Markers of bone and calcium metabolism following gastric bypass and laparoscopic adjustable gastric banding.

BACKGROUND: Several studies have suggested that morbid obesity is associated with vitamin D deficiency and elevated parathyroid hormone (PTH). Studies have also suggested that there is an increase in vitamin D deficiency, bone resorption, and elevated PTH after gastric bypass surgery. Few studies have evaluated markers of bone and calcium metabolism after laparoscopic adjustable gastric banding or compared these results to those after gastric bypass.

METHODS: Data on all patients undergoing primary gastric bypass (GBP; n = 979) and laparoscopic adjustable gastric banding (LAGB; n = 269) procedures at a tertiary-referral center from June 1996 through March 2005 were reviewed from a prospective database. Only patients with 25OH vitamin D levels available were included in this study (n = 534; GBP = 403, LAGB = 131). All patients were advised to take at least 1,200 mg calcium and 800-1,200 IU of vitamin D daily before and subsequent to their operation. Markers for bone metabolism [25OH Vitamin D, corrected serum calcium, alkaline phosphatase (AP), and PTH] were evaluated preoperatively and 3, 6, 12, and 24 months postoperatively. An analysis of variance and chi-square were performed to determine differences between the operative groups. Linear regression analysis was performed to evaluate the relationship between preoperative body mass index (BMI) and 25OH vitamin D and PTH levels and between percent excess weight loss and 25OH vitamin D and PTH after surgery.

RESULTS: Sixty-four percent of all patients presented with vitamin D deficiency (<20 ng/ml) and 14% presented with elevated PTH preoperatively. Mean 25OH vitamin D levels and AP levels increased significantly after GBP surgery (vitamin D, 17 to 25 ng/ml 12 months post-op; AP, 80 to 90 IU/L 24 months post-op). Corrected calcium levels remained within normal limits and showed no change over time after both procedures. AP levels significantly increased from 76 IU/l preoperatively to 82 IU/l 6 months after LAGB surgery and then decreased to 59 IU/l 24 months after LAGB surgery. Linear regression analysis of preoperative vitamin D, PTH, and BMI values showed a significant positive relationship between initial BMI and PTH (r = 0.29) and a significant negative relationship between vitamin D and initial BMI (r = -0.19). A significant positive linear relationship between vitamin D and percent excess weight loss was evident 12 and 24 months after GBP surgery (r = 0.39 and 0.57, respectively). A negative relationship was evident between PTH and vitamin D 6 months after GBP surgery (r = -0.35) and 12 months after LAGB surgery (r = -0.61).

CONCLUSIONS: These findings suggest that morbid obesity is associated with vitamin D deficiency, and elevated PTH and with adequate supplementation, GBP, and particularly LAGB, patients can improve their bone metabolism abnormalities related to obesity. Furthermore, adequate supplementation for GBP patients may attenuate the increased risk for bone loss associated with malabsorption from the bypass.