Vitamin D precursors are formed in the skin in the presence of sunlight, converted into the active form of vitamin D in the kidneys and carried by the blood to the rest of the body (Adams 2010). It is also produced by immune system cells as part of the body’s defenses against disease.
People store up vitamin D in blood and fat during sunny months and draw down that supply through the fall and winter until it is depleted. Many people can synthesize enough vitamin D from 10 to 30 minutes of midday sun exposure to arms and legs without sunscreen several times a week.
Sunscreen use combined with too little outdoor time can contribute to vitamin D deficiency. Ultraviolet light-induced vitamin D production can be inhibited by deep skin pigmentation, indoor lifestyles, older age, strict sun avoidance and other factors (Lucas 2006). People most at risk have dark skin, spend little time in the sun, are obese or live in the northern half of the country, where the body can’t make enough vitamin D in fall and winter months.
Because people with darker skin make less vitamin D from sunshine, the federal Centers for Disease Control and Prevention has found that Mexican Americans and African Americans are two to three times more likely than Caucasians to have low vitamin D levels (CDC 2011). Increased use of sun protection and higher average body weight are also associated with lower vitamin D levels (Looker 2008).
Experts agree that too many people in the United States don’t get enough vitamin D in their daily lives (AMA 2009, IOM 2010, CDC 2012). An estimated 8 percent of Americans have serious vitamin D deficiency. Another 25 percent are “at risk of inadequacy” (CDC 2011). Seventy percent of breastfed babies are vitamin D deficient at one month of age (Wagner 2010), when such a deficiency can be particularly harmful because of vitamin D’s role in growth and development.
In human epidemiological studies, low vitamin D levels have been associated with increased cardiovascular mortality, colon cancer mortality and breast cancer risk and have been tentatively linked to skin cancer, metabolic disease, hypertension and obesity (Adams 2010; Grant 2009; Tang 2010). People with inadequate vitamin D are more susceptible to upper respiratory tract infections and other microbe-borne infections (Ginde 2009).
Vitamin D promotes intestinal calcium and phosphate absorption and calcium/phosphate release from the bone. Vitamin D deficiency is associated with rickets in children and osteoporosis in adults (Papandreou 2010). Calcium strengthens bones, but only in the presence of vitamin D – too little of either can lead to osteoporosis, fractures and other bone problems.
Physicians and scientists around the world agree that sun protection is essential to prevent skin cancers and reduce their toll on human health and well-being and health care costs (Gordon 2009). Yet strict sun protection has been shown to cause vitamin D deficiency (Norval 2009; Reichrath 2009).
UV light exposure, whether from the sun or from artificial tanning, is the most important environmental risk factor for skin cancer (International Agency for Research on Cancer 2001b). Over the past several decades, the number of new cases of the three most common forms of skin cancer — basal cell carcinoma, squamous cell carcinoma and malignant melanoma — has been steadily rising in the U.S. and other countries (Aceituno-Madera 2010; Gloster 1996).
The American Academy of Dermatology’s 2009 Position Statement on vitamin D concludes that “there is no scientifically validated, safe threshold level of UV exposure from the sun that allows for maximal vitamin D synthesis without increasing skin cancer risk.” The academy recommends increased intake of foods naturally rich in vitamin D, vitamin D-fortified foods and vitamin D supplements (AAD 2009). But vitamin D is so vital to health that the World Health Organization considers sun exposure essential if vitamin D is not available from food and supplements, in spite of the skin cancer risk (WHO 2006, Lucas 2008). Fortunately, supplements are readily available in the U.S., so that people can stay safe in the sun and still maintain healthy levels of this important vitamin.
Experts recommend getting your vitamin D levels tested by your doctor, particularly if you are at risk for deficiency, and taking supplements if needed. Some foods contain vitamin D, but not enough to meet daily requirements. Getting too much vitamin D causes side effects, so take only what your doctor recommends. The body regulates the amount it makes in the sun but cannot protect against excess vitamin D from supplements.
In 2010, the Institute of Medicine of the U.S. National Academies updated its dietary guidelines to recommend 600 International Units of vitamin D daily for people ages 1 to 70 and 800 IU for those past 70 (IOM 2010). This recommendation suggests that vitamin D supplementation is necessary for many Americans.
Consult your physician to determine if you are at risk for vitamin D deficiency. In that case, you may need a blood test and regular or seasonal vitamin D supplementation.