r = -0.18, P = 0.35). In conclusion, SNS activity is a determinant of EE in Caucasian but not in Pima Indian men.