January 2010 Issue

Vitamin D — Is It a Weapon in Disease Prevention?
By Jasmin Ilkay, MPH, RD
Today’s Dietitian
Vol. 12 No. 1 P. 12

Vitamin D has been touted to prevent everything from cancer to hypertension and now swine flu. While it may not be a cure-all vitamin, a deficiency in this nutrient may indeed be a risk factor for disease. The increase in the number of individuals being diagnosed with vitamin D deficiency is a problem that some researchers suspect coincides with the development of certain conditions, such as diabetes. Current research on vitamin D's potential as a disease-fighting weapon is limited but in some areas it appears promising.

Vitamin D Deficiency Resurfaces
Scientists discovered vitamin D in the early 20th century while looking for a cure for rickets. When the United States began its milk fortification program in the 1930s, rickets became almost nonexistent. Then, about 10 years ago, physicians noticed an increase in the number of infants diagnosed with rickets. The two main reasons were believed to be the increased use of sunscreen and the increased prevalence of breast-feeding. While breast-feeding is nutritionally superior to formula feeding and breast milk alone is not inherently deficient in vitamin D, a breast-feeding mother who has low levels of vitamin D, inadequate sun exposure, and a low dietary intake of vitamin D may produce breast milk with low levels of the nutrient.

Because rickets is easily prevented, in 2008, the American Academy of Pediatrics decided to up vitamin D intake recommendations from 200 IU/day to 400 IU/day for infants, children, and adolescents.

The increased risk of vitamin D deficiency is not limited to infants. Many individuals are at risk, including older adults, people with limited sun exposure, people with dark skin, those with fat malabsorption conditions, and people who are obese or have undergone gastric bypass surgery.1 In fact, an editorial published by the American Academy of Family Physicians stated that the prevalence of vitamin D deficiency has been reported to affect 21% to 58% of adolescents and adults in the United States.

The increased incidence and risk of vitamin D deficiency has motivated researchers to further evaluate intake requirements and the vitamin's role in preventing chronic diseases.

Vitamin D's Role in Disease Prevention

Heart Disease
According to Framingham Heart Study researchers, a deficiency in vitamin D is associated with an increased risk of cardiovascular disease (CVD).2 A recent article published on WebMD highlighted the results of a study conducted at Johns Hopkins that linked low levels of vitamin D and estrogen to an increased risk for both heart disease and osteopenia in 1,000 men. Researchers with that study are now analyzing blood samples in women to determine whether the results are similar. While more clinical trials are necessary, vitamin D shows promise for helping prevent CVD.

Diabetes
Many epidemiological studies have confirmed the inverse relationship between serum 25-hydroxyvitamin D levels and both type 1 and type 2 diabetes, as well as insulin resistance. A cohort study conducted in Finland and published in Diabetes Care followed more than 10,000 children for 30 years. Those who took at least 2,000 IU of vitamin D daily had an 80% lower risk of developing diabetes than those who did not take vitamin D.3 Clinical trials are currently under way to further evaluate vitamin D's effectiveness in preventing diabetes.

Cancer
Laboratory, animal, and epidemiological data suggest that vitamin D supplementation has the potential to prevent many cancers, including those of the colon, breast, and prostate. A recent clinical trial conducted in Nebraska found that postmenopausal women supplemented daily with calcium (1,400 to 1,500 mg) and vitamin D3 (1,100 IU) had a significantly lower incidence of cancer over four years compared with women who took a placebo.1 However, whether the decreased risk was due to vitamin D, calcium, or both is unclear.

Some studies do not recognize vitamin D as a protective factor against cancer. For example, an analysis of more than 16,000 National Health and Nutrition Examination Survey III participants found cancer mortality to be unrelated to vitamin D status.1 The National Cancer Institute states that it is unclear whether vitamin D is associated with cancer risk; currently, the institute does not recommend the use of vitamin D supplements to reduce the risk of cancer.

Other Diseases
Some researchers also believe vitamin D deficiency is a risk factor for hypertension, multiple sclerosis, autism, Alzheimer's disease, and swine flu. At this time, there is little or no evidence supporting these claims.

Dietary Reference Intakes
In 1997, the Food and Nutrition Board established an Adequate Intake (AI) for vitamin D that should maintain bone health and calcium metabolism in healthy people.1 Currently, there is not enough scientific evidence to develop a Recommended Dietary Allowance (RDA) that would meet the needs of nearly all healthy people. (The accompanying table details the current AIs for vitamin D.) The Upper Limit (UL) for vitamin D for infants younger than 12 months and anyone older than 1 year is 1,000 IU and 2,000 IU, respectively.

Since the development of the Dietary Reference Intake (DRI), it is apparent that a great deal of new research has been published to justify reevaluating adequate vitamin D intake for healthy populations.1 As mentioned, in 2008, the American Academy of Pediatrics issued recommended intakes for vitamin D that exceed those of the Food and Nutrition Board.1 The National Academy of Sciences also recommends 400 IU for people aged 50 and older and 600 IU for people aged 70 and older. In response, the Food and Nutrition Board created an expert committee in 2008 to review the DRIs for vitamin D and calcium. It's also worth noting that the usual dose used in a majority of studies having to do with vitamin D supplementation gave close to the UL (1,000 to 2,000 IU) of the vitamin daily to participants.

How to Get Vitamin D
What makes vitamin D unique is the body's ability to produce it on its own. When UV rays from the sun hit the cells of our skin, a certain form of cholesterol (7-dehydrocholesterol) can be converted into a form of vitamin D. However, season, geographic latitude, time of day, cloud cover, smog, skin melanin content, and sunscreen all affect UV radiation exposure and vitamin D synthesis. Combine that with the unknown risks of skin cancer from regular sun exposure, and many health professionals recommend supplementation.

Due to the risk of skin cancer, the American Academy of Dermatology recommends against intentional sun exposure for preventing vitamin D deficiency due to the risk of skin cancer. On the other hand, Michael F. Holick, PhD, MD, director of the Vitamin D, Skin and Bone Research Laboratory and author of The UV Advantage, stated in an interview that there are benefits to sensible exposure to sunlight and there is essentially no evidence that it will increase one's risk of cancer.3 Sensible is the operative word here, as he defines sensible as no more than five to 10 minutes of sun exposure without sunscreen on the arms and legs or face and arms two to three times per week.3 However, Americans living north of southern California or Florida may want to supplement between the months of November and February.1

Of course, vitamin D is also found in food, with some of the best sources being cod liver oil, salmon, mackerel, tuna, sardines, milk, and vitamin D-fortified foods (eg, soy milk, cereal). Most experts agree that many people will not be able to meet the AI focusing solely on food sources, hence the recommendation to supplement.

Should RDs recommend vitamin D supplements to prevent a deficiency? The reality is that some people may have to rely on vitamin D supplements to prevent a deficiency. However, vitamin D can be stored in the body and long-term supplementation of more than 2,000 IU daily could cause a toxicity for anyone older than 1 year (1,000 IU in infants).

Conclusion
At this time, there are not enough data available to verify vitamin D's role in preventing or treating any of the diseases discussed. Current research is promising in the area of heart disease and diabetes and more research is under way. The need to reevaluate the DRI recommendations for vitamin D is warranted.

Until the DRIs are updated, spending a “sensible” amount of time in the sun and planning the diet wisely should prevent vitamin D deficiency in most healthy people.  Those individuals at risk for a vitamin D deficiency should also add a multivitamin to their daily regimen.

— Jasmin Ilkay, MPH, RD, is a lecturer for the human nutrition and food science department at the California State Polytechnic University in Pomona and a freelance writer specializing in dietary supplements, child and family nutrition, and eating disorders.

References
1. National Institutes of Health Office of Dietary Supplements. Dietary supplement fact sheet: Vitamin D. Last updated November 13, 2009. Available at: http://dietary-supplements.info.nih.gov/factsheets/vitamind.asp. Accessed November 2009.

2. American Heart Association. Lack of vitamin D may increase heart disease risk. January 7, 2008. Available at: http://www.americanheart.org/presenter.jhtml?identifier=3052800. Accessed November 2009.

3. Liebman B. Soaking up the D’s, interview with Michael F. Holick. Nutrition Action Healthletter. December 2003.

Adequate Intakes for Vitamin D1