Vitamin D is actually a hormone and not technically a vitamin: a vitamin is defined as a substance that is not made naturally by the body but must be supplied in the diet to maintain life processes. Vitamin D, however, is predominantly made by the action of ultraviolet light (sunlight) on 7-dehydrolcholesterol, the vitamin D precursor that is found in the skin. We only get very small amounts from our diet in vitamin D fortified foods.
Ninety per cent or more of our vitamin D comes from the sun. A major cause of deficiency is insufficient sun exposure of the skin. This can occur especially in northern climates where people don’t spend much time outdoors; but even in countries near the equator, women in particular often have much of their skin area covered for cultural reasons, and the use of sunscreen also blocks the synthesis of vitamin D in the skin.
Vitamin D is metabolized by the liver to 25-hydroxyvitamin D, which circulates in the blood until it is needed. Enzymes in the kidneys metabolize it further to form the highly active hormone that is involved in essential biochemical processes throughout the body. 25-hydroxyvitamin D status is therefore an important screening blood test, especially if you spend much of your time indoors, or live in a colder climate. Testing can be used to measure and monitor vitamin D supplementation to ensure you are getting the right amount for optimum health.
A useful website for more information about vitamin D is www.vitamindcouncil.org.
Who is at Risk?
People with Limited Sunlight Exposure
People living at northern latitudes or who have limited sunlight exposure because of their working environment or cultural dress rules may have low vitamin D levels. In some people at high risk for sun-related skin damage and/or cancer it might be better to continue avoiding the sun while taking vitamin D supplements. Consult with your clinician as to what is best for you.