Vitamin D2 (ergocalciferol) is the form often added to milk and other foods, and most often used in nutrition supplement. Vitamin D3 (cholecalciferol) is made by the body and is found in some foods. Active vitamin D functions as a hormone as it sends a message to the intestines to increase the absorption of calcium and phosphorus; as such vitamin D is critically important for the development, growth, and maintenance of a healthy body, from birth until death, in humans. The major biologic function of vitamin D is to maintain normal blood levels of calcium and phosphorus. It helps to form and maintain strong bones by promoting calcium absorption. It also works alongside a number of other vitamins, minerals, and hormones to promote bone mineralisation. Without vitamin D, bones can become thin, brittle, or misshapen. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults, two forms of skeletal diseases that weaken bones.
Sun exposure is perhaps the most important source of vitamin D because exposure to sunlight provides most humans with their vitamin D requirement. About 90% of the daily recommended intake of 10 micrograms or 400 international units is obtained from the action of sunlight on the skin. UV rays (particularly UVB rays) from the sun trigger an inactive form of vitamin D to be formed in the skin. This is taken up by the blood stream, to be stored in muscles and body fat; it also passes through the liver and kidneys and becomes an active form of vitamin D, which is then used by the body. Since vitamin D can only be produced by the skin during the summer months via this route, the body's ability to store vitamin D is extremely essential. The remaining 10% of the recommended intake of vitamin D is supplied by diet.
Factors such as the season of the year, latitude, time of day, cloud cover, smog, and sun screens all affect the amount of ultraviolet ray exposure we receive from the sun and can reduce our supply of vitamin D. In countries where exposure to the sun is reduced by head to feet clothing for religious or other reasons, vitamin D under nutrition is a problem. Elderly people who stay inside and have poor diets may also have a sub clinical deficiency of vitamin D.
An initial exposure of about 10-15 minutes to sunlight allows adequate time for Vitamin D synthesis and should be followed by application of a sunscreen with a sun protection factor SPF of at least 15 to protect the skin. This is because sunscreens with an SPF of 8 or greater will block UV rays that produce vitamin D, but it is still important to routinely use sunscreen to help prevent skin cancer and other negative consequences of excessive sun exposure. Ten to fifteen minutes of sun exposure at least two times per week to the face, arms, hands, or back without sunscreen is usually sufficient to provide adequate vitamin D. It is very important for individuals with limited sun exposure to include good sources of vitamin D in their diet.
