Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique because it can be ingested as cholecalciferol (vitamin D3) or ergocalciferol (vitamin D2) and because the body can also synthesize it (from cholesterol) when sun exposure is adequate. 
Vitamin D has several important functions, it:
- helps regulate the amount of calcium and phosphate in the body. These nutrients are needed to keep bones and teeth healthy.
A lack of vitamin D can lead to bone deformities such as rickets in children and bone pain and tenderness as a result of osteomalacia in adults.