RESPIRATORY QUOTIENT IS INVERSELY ASSOCIATED WITH MUSCLE SYMPATHETIC-NERVE ACTIVITY

The relative amounts of the macronutrients oxidized by an individual a re reflected in the respiratory quotient (RQ), which varies inversely with lipid oxidation. A high RQ, indicating a relatively low lipid oxi dation, and a low activity of the sympathetic nervous system have both been identified as risk factors for body weight gain. The stimulatory effect of norepinephrine on lipid oxidation suggests that low lipid o xidation may contribute to the relationship between low sympathetic ne rvous activity and body weight gain. The purpose of the present study was to determine whether low basal muscle sympathetic nerve activity ( MSNA), a direct measure of sympathetic nervous outflow, is independent ly associated with low lipid oxidation. Intraneural recordings of basa l MSNA were performed in 39 healthy, nondiabetic males, 19 Caucasians (mean +/-SD, 33 +/- 9 yr,91 +/- 23 kg, and 28 +/- 11% body fat) and 20 Pima Indians (30 +/- 5 yr, 94 +/- 25 kg, and 35 +/- 8% fat) immediate ly after measurement of 24-h RQ in a respiratory chamber. Basal MSNA, energy balance, and age were independent determinants of 24-h RQ, toge ther explaining 45% of its variability. Accordingly, 24-h RQ adjusted for energy balance and age was inversely related to MSNA (r = -0.41; P = 0.01). Race, percent body fat, and fasting plasma insulin were not independent determinants of 24-h RQ. Although MSNA explained only a li mited part of the variability in 24-h RQ, the results support the hypo thesis that an effect on lipid oxidation contributes to the demonstrat ed relationship between low activity of the sympathetic nervous system and body weight gain.