Hollstein T, et al. - Since previous studies proposed that 24-hour energy expenditure (24EE) might also be influenced by sympathetic nervous system activity by catecholamine (norepinephrine, epinephrine) secretion, researchers explored the connection between catecholamines and energy expenditure in 202 individuals from a heterogeneous population of mixed ethnicities. Candidates (n = 202, 33% female, 14% Black, 32% Caucasian, 41% Native Americans, 11% Hispanic, age: 36.9±10.3 years (mean ± SD), percentage body fat: 30.3 ± 9.4) resided in a whole-room calorimeter over 24-h during carefully controlled energy balance conditions to measure 24EE and its components: sleeping metabolic rate (SMR), awake-fed thermogenesis (AFT), and spontaneous physical activity (SPA). Both catecholamines were linked to 24EE and SMR in separate analyses after adjustment for age, sex, ethnicity, fat mass, fat-free mass, calorimeter room and temperature, and physical activity. Neither epinephrine nor norepinephrine were correlated with adjusted AFT but epinephrine was related to adjusted SPA. The data present compelling proof that sympathetic nervous system activity, mediated by norepinephrine, is a determinant of human energy expenditure during unstressed, eucaloric conditions.