Obesity is an ever-increasing disease in our environment, and a major risk factor for the development of other chronic diseases that increase morbidity and mortality. Bariatric surgery is an effective treatment for obesity, since it not only allows a significant and sustained loss of weight, but also an important reduction of its co-morbidities. However, this treatment is not free of complications, since along with weight loss, frequent nutritional and metabolic deficiencies have been described. These complications are particularly frequent when mixed surgical procedures with a restrictive and malabsortive component such as Roux-en-Y gastric bypass and biliopancreatic diversion are performed. The nutritional deficiencies observed include, to a greater or lesser extent, malabsorption of fat and liposoluble vitamins, as well as other micronutrients such as calcium. As a result, disorders of bone mineral metabolism with skeletal manifestations that include bone mass reduction, increased bone turnover and defective bone mineralization may develop.