Currently, the two most common types of weight loss surgery performed include sleeve gastrectomy and Roux-en-Y gastric bypass (RYGB). Sleeve gastrectomy involves removing a large portion of the stomach so that its capacity is significantly decreased (to about 20%), reducing the ability to consume large quantities of food. Also, the procedure leads to marked reductions in ghrelin (an appetite-stimulating hormone), and some studies have reported increases in glucagon-like peptide 1 (GLP-1) and peptide YY (PYY), hormones that induce satiety. Gastric bypass involves creating a small stomach pouch and rerouting the small intestine so that it bypasses much of the stomach and also the upper portion of the small intestine. This reduces the amount of food that can be consumed at any time, increases levels of GLP-1 and PYY, and reduces absorption of nutrients with resultant weight loss. Less common bariatric surgeries include gastric banding and biliopancreatic diversion with duodenal switch (BPD-DS). Gastric banding involves placing a ring in the upper portion of the stomach, and the size of the pouch created can be altered by injecting more or less saline through a port inserted under the skin. BPD-DS includes sleeve gastrectomy, resection of a large section of the small intestine, and diversion of the pancreatic and biliary duct to a point below the junction of the ends of the resected gut.
Weight loss surgery is currently recommended for people who have a body mass index greater than or equal to 35 regardless of obesity-related complication and may be considered for those with a BMI greater than or equal to 30. BMI is calculated by dividing the weight (in kilograms) by the height (in meters). In children and adolescents, weight loss surgery should be considered in those with a BMI greater than 120% of the 95th percentile and with a major comorbidity or in those with a BMI greater than 140% of the 95th percentile.
What impact does weight loss surgery have on bone?
Multiple studies in both adults and teenagers have demonstrated that sleeve gastrectomy, RYGB, and BPD-DS (but not gastric banding) are associated with a decrease in bone density, impaired bone structure, and reduced strength estimates over time (; ; ). The relative risk for fracture after RYGB and BPD-DS is reported to be 1.2-2.3 (that is, 20%-130% more than normal), whereas fracture risk after sleeve gastrectomy is still under study with some conflicting results. Fracture risk starts to increase 2-3 years after surgery and peaks at 5-plus years after surgery. Most of the data for fractures come from studies in adults. With the rising use of weight loss surgery, particularly sleeve gastrectomy, in teenagers, studies are needed to determine fracture risk in this younger age group, who also seem to experience marked reductions in bone density, altered bone structure, and reduced bone strength after bariatric surgery.
What contributes to impaired bone health after weight loss surgery?
The deleterious effect of weight loss surgery on bone appears to be caused by various factors, including the massive and rapid weight loss that occurs after surgery, because body weight has a mechanical loading effect on bone and otherwise promotes bone formation. Weight loss results in mechanical unloading and thus a decrease in bone density. Further, when weight loss occurs, there is loss of both muscle and fat mass, and the reduction in muscle mass is deleterious to bone.
Other possible causes of bone density reduction include reduced absorption of certain nutrients, such as calcium and vitamin D critical for bone mineralization, and alterations in certain hormones that impact bone health. These include increases in parathyroid hormone, which increases bone loss when secreted in excess; increases in PYY (a hormone that reduces bone formation); decreases in ghrelin (a hormone that typically increases bone formation), particularly after sleeve gastrectomy; and decreases in estrone (a kind of estrogen that like other estrogens prevents bone loss). Further, age and gender may modify the bone consequences of surgery as outcomes in postmenopausal women appear to be worse than in younger women and men.
Preventing bone density loss
Given the many benefits of weight loss surgery, what can we do to prevent this decrease in bone density after surgery? It’s important for people undergoing weight loss surgery to be cognizant of this potentially negative outcome and to take appropriate precautions to mitigate this concern.
We should monitor bone density after surgery with the help of dual energy x-ray absorptiometry, starting a few years after surgery, particularly in those who are at greatest risk for fracture, so that we can be proactive about addressing any severe bone loss that warrants pharmacologic intervention.
More general recommendations include optimizing intake of calcium (1,200-1,500 mg/d), vitamin D (2,000-3,000 IUs/d), and protein (60-75 g/d) via diet and/or as supplements and engaging in weight-bearing physical activity because this exerts mechanical loading effects on the skeleton leading to increased bone formation and also increases muscle mass over time, which is beneficial to bone. A progressive resistance training program has been demonstrated to have beneficial effects on bone, and measures should be taken to reduce the risk for falls, which increases after certain kinds of weight loss surgery, such as gastric bypass.
Meeting with a dietitian can help determine any other nutrients that need to be optimized.
Though many hormonal changes after surgery have been linked to reductions in bone density, there are still no recommended hormonal therapies at this time, and more work is required to determine whether specific pharmacologic therapies might help improve bone outcomes after surgery.
Dr. Misra is chief of the division of pediatric endocrinology, Mass General for Children; associate director, Harvard Catalyst Translation and Clinical Research Center; director, Pediatric Endocrine-Sports Endocrine-Neuroendocrine Lab, Mass General Hospital; and professor, department of pediatrics, Harvard Medical School, Boston.
A version of this article originally appeared on.