Vitamin D is a prohormone—something the body makes and converts to a hormone—that has a role in several important bodily processes. Known as the “sunshine vitamin,” vitamin D is made by the body as a response to sun exposure. When sunlight hits the skin, the body produces a substance that the liver and kidneys convert to forms usable by various organs and systems. Vitamin D is found in a variety of foods, including eggs and milk. But much of the world’s population is deficient in vitamin D—up to 1 billion people worldwide, and 40% of Americans.