Bone mass decreases in morbidly obese women after long limb-biliopancreatic diversion and marked weight loss without secondary hyperparathyroidism. A physiological adaptation to weight loss?

OBJECTIVE

To investigate the effect of marked weight loss after long limb-biliopancreatic diversion (BPD-LL) on bone mass and serum calcium, 25-OH-vitamin D, and PTH levels in relation to calcium supplementation.

BACKGROUND DATA

BPD is the most effective type of bariatric surgery, but it is followed by bone mass loss, mainly attributed to calcium and vitamin D malabsorption leading to secondary hyperparathyroidism.

METHODS

Two groups, each consisting of 26 healthy, morbidly obese, premenopausal women, underwent BPD-LL. Both groups received high-calcium diets, 200 IU of vitamin D and 100 mg elemental calcium daily and group B, in addition to an extra 2 g of calcium. Bone density and biochemical markers were measured before and 12 months after BPD-LL.

RESULTS

One year after surgery, both groups had lost an average of 55 kg of body weight; albumin-corrected calcium concentration did not change. 25-OH-vitamin D levels in group A were 17.34 +/- 8.90 pre- and 20.51 +/- 14.71 microg/L postoperatively (p = 0.058), and in group B, 15.70 +/- 9.46 and 13.52 +/- 8.16 microg/L (p = 0.489), respectively. PTH levels in group A were 38.5 +/- 12.2 before and 51.2 +/- 32.8 pg/ml after surgery (p = 0.08), and in group B, 48.1 +/- 26.3 and 52.9 +/- 29.2 pg/ml (p = 0.147), respectively. Bone formation markers (alkaline phosphatase, osteocalcin and procollagen type 1), as well as the bone resorption marker CTx, increased significantly in both groups. Bone mineral density T score was 0.862 +/- 0.988 in group A and 0.851 +/- 1.44 in group B and declined postoperatively to -0.123 +/- 1.082 and 0.181 +/- 1.285, respectively.

CONCLUSIONS

Marked weight loss after BPD-LL leads to increased bone turnover and normalization of the increased bone mass without calcium or vitamin D malabsorption and without the appearance of secondary hyperparathyroidism. We conclude that the bone mass reduction is a normal adaptation to the decreased loading of the bone following weight loss.