['To measure changes in resting metabolic rate (RMR) and body composition in obese subjects following massive weight loss achieved via bariatric surgery or calorie restriction plus vigorous exercise.', 'Body composition and RMR were measured in 13 pairs of obese subjects retrospectively matched for sex, body mass index, weight, and age who underwent either Roux-en-Y gastric bypass surgery (RYGB) or participated in "The Biggest Loser" weight loss competition (BLC).', 'Both groups had similar final weight loss (RYGB: 40.2\u2009±\u200912.7 kg, BLC: 48.8\u2009±\u200914.9 kg; P\u2009=\u20090.14); however, RYGB lost a larger proportion of their weight as fat-free mass (FFM) (RYGB: 30\u2009±\u200912%, BLC: 16\u2009±\u20098% [P\u2009<\u20090.01]). In both groups, RMR decreased significantly more than expected based on measured body composition changes. The magnitude of this metabolic adaptation was correlated with the degree of energy imbalance (r\u2009=\u20090.55, P\u2009=\u20090.004) and the decrease in circulating leptin (r\u2009=\u20090.47, P\u2009=\u20090.02).'] Is metabolic adaptation following massive weight loss related to the degree of energy imbalance and changes in circulating leptin?