Vitamin D, one of the 13 essential vitamins, enhances calcium absorption, supports the immune system, helps nerve and muscle function and regulates protein production. The National Institute of Health Office of Dietary Supplements recommends that anyone up to age 50 intake 5 micrograms or 200 IU per day. Although few foods naturally contain vitamin D, fortified foods such as milk, orange juice and breakfast cereals serve as good sources. The ultraviolet rays of the sun trigger the production of vitamin D in the skin.
Types
Vitamin D exists in several different forms. Plants such as yeast produce vitamin D-2 while animals produce vitamin D-3. While many foods and supplements contain vitamin D-2, research published in the October 2006 issue of "American Journal of Clinical Nutrition" reveals that vitamin D-3 may actually be more potent than D-2. Once in the body, the kidneys convert vitamin D-2, vitamin D-3 and pre-vitamin D-3 produced in the skin into the active form of vitamin D known as calcitriol.
Vitamin D Production
The skin contains a substance known as 7-dehydrocholesterol. The ultraviolet rays from the sun trigger the conversion of 7-dehydrocholesterol into pre-vitamin D-3. The body then changes pre-vitamin D-3 into vitamin D-3, which the kidneys then process into calcitriol the body can utilize.
Adequate Sun Exposure
The medical community warns against unprotected or excessive sun exposure, which increases the risk for skin cancer. In the United States doctors diagnose more than 1 million cases of non-melanoma skin cancer with most of those related to sun exposure, according to the American Cancer Society. Despite these dangers, the human body needs sun to produce vitamin D. Researchers suggest that getting five to 30 minutes of sun exposure at least twice a week leads to sufficient vitamin D synthesis, according to the National Institutes of Health Office of Dietary Supplements. Because most people fail to cover all their exposed skin all the time, most people do meet their vitamin D requirements with sun exposure.
Environmental Influences
Because the intensity of the sun's rays varies based on location, the ability of the sun to induce vitamin D production varies. Those who live in the far northern latitudes may suffer from vitamin D deficiency for up to six months each year due to decreased sun intensity. Cloud cover also influences the intensity of the sun, with complete cloud cover causing a 50 percent reduction in ultraviolet rays, according to the National Institutes of Health Office of Dietary Supplements. Other factors including shade and pollution can cause a lack of vitamin D production in the skin.
Skin Influences
Adults over the age of 50 have an increased risk of developing a lack of vitamin D due to their skin failing to produce vitamin D even with adequate sun exposure. Therefore the Food and Nutrition Board recommends those ages 51 to 70 get 10 micrograms or 400 IU per day while those over 71 get 15 micrograms or 600 IU, according to the National Institutes of Health Office of Dietary Supplements. The more pigment in the skin, meaning those with darker skin, means less absorption of the sun's rays, which decreases vitamin D production, as described by the University of Maryland Medical Center.
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