Vitamin D is a fat-soluble vitamin that is important for many bodily functions, including bone health, immune system function, and muscle function. Vitamin D is unique in that the body can produce it in response to sunlight, but it is also found in some foods, including fatty fish and fortified foods such as milk and cereal. A deficiency in vitamin D can lead to health problems such as weakened bones, increased risk of falls, and weakened immune function.