Physiological increments in epinephrine stimulate metabolic rate in humans

Markedly elevated plasma epinephrine is known to increase metabolic rate (MR), but such levels of epinephrine are encoutnered infrequently in normal free-living subjects. We studied whether epinephrine levels common in usual daily activities can affect MR and thus possibly regulate carloric expenditure. To aid definition of a MR threshold, we first measured the hourly and daily variation in MR within individuals by measuring the MR of four individuals by indirect calorimetry for 6 h on six separate occasions without any intervention. We found that hour-to-hour variation (2.0 ± 0.9%) and the day-to-day variation (2.7 ± 0.9%) were low, thus allowing confident detection of small increments in metabolic rate during epinephrine infusion. To define a threshold for epinephrine's effect to increase MR, we studied five normal-weight postabsorptive young men on four separate occasions. During the 1st h of each 5-h study period, saline was infused intravenously. Then, during the subsequent 4 h, subjects received intravenous infusion of saline or epinephrine at 0.1, 0.5, and 1.0 μg/min (randomized). A significant increase in MR (3.6 ± 1.0% SE) was measured with the lowest epinephrine infusion rate (venous plasma concentration, 94 ± 32 pg/ml). The increases in MR correlated (r = 0.85, P < 0.001) with increases in plasma epinephrine. The threshold concentration (upper 95% confidence limit) of epinephrine to affect MR was 90 pg/ml, a concentration frequently occurring in daily life. Thus epinephrine may play an important role in weight maintenance by affecting energy expenditure.