A low metabolic rate for a given body size and a low fat versus carbohydrate oxidation ratio are known risk factors for body weight gain, but the underlying biological mechanisms are poorly understood. Twenty-four-hour energy expenditure (24EE), sleeping metabolic rate (SMR), 24-hour respiratory quotient (24RQ), and forearm oxygen uptake were compared with respect to the proportion of skeletal muscle fiber types and the enzyme activities of the vastus lateralis in 14 subjects (seven men and seven women aged 30 +/- 6 years [mean +/- SD], 79.1 +/- 17.3 kg, 22% +/- 7% body fat). The following enzymes were chosen to represent the major energy-generating pathways: lactate dehydrogenase (LDH) and phosphofructokinase (PFK) for glycolysis; citrate synthase (CS) and beta-hydroxyacl-coenzyme A dehydrogenase (beta-OAC) for oxidation; and creatine kinase (CK) and adenylokinase (AK) for high-energy phosphate metabolism. Forearm resting oxygen uptake adjusted for muscle size correlated positively with the proportion of fast-twitch muscle fibers (IIa: r = .55, P = .04; IIb: r = .51, P = .06) and inversely with the proportion of slow oxidative fibers (I: r = -.77, P = .001). 24EE and SMR adjusted for differences in fat-free mass, fat mass, sex, and age correlated with PFK activity (r = .56, P = .04 and r = .69, P = .007, respectively). 24RQ correlated negatively with beta-OAC activity (r = -.75, P = .002). Our findings suggest that differences in muscle biochemistry account for part of the interindividual variability in muscle oxygen uptake and whole-body energy metabolism, ie, metabolic rate and substrate oxidation.