Gastric bypass surgery is indicated for the surgical treatment of morbid obesity by making the stomach smaller and allowing food to bypass proximal part of the small intestine, where many minerals and vitamins are most absorbed. Food (and calorie) intake is dramatically reduced because of the new small stomach. Many patients who have bypass surgery develop nutritional deficiencies because they do not adhere to their post-operative guidelines concerning the need for nutritional supplements. Malabsorptive operations like gastric bypass surgery pose a particular risk for multiple nutritional deficiencies, including vitamin D malabsorption. (3) Vitamin D is ingested in the diet, from supplements, as well as synthesized naturally in the skin following UVB irradiation from the sun. As sunlight enters the atmosphere, all radiation below 290 nm is absorbed by the stratospheric ozone layer. Radiation between 290 and 315 nm is responsible for converting 7-dihydrocholesterol (provitamin D3) in the epidermis and dermis to previtamin D3. Once formed, previtamin D3 undergoes a thermally induced isomerization to vitamin D3. Vitamin D3 enters the circulation and is metabolized in the liver to 25-hydroxyvitamin D3 [25(OH)D] and then in the kidney to 1,25-dihydroxyvitamin D3 [1,25(OH)2D] by the 25-hydroxyvitamin D3-1á-hydroxylase [1á-OHase]. 25(OH)D is the major circulating form of vitamin D and is routinely measured in the blood to determine the vitamin D status of a patient.