Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Although vitamin D is commonly called a vitamin, it is not in the sense an essential dietary vitamin as it can be synthesized in adequate amounts by all mammals from sunlight, though it otherwise fits the description of a vitamin being an organic compound required as a vital nutrient in tiny amounts by an organism. Learn more about vitamin D, its chemical properties and metabolic functions in this book.