Changes in body composition and resting energy expenditure during the first year after Roux-en-Y or one-anastomosis gastric bypass

Resting energy expenditure (REE) declines during weight loss because of both organ-tissue mass loss and adaptive thermogenesis, but their relative contributions after bariatric surgery remain unclear. Our primary aim was to quantify these components during the first postoperative year and secondarily to compare Roux-en-Y gastric bypass (RYGB) with one-anastomosis gastric bypass (OAGB). Forty-two adults with obesity (RYGB, <i>n</i> = 21; OAGB, <i>n</i> = 21) underwent dual-energy X-ray absorptiometry (DXA) before surgery and at 6 and 12 mo, and organ-tissue masses were estimated using MRI-derived prediction equations. A smaller subset of 29 participants (RYGB, <i>n</i> = 15; OAGB, <i>n</i> = 14) had valid indirect calorimetry measurements for REE analysis. Participants lost 27.3 kg (21%) of baseline body weight at 6 mo and 32.3 kg (25%) at 12 mo, with no significant differences between procedures in the DXA-measured body composition outcomes or calculated organ-tissue masses. Measured REE decreased by 383 kcal/day at 6 mo (<i>P</i> < 0.001), of which 168 kcal/day [95% confidence interval (CI): 59-276, <i>P</i> = 0.004] was attributed to adaptive thermogenesis. At 12 mo, REE remained 315 kcal/day below baseline (<i>P</i> < 0.001), but adaptive thermogenesis was not statistically detectable (89 kcal/day; 95% CI: -19 to 198, <i>P</i> = 0.11). Adaptive thermogenesis did not significantly differ between procedures. In conclusion, ∼44% of the early 6-mo postoperative decline in REE was associated with adaptive thermogenesis, with the remaining ∼56% associated with organ-tissue mass loss. Adaptive thermogenesis was no longer clearly evident at 12 mo. RYGB and OAGB resulted in similar changes in body composition and REE.<b>NEW & NOTEWORTHY</b> Resting energy expenditure declines after bariatric surgery, but the relative roles of tissue loss and adaptive thermogenesis are unclear. Using DXA-based organ-tissue estimates with indirect calorimetry, we show that adaptive thermogenesis accounted for ∼44% of the 6-mo postoperative REE decline, with the remainder due to tissue loss. Adaptive thermogenesis was not robustly detectable at 12 mo. Roux-en-Y and one-anastomosis gastric bypass show similar body composition and REE changes.