Serum Bioavailable Vitamin D Concentrations and Bone Mineral Density in Women After Obesity Surgery

Introduction Low bone mass after obesity surgery may arise as a consequence of chronic malabsorption of calcium and vitamin D. However, we have not found any role of serum 25-hydroxyvitamin D or of polymorphisms in the vitamin D receptor gene in previous studies. Purpose To investigate the circulating bioavailable 25-hydroxyvitamin D in women after bariatric procedures and its association with bone mass. Patients and Methods The study consisted of 91 women on follow-up for 7 ± 2 years after bariatric surgery. We measured bone mineral density (BMD), serum parathormone (PTH), 25-hydroxyvitamin D, and vitamin D binding protein (VDBP). All patients were genotyped for two variants in the coding region of VDBP (rs4588 and rs7041). Bioavailable 25-hydroxyvitamin D was calculated in double homozygotes. Results We found a negative correlation between bioavailable 25-hydroxyvitamin D and PTH ( r = −0.373, P = 0.018), but not with BMD at lumbar spine ( r = −0.065, P = 0.682) or hip ( r = −0.029, P = 0.857). When adjusting by age, similar results were found for PTH ( r = −0.441, P = 0.005), BMD at lumbar spine (r = −0.026, P = 0.874) and hip ( r = −0.096, P = 0.561). After multivariate linear regression, forcing bioavailable 25-hydroxyvitamin D into the model resulted in a weak significant association with BMD at the lumbar spine ( β = − 0.247, P = 0.025). Conclusions Serum bioavailable 25-hydroxyvitamin D concentrations are not associated with bone mass loss after bariatric surgery in women. The negative association with serum PTH levels suggests that vitamin D supplementation partly improves secondary hyperparathyroidism, yet other mechanisms may contribute to low bone mass after bariatric surgery.