Vitamin D is actually a fat-soluble prohormone, and there are several forms of vitamin D. The two primary forms are vitamin D2 (ergocalciferol) and vitamind3   (cholecalciferol). Vitamin D2 is made by plants. Vitamin D3 is made by humans when skin is exposed to ultraviolet B (UVB) rays from sunlight.

It's well known that vitamin D helps the body absorb calcium and maintain proper blood levels of calcium and phosphorus, thereby helping bones and teeth stay strong and healthy. But recent research indicates vitamin D does much more than that.

There's a growing body of evidence that vitamin D may be helpful in the prevention and treatment of certain cancers, heart disease,high blood pressure, diabetes, fibromyalgia, multiple sclerosis, rheumatoid arthritis, chronic pain and obesity. It also looks like vitamin D helps strengthen the immune system and general mood swings.