Vitamin D is not one but several vitamins: fat-soluble steroids or Prohormones, that exist in five forms: vitamins D1, D2, D3, D4, and D5. The two forms that appear to be of most significance for humans are vitamin D2 (also called ergocalciferol) and vitamin D3 (termed cholecalciferol).
Cholecalciferol is the naturally occurring form of vitamin D, and is formed by cells located in the human skin when the rays of the sun fall on the bare skin. The estimated dietary requirement of vitamin D for an adult male is at least 5.0 µg/day.
However, studies done in Australia suggest that the average estimated dietary intake of vitamin D for men in this country is 2.6–3.0 µg/day - and for women it is thought to be a little less (2.0–2.2 µg/day).
The Function of Vitamin D
Vitamin D is vital for building strong bones, muscles and teeth. Most of our vitamin D is produced when ultraviolet rays of sun hit our skin. Thus, lack of exposure to sunlight - as can happen in the relatively sunless days of winter, particularly if one wears clothes that cover most of the body's surface, can lead to vitamin D deficiency , which can cause rickets in children and osteomalacia in older people.
Food Sources of Vitamin D
Vitamin D is contained in small amounts in some foods - for example eggs and fish (especially fish with a high fat content such as sardines, salmon, herring and mackerel). Certain foods such as milk and margarine are supplemented by the addition of vitamin D during the manufacturing process. In the United States, manufacturers voluntarily fortify a large number of foods such as milk, margarine, breakfast cereals and bread with vitamin D. However, it is not easy to obtain adequate vitamin D through diet alone - in fact most of us get less than a quarter of the vitamin D we need from our food.
Sun exposure and vitamin D
Most people with fair to olive skin receive enough vitamin D through natural exposure of their skin to sunlight (even if they apply sunscreen). However, folk with naturally dark skin need about five times the exposure time to the same degree of sunlight in order to absrob enough sunlight for adequate vitamin D production. Modern lifestyles - where many men leave home for work early in the morning and return home after dark, travelling in closed vehicles to and from work - result in limited sunlight exposure particularly during winter. Certain population groups, such as the elderly and dark-skinned men, are at particular risk for vitamin D deficiency.
Disclaimer: The information contained in this article is for educational purposes only and should not be used for diagnosis or to guide treatment without the opinion of a health professional. Any reader who is concerned about his or her health should contact a doctor for advice.
Further reading: Vitamin D status of Australians (2002)
Sanjiva Wijesinha - Dr. Sanjiva Wijesinha, Associate Professor at Monash University medical school, writes on health, travel and medical topics.
