Vitamin D—Is it really something I need to worry about?

Carol L. Wagner, M.D.



What if you live in a place with abundant sunshine year around and you spend 5-9 hours a day outside? What could you expect your body to produce in abundance? Why vitamin D, of course! Unlike all the other vitamins, vitamin D is the only one that is mainly produced by the body via a chemical reaction that takes place in the skin when exposed to sunlight; hence its name—the “sunshine vitamin.”


Each time ultraviolet light of a certain wavelength (UV-B) hits the upper layer of skin at the correct angle, vitamin D is formed. A series of chemical alterations to the parent compound vitamin D occurs in the body, transforming vitamin D into 25-hydroxyvitamin D and then into its most active form—1,25-dihydroxy-vitamin D, which is a hormone that is utilized by every cell in the body (1). In fact, cells can take in both vitamin D itself and 25-hydroxyvitamin D to make the active form. No longer thought to be important for just calcium and bones, vitamin D has effects on far more systems in the body, the most striking of which is the immune system.


First discovered in the early 1920s by Mellanby and others as the cure for childhood rickets, vitamin D was originally thought only necessary for children (2). There are two forms of vitamin D—the form we make in our skin, called cholecalciferol or vitamin D3, and the form found in plants, called ergocalciferol or vitamin D2. One teaspoon of cod liver oil (vitamin D3) was found to prevent rickets, and by the 1950s the occurrence of rickets in the United States had been drastically reduced (3). In those days, however, individuals spent more time outdoors and ate foods that had greater amounts of vitamin D. It would be several decades later that the connection between vitamin D and infection was solidified.


Beginning in the late 1970s, a resurgence of rickets was seen, particularly in darker pigmented, breastfeeding young children (4, 5). What happened? People were spending less time outdoors, and when outdoors, were more likely to cover their skin with sunscreens or protective clothing. The drawbacks of too much sunlight on unprotected skin are well documented, yet each time we use sunscreen we are blocking the natural process within our skin to make vitamin D. That is how we evolved. There was a rapid loss of darker pigment as some ancestors moved to higher latitudes that maximized vitamin D production in the skin with what little sunlight there was. But, with lifestyle changes and our greater mobility, we are individuals of a variety of pigments living in a myriad of places and the amount of sunlight that our skin sees varies considerably.


What has this change in lifestyle caused? We are more likely to be vitamin D deficient. In two recent studies involving more than a thousand pregnant women, we found that more than 70% of African American women, 1/3 of Hispanic women and 12% of European Americans were vitamin D deficient (6). Others have found similar rates of deficiency among other adults and children. The importance of vitamin D is especially true during pregnancy and breastfeeding as the mother is the sole source of vitamin D for her baby (9).


What are the implications of such deficiency? We certainly know that with severe vitamin D deficiency there can be serious problems with calcium levels in the body, as well as a weakening of bones in children (called rickets) and osteoporosis in adults. We also know that vitamin D deficiency can impact our ability to fight bacterial infections such as tuberculosis and can render us at higher risk for autoimmune diseases such as multiple sclerosis and certain cancers such as prostate, breast and colon cancer (10).


What should we do to prevent vitamin D deficiency? Going out in the sun is a sure way to get enough vitamin D, but there are certain risks. Limiting your sunlight exposure to 10-15 minutes before you add sunscreen could be a safe way to obtain an adequate supply of vitamin D. During winter months, only the most southern parts of the US have enough sunlight at the proper angles to trigger vitamin D production in the skin. If you work or go to school and spend most of the daylight hours indoors, then you are not able to make vitamin D.


Most Americans only get about 200 IU vitamin D per day from their diets, most being in the form of vitamin D3. Infants and young children require about 400 IU vitamin D per day. The amount that is needed to achieve normal levels of vitamin D in the body varies depending on your sunshine exposure, the season, where you live, and how much you weigh. Because vitamin D is a fat-soluble vitamin, you can take too much, but that amount is well above the 4000 IU/day that is safe for pregnant women. It is best to have your vitamin D level checked to know exactly what you need. You can ask your doctor to measure a total circulating 25-hydroxy-vitamin D level.


No matter how you get your vitamin D—sunshine or vitamin D supplement—it is essential that you have a normal, healthy reserve in your body all year long so that your body can function at its peak. Like any other hormone in the body, there is a certain range that is best for your health. To find out more, ask your health care provider about vitamin D!





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14. Food and Nutrition Board. Standing Committee on the Scientific Evaluation of Dietary Reference Intakes. Dietary Reference Intakes for Vitamin D and Calcium. Washington, D.C.: National Academy Press; 2010.