Although vitamin D is known to be stored in fat tissue, researchers are not sure about the role that fat tissue plays in vitamin D metabolism. This study will help develop the methodology necessary to further investigate the role of fat tissue in vitamin D metabolism and will assess the relationship between levels of vitamin D in fat tissue and in blood. 50 subjects who are referred for gastric bypass surgery will be enrolled in this study. Subjects will complete questionnaires about their medical history, travel history and food intake. Prior to or during surgery, subjects will be asked to provide a blood sample for selected chemistries related to vitamin D metabolism. During gastric bypass surgery, the surgeon will collect small pieces of fat tissue from the fat under the skin and within the abdomen by surgical biopsy. These samples will be used to refine the methodology for determining the levels of vitamin D in blood and fat tissue and for comparing levels of vitamin D in various tissues.