Introduction: Bariatric surgery (BS) rates are rising as a consequence of the increase in obesity and its associated diseases. Weight loss following BS positively impacts metabolic health, however, fracture rates following BS are increased. A possible mediator of the increased fracture risk following BS could be bone marrow adipose tissue (BMAT), since high BMAT is also associated with the increased fracture risk in anorexia nervosa, a disease characterized by extreme weight loss. However, the effect of BS on BMAT reported in the literature has been inconsistent, possibly due to the heterogeneity of the patient populations. Therefore, the aim of this study is to determine the effect of BS induced weight loss on BMAT and bone mineral density in postmenopausal women.

Methods: We included 14 postmenopausal, non-diabetic obese women, scheduled for Roux-en-Y gastric bypass surgery (RYGB). Exclusion criteria were metabolic bone disease, bone marrow disease and diabetes mellitus. Outcome parameters were bone marrow fat signal fraction (BMAT), measured by quantitative chemical shift imaging of the lumbar vertebra (L3-5) and volumetric bone mineral density (vBMD) of L3-4, measured by quantitative CT. Two baseline measurements were scheduled before surgery and two follow up measurements were scheduled at 3 months and 12 months post-surgery. Data were analyzed by linear mixed model.

Results: BMAT was negatively associated with vBMD at baseline (R² = 0.41 p = 0.005). Body weight decreased after surgery from 106±15 [baseline] to 91±13 [3 months] and 73±8 kg [12 months] (p<0.001). BMAT decreased after surgery from 52% [baseline] to 50% [3 months, p=0.053] and 47% [12 months, p<0.001]. vBMD decreased after surgery from 104±27 [baseline] to 95±21 [3 months, p=0.006] and 95±25 mg/cm³ [12 months, p=0.178]. Calcium, PTH and vitamin D did not change after surgery.

Conclusion: We show a decrease in BMAT 12 months after RYGB and a decrease in vBMD 3 months after RYGB. Our results are inconsistent with