Respiratory Quotient Is Inversely Associated with Muscle Sympathetic Nerve Activity

The relative amounts of the macronutrients oxidized by an individual are reflected in the respiratory quotient (RQ), which varies inversely with lipid oxidation. A high RQ, indicating a relatively low lipid oxidation, 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 oxidation may contribute to the relationship between low sympathetic nervous 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 independently associated with low lipid oxidation. Intraneural recordings of basal 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) immediately after measurement of 24-h RQ in a respiratory chamb...