Gastric bypass procedures (GBP) are the mainstay of treatment for morbid obesity. The weight loss and improvement in comorbidities have been well studied, but long term side effects of the operation are not known. In particular little data exists on what happens to bone mineral density (BMD) once patients have undergone gastric bypass procedures, excluding the duodenum from its role in calcium absorption.

Prospective data on BMD, calcium, parathyroid hormone (PTH) and vitamin D was collected preoperatively and at yearly intervals. We reviewed our data to determine the effects of gastric bypass on BMD and calcium metabolism.

230 Patients had preoperative BMD scans performed. Most patients had normal BMD pre-op and remained normal. Preoperatively 15 patients were osteopenic while 3 patients subsequently developed osteopenia within the first year postop. One patient who had osteopenia preoperatively had an increase in BMD and at one year post-op was no longer osteopenic. No patient had or developed osteoporosis. At one year total forearm (TF) BMD decreased by 0.55% (n=91;p=.03) and radius BMD (RB) had increased overall by 1.85% (n=23;p=.008) while both total hip (TH) and lumbar spine (LS) decreased by 9.27% (n=22;p<.001) and 4.53% (n=31;p<.001) respectively. By the second year post-op BMD in the TF decreased by 3.62% (n=14;p<.001) whereas RB remained unchanged. Although TH and LS had a significant decrease at one year, by year 2 both TH and LS had only slight decrease in BMD that were not significantly different from preop. By post-op years three and four the trend was toward an increase in BMD. Mean serum calcium decreased from 9.8mg/dL to 9.2 during the first year post-op(ns) and further decreased to 8.8(ns) by year 2. Mean PTH increased from 59.7pg/mL(nl 10-65 pg/mL) pre-op to 63.1 at year 1 (ns) and continued to increase to 64.7 by year 2(ns). No difference was noted between mean 25-hydroxy Vitamin D levels preoperatively(25.2nmol/L), at 1 year(34.4), and at two years(35.4).

Our data strongly suggests that bone loss is highest the first year after GBP with a stabilization and in some cases an increase in bone density after the first year. The clinical significance of such an early decrease in BMD remains to be elucidated. As such, prospective trials are needed to determine the long-term amount of bone mineral loss and ultimately the clinical significance of such BMD decreases.