To examine the factors associated with weight loss following ileogastrostomy, (i) energy expenditure, (ii) body composition and (iii) urinary and fecal energy losses were examined in 11 morbidly obese subjects (36 +/- 3 y, 43.7 +/- 0.9 kg/m2 BMI, 47.0 +/- 1.3% body fat (BF) (mean +/- SEM) prior to and after this surgical procedure. Respiratory gas exchange analysis was used to measure changes in basal energy expenditure (BEE) and thermic effect of food (TEF). Total energy expenditure (TEE) was determined by doubly labeled water (DLW) method over 14 d during 6-8 weeks after surgery. Body composition was assessed using isotope dilution space (IDS) method. Illegastrostomy did not significantly influence BEE levels but TEF declined following surgery (p = 0.001). There was a significant decline (p < 0.0001) in fat-free mass (FFM) and fat mass (FM). Weight loss was closely associated with TEE (r = 0.719, p = 0.019, n = 10), and fecal energy content (r = 0.808, p = 0.052, n = 6) but not with urinary energy loss (r = 0.011, p = 0.983, n = 6). The results indicate that ileogastrostomy blunts TEF levels and both FFM and FM declined following surgery. Total energy expenditure and fecal energy losses appear to be important determinants in the extent of weight loss following ileogastrostomy.