The very popular Vitamin D is more akin to a hormone rather than a true vitamin. The active form is known as calcitrol and it is stable despite heat and oxidation. Its primary role is the regulation of calcium absorption and stimulation of bone mineralization.
There are many tissues in the body that benefit from this vitamin. Recently, Vitamin D has been shown to be essential in immune function. It also appears to play a role in keeping the brain healthy later in life, it reduces respiratory infections, aids in insulin secretion and helps alleviate symptoms of depression.
Deficiency of Vitamin D often appears as rickets in children and osteomalacia in adults. These conditions cause muscle weakness and achy bone pain. In extreme cases, bones appear deformed and porous.
Vitamin D can be produced by the human body given sufficient sunlight exposure. Thus, the best source is sunlight. Ultraviolet rays stimulate endogenous production of Vitamin D. Just ten minutes a day provide approximately 400 IU of cholecalciferol. Prolonged exposure is not needed, and it is important to remember that there is always a risk of skin cancer from excessive sunbathing.
Best dietary sources are fatty fish such as sardines and salmon, as well as liver and eggs. Vitamin D in these foods is primarily found as vitamin D3. Vegetables are generally low in Vitamin D with the exception of mushrooms, which provide vitamin D2. Currently, fortified foods provide most of the vitamin D in the American diet.
Dosage guidelines for adequate supplementation released by the Institute of Medicine (IOM) in 2010, indicate a RDA of vitamin D of 600 international units (IU) for children and adults alike. The RDA for adults older than 70 is 800 IU to optimize bone health.
The upper limit intake of Vitamin D is currently set at 4000 IU for healthy adults. A slight risk for toxicity appears with higher daily doses. Toxicity can manifest itself as recurrent urinary tract infections, decreased appetite, weight loss and gastrointestinal complaints. Endogenous Vitamin D synthesis from sunlight does not produce any kind of toxicity.