[How to identify the risk for osteoporosis after bariatric surgery].

Gastric bypass often reduces the protein and calcium consumption and causes deficiencies in vitamin D with secondary hyperparathyroidism. As a consequence, it is a risk for osteoporosis. It is therefore necessary to regularly evaluate protein consumption and the levels of vitamin D and PTH. In case of deficiency, supplements must be given in order to obtain PTH level < 50 ng/l. The interpretation of the results of the bone densitometry is difficult during rapid weight loss due to the presence of technical artifacts related to the reduction of the subcutaneous fat thickness, while from the second year postoperative, densitometry remains the gold standard of investigation to follow the evolution of the bone state after gastric bypass.