Effects of vertical-banded gastroplasty on bone and mineral metabolism in obese patients

The surgical treatment of obesity can have adverse effects on bone, but there are few published data on the effects of vertical-banded gastroplasty. Serial measurements of bone mineral density at the lumbar spine and three upper femoral sites, using dual-energy X-ray absorptiometry, and also of biochemical indices of bone and mineral metabolism at intervals up to 2 years after operation were performed in 18 patients with morbid obesity who had vertical-banded gastroplasty. Bone mineral density measurements were also made in age- and sex-matched non-obese controls. Bone density before operation was significantly greater in the obese than in the controls (P <0.02 at all sites). The obese patients lost weight rapidly after vertical-banded gastroplasty (mean weight loss 29 kg at 1 year, P <0.001). This was accompanied by a measurable loss of bone density from the trochanter and Ward's triangle sites in the upper femur (P <0.05), but not from the lumbar spine. Bone density values remained stable over 14 months in the controls. Hydroxyproline excretion increased significantly (P <0.005), indicating an increase in bone resorption. Alkaline phosphatase levels decreased significantly (P <0.001), but this probably represents the reversal of hepatic steatosis. There was no evidence of hyperparathyroidism or vitamin D deficiency. In conclusion, vertical-banded gastroplasty causes modest bone density loss from femoral sites, but not the lumbar spine. The difficulties of assessing bone density changes in the obese are discussed.