FILOZOF, CLAUDIA M., CARLOS MURÚ A, MARTA P. SANCHEZ, CARLOS BRAILOVSKY, MARIO PERMAN, CLAUDIO D. GONZALEZ, AND ERIC RAVUSSIN. Low plasma leptin concentration and low rates of fat oxidation in weight-stable post-obese subjects. Obes Res. 2000;8:205-210. Objective: A low resting metabolic rate for a given body size and composition, a low rate of fat oxidation, low levels of physical activity, and low plasma leptin concentrations are all risk factors for body weight gain. The aim of the present investigation was to compare resting metabolic rate (RMR), respiratory quotient (RQ), levels of physical activity, and plasma leptin concentrations in eight post-obese adults (2 males and 6 females; 48.9 Ϯ 12.2 years; body mass index [BMI]: 24.5 Ϯ 1.0 kg/m 2 ; body fat 33 Ϯ 5%; mean Ϯ SD) who lost 27.1 Ϯ 21.3 kg (16 to 79 kg) and had maintained this weight loss for Ն2 months (2 to 9 months) to eight age-and BMI-matched control never-obese subjects (1 male and 7 females; 49.1 Ϯ 5.2 years; BMI 24.4 Ϯ 1.0 kg/m 2 ; body fat 33 Ϯ 7%). Research Methods and Procedures: Following 3 days of weight maintenance diet (50% carbohydrate and 30% fat), RMR and RQ were measured after a 10-hour fast using indirect calorimetry and plasma leptin concentrations were measured using radioimmunoassay. Levels of physical activity were estimated using an accelerometer over a 48-hour period in free living conditions. Results:After adjustment for fat mass and fat-free mass, post-obese subjects had, compared with controls, similar levels of physical activity (4185 Ϯ 205 vs. 4295 Ϯ 204 counts) and similar RMR (1383 Ϯ 268 vs. 1430 Ϯ 104 kcal/day) but higher RQ (0.86 Ϯ 0.04 vs. 0.81 Ϯ 0.03, p Ͻ 0.05). Leptin concentration correlated positively with percent body fat (r ϭ 0.57, p Ͻ 0.05) and, after adjusting for fat mass and fat-free mass, was lower in post-obese than in control subjects (4.5 Ϯ 2.1 vs. 11.6 Ϯ 7.9 ng/mL, p Ͻ 0.05). Discussion: The low fat oxidation and low plasma leptin concentrations observed in post-obese individuals may, in part, explain their propensity to relapse.