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

Participants lost substantial body weight at both 6 and 12 months postoperatively, with no significant differences between surgical procedures.

• Participants lost 27.3 kg (21%) of baseline body weight at 6 months and 32.3 kg (25%) at 12 months.
• No significant differences were found between RYGB and OAGB in DXA-measured body composition outcomes or calculated organ-tissue masses.
• Study included 42 adults with obesity (RYGB, n=21; OAGB, n=21) who underwent DXA before surgery and at 6 and 12 months.
• Organ-tissue masses were estimated using MRI-derived prediction equations applied to DXA data.

Measured resting energy expenditure decreased significantly at 6 months postoperatively, with approximately 44% of the decline attributed to adaptive thermogenesis.

• Measured REE decreased by 383 kcal/day at 6 months (P < 0.001).
• Of this decline, 168 kcal/day was attributed to adaptive thermogenesis (95% CI: 59–276, P = 0.004).
• The remaining ~56% of the REE decline was associated with organ-tissue mass loss.
• REE analysis was conducted in a subset of 29 participants (RYGB, n=15; OAGB, n=14) with valid indirect calorimetry measurements.

At 12 months postoperatively, REE remained below baseline but adaptive thermogenesis was no longer statistically detectable.

• REE remained 315 kcal/day below baseline at 12 months (P < 0.001).
• Adaptive thermogenesis at 12 months was estimated at 89 kcal/day (95% CI: -19 to 198, P = 0.11), which was not statistically significant.
• The transition from statistically significant adaptive thermogenesis at 6 months to non-detectable at 12 months suggests attenuation of this metabolic response over time.

Adaptive thermogenesis did not differ significantly between the Roux-en-Y gastric bypass and one-anastomosis gastric bypass procedures.

• No statistically significant differences in adaptive thermogenesis were found between RYGB and OAGB at either time point.
• RYGB and OAGB also resulted in similar changes in body composition across the study period.
• Both procedures showed similar DXA-measured body composition outcomes and calculated organ-tissue masses.

The study used a combination of DXA-based organ-tissue mass estimates and indirect calorimetry to partition the REE decline into its mechanistic components.

• Dual-energy X-ray absorptiometry (DXA) was performed before surgery and at 6 and 12 months postoperatively.
• Organ-tissue masses were estimated using MRI-derived prediction equations applied to DXA measurements.
• Indirect calorimetry was used to measure REE in a subset of 29 participants.
• Adaptive thermogenesis was quantified as the difference between measured REE and predicted REE based on organ-tissue mass changes.