OBJECTIVE: To investigate the effect of surgically induced weight loss on energy, substrate and protein metabolism of morbidly obese patients. DESIGN: A prospective, clinical intervention study of morbidly obese patients before and after surgical treatment. SUBJECTS: Eight morbidly obese patients (BMI 47.88 +/- 7.03). METHODS: Total energy expenditure (TEE; doubly labeled water method), sleeping metabolic: rate (SMR; respiration chamber), body composition (deuterium oxide component of doubly labeled water), substrate metabolism (48 h dietary records, 48 h urine collection and gaseous exchange in the respiration chamber) and whole body protein turnover (primed-continuous infusion of L-[1-C-13]-leucine) were measured before, 3 and 12 months after vertical banded gastroplasty (VBG). RESULTS: The TEE decreased as a result of a decreased SMR (64%) and non-SMR (36%; P = 0.0011, SMR as a function of fat-free mass (FFM) decreased after weight loss (P <0.05). The physical activity index (PAI), defined as TEE/SMR, was low and was not influenced by weight loss. Protein and carbohydrate oxidation decreased significantly after VBG (P <0.05), although 3 months after VBG protein oxidation did not decrease enough to prevent loss of FFM. The energy used for protein turnover was approximately 24% of SMR and did not change after weight loss. CONCLUSIONS: Compensatory processes that oppose weight loss of morbidly obese patients exist, as demonstrated by the disproportional reduction of SMR, and a low PAI, Protein turnover is not a major contributor to the disproportional reduction of SMR.