Vitamin D endocrine system. Conceptual image of human skeletons with capsules (orange), pancreatic tissue (pink) and a molecular model (yellow) of a form of vitamin D, with question marks representing the health claims relating to the vitamin D endocrine system. Vitamin D's role in the endocrine system includes aspects of calcium metabolism, the immune system, the pancreas, and fat and muscle tissues. This vitamin has an important role in insulin release, and hence on diseases such as diabetes. The molecule shown here is vitamin D3 (cholecalciferol).