Vitamin D is fat-soluble in the body. It is found naturally only in fish and fish oils, and is added to foods such as milk. It is the only vitamin that can be made in the body through sunlight action. Some medical sources deem vitamin D to be a hormone rather than a vitamin, as it is produced by all vertebrate species exposed to sunlight.

Vitamin D has a role in the proper absorption of calcium and phosphorous, is essential in normal cell growth, and is needed for a healthy nervous system as calcium is vital for normal nerve impulses and muscle contractions.

Exposure to sunlight is one of the main sources of vitamin D. Ultraviolet radiation is what stimulates the body to create vitamin D. Babies under 12 months of age have been shown to have vitamin D stored in their bodies from when they were in the womb. People living in the higher latitudes (areas more than 52 degrees) are not exposed to enough UV radiation in the winter months for sufficient vitamin D production, and so fortified foods are necessary. Anything that blocks UV rays (such as sunscreen or smog) can block the body's production of vitamin D.

A deficiency in vitamin D can cause rickets in children and osteomalacia in adults.