Vitamin D is an essential vitamin required for the proper formation of bones and teeth, and is involved in maintaining proper metabolism of calcium and phosphorus in the body. It is also well known for its ability to enhance the absorption and utilization of dietary and supplemental calcium.
      The vitamin is made up of two compounds, ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). Both are similar biochemically, though cholecalciferol is absorbed in the body more rapidly and more completely. This is a fat-soluble vitamin, and body stores remain higher and longer than for the water-soluble vitamins.

      The quantification of vitamin D is fairly straightforward. Although this vitamin was formerly measured in International Units (IU), the currently recognized measurement is micrograms (mcg) of cholecalciferol. For food products and supplements still using the IU system of measurement, the conversion is quite simple. 1 mcg = 40 IU of vitamin D (as cholecalciferol). Stated another way, 1 IU = .025 mcg ( 1 / .025 = 40 ).

      The most common source of vitamin D for most individuals is fortified foods, such as milk, cereals and grains, desserts, and even some candies. Other sources include egg yolks (from chickens that are fed vitamin D), the flesh of fatty fish (i.e., herring and mackerel), and some fish liver oils.
Although obviously not a food source, vitamin D is also formed in the skin in normal individuals as a result of exposure to the sun. This is a primary source of the vitamin for many individuals. As to the potential of skin cancer from excessive solar exposure, studies indicate that as little as 10-15 minutes a day, two to three times a week with just face and arms exposed is adequate to provide most individuals with sufficient vitamin D. It should also be noted that the body has a natural defensive system that prevents over production of the vitamin in cases of excessive exposure to sunlight.

      The most common result of vitamin D deficiency in children is stunted bone growth, and a disorder known as rickets (bone malformations), and in adults, osteomalacia, a weakening of bone structure leading to ease of fractures and muscle spasms. Both are relatively unknown in the developed world, as a result of adequate dietary vitamin D sources and normal exposure to sunlight.

      Supplemental vitamin D (or one of its biochemical derivatives) is usually prescribed by a physician, most commonly in pediatric cases for bone disorders resulting from deficiency of the vitamin, and in adults to a lesser degree for similar conditions. Additionally, vitamin D is prescribed in cases where a specific disease process impairs the absorption of dietary vitamin D, such as Crohn's disease, Whipple's disease and sprue.
      Additionally, vitamin D is occasionally prescribed in some cases of hypoparathyroidism (decreased function of the parathyroid gland), primarily because of the vitamin's ability to affect calcium and phosphate metabolism, which in turn affect parathyroid metabolism.
      Although vitamin D as a dietary supplement has been suggested for a variety of unrelated conditions, there is no clear evidence that it has any significant beneficial effect other than to improve an otherwise vitamin D deficient diet. In this regard, it has been reported in a number of studies over the past ten years that some 70% of milk products sampled in the U.S. and Canada (the primary source of the vitamin for most individuals) did not contain the stated amount of vitamin D on their labeling. Wisdom would suggest that a multivitamin containing vitamin D be considered as part of the daily diet.

      The primary health risk of excessive vitamin D intake is a number of disorders that appear to be related to the vitamin's enhancement of calcium and, to a lesser degree, phosphorus absorption and utilization. Please see health risks for calcium and health risks for phosphorus for more information. Individuals who also consume supplemental fish oil (omega-3, other fatty acids, and cod liver oil, in particular) are at greater risk of a higher retention of fat soluble vitamins, including vitamin D.

      Numerous studies have suggested that there is a need to increase vitamin D intake in the elderly to assist in the maintenance of normal calcium metabolism and general bone health. It was for this reason the most recent NAS study has recommended such an increase for individuals over the age of 50 (see Reference Values for Vitamin D above).