SAT-518 Long-term Impact of Gastric Bypass on Bone Density >10 years after Bariatric Surgery: Exceeding Physiologic Expectations

Introduction: In the initial years after bariatric surgery, Roux-en-Y gastric bypass (RYGB) leads to high-turnover bone loss and an increased risk of fractures. The long-term effects of RYGB on bone density and microarchitecture have not been defined beyond 5 years. Furthermore, there remains an open question of whether bone loss exceeds physiologic expectations for surgical weight loss. Thus, we evaluated bone mineral density (BMD) and bone microarchitecture in adults who received RYGB at least 10 years ago compared to BMI-matched controls. Methods: In this ongoing study, we recruited 24 adults who received RYGB ≥10 years ago, along with nonsurgical controls who were matched 1:1 by age, sex, race/ethnicity, and current BMI. Total body composition and areal BMD (aBMD) of the lumbar spine and proximal femur were assessed by dual energy x-ray absorptiometry (DXA). Volumetric BMD (vBMD) and bone microarchitecture at the distal radius and tibia were evaluated with high resolution peripheral QCT. Serum calcium, 25-OH vitamin D, and PTH were available in 2/3 of subjects; additional analyses are ongoing. Paired t-tests were used to compare outcomes between groups. Results: By design, the RYGB group was well-matched with controls for BMI (33 ± 6 kg/m2), age (56 ± 10 yr), sex (88% female), and race (65% white). Fat mass and lean mass were similar between RYGB and controls. Mean time since surgery was 13.7 ± 2.2 yrs in the RYGB group, and self-reported weight loss was 45 ± 19 kg. As compared to BMI-matched controls, RYGB patients had 17% lower femoral neck aBMD (p<0.001) and 13% lower total hip aBMD (p<0.001), but lumbar spine aBMD was not significantly different between groups. RYGB patients had lower cortical and trabecular vBMD than BMI-matched controls at both the radius and tibia (p≤0.02 for all), leading to a total vBMD that was 27% lower at the radius (p<0.001) and 20% lower at the tibia (p<0.001). Cortical area and thickness were significantly lower in the RYGB group at the radius and tibia (p<0.05 for all). In addition, the RYGB group had lower trabecular number and thickness, wider trabecular separation, and increased trabecular heterogeneity at both the radius and tibia (p≤0.02 for all). Serum calcium and 25-OH vitamin D levels were similar to BMI-matched controls, although PTH levels tended to be higher in the RYGB group (71 vs 52 pg/mL, p=0.07). Conclusions: Adults who received RYGB at least 10 years ago have femoral and peripheral BMD that is lower than expected given their new weight set point, despite comparable serum calcium and vitamin D levels as BMI-matched controls. Furthermore, both the radius and tibia showed a similar magnitude of deterioration in bone microarchitecture, suggesting that bone loss is driven by a systemic process rather than confined to weight-bearing sites. These data indicate that the effects of RYGB on bone are pathophysiologic, and may be attributed to factors beyond skeletal unloading.