There are several mechanisms at play after sleeve gastrectomy, gastric bypass, and other malabsorptive procedures (such as the duodenal switch) that increase the risk of bone loss in these bariatric patients.
1.Calcium malabsorption in the gut. With the alteration in the intestines, less calcium is able to be absorbed in procedures that involve bypassing the first portion of small intestines (the duodenum). This means there must be increased calcium intake over the recommended daily intake doses for people with normal absorption. Theoretically, those who had the sleeve gastrectomy do not have malabsorption and therefore should absorb calcium normally and regular RDA intake should be sufficient. However, multiple studies comparing bone loss between bypass and sleeve gastrectomy demonstrate that bone loss exists in both groups as compared to controls (non-bariatric patients). This may be due to the other mechanisms for bone loss after bariatric surgery.
2.“Mechanical unloading” occurs after weight loss. When an individual is carrying more weight (as in obesity), their bones generate more new bone cells to strengthen the bones in order to support the increased body weight. When the weight is reduced, more bone turnover occurs, with less new generation due to less perceived need. This results in decreased bone mass.
3.Low vitamin D status. This is very prevalent both pre-surgery and post-surgery and is due to the digestive enzymes in the gut mixing with food further down in the intestines after gastric bypass, which decreases vitamin D absorption. Vitamin D levels must be adequate in order for calcium absorption to occur.
4.Aging and hormonal changes with aging. Decreased estrogen production in women (and men to a lesser degree) affect bone mass negatively. Decreased estrogen levels in the body cause less new bone formation which increases risk for osteopenia and osteoporosis.
It is recommended that bariatric patients consume 1000-1200 mg daily of calcium through a combination of food and supplements. Vitamin D should be included or taken separately to ensure adequate calcium absorption. It is important to note that the body can only absorb approximately 400 mg of calcium at one time, so the doses should be divided and taken at different times of the day. Taking 1000 mg at one time will not be helpful, and can actually be harmful. Follow your doctors’ recommendation for calcium dosing as certain people may require more or less than the recommended post bariatric surgery dosing.
Guidelines recommend bone density testing for those at risk of osteopenia. Including blood levels of parathyroid hormone (PTH) is recommended as this is a much more sensitive indicator of the need for more calcium (or vitamin D) intake than the serum calcium level seen on routine bloodwork. The kidneys regulate blood levels of calcium since it is important for conduction of the heart. Therefore, calcium levels are unlikely to be “low” even when the actual need for increased calcium may be present.