Vitamin D has an illustrious history as a major bone-building nutrient required for calcium absorption and bone growth. But recent revelations about D’s roles in the body have led scientists to view this nutrient in a whole new light.
?Scientists have discovered that essentially every cell in the body has receptors for active vitamin D, which wouldn’t be there if they didn’t have a function,? says noted vitamin D researcher Michael Holick, Ph.D., M.D., of Boston University. A large and expanding body of research has suggested potential links far beyond bones, such as between vitamin D status and immune diseases like multiple sclerosis, type 1 diabetes and rheumatoid arthritis, as well as between D and chronic diseases like cardiovascular disease and type 2 diabetes.
One of the most impressive and fastest growing areas of vitamin D research is D’s possible role in preventing cancer, especially of the breast, colon and prostate. Moreover, a number of scientists think D may have a role in surviving cancer, not just preventing it.
As a result of all these connections, many experts believe that current recommendations for vitamin D intake are too low, possibly far too low given modern lifestyles lived mostly indoors and public health initiatives that encourage the use of sunscreen, which blocks the production of vitamin D in the skin. EN looks at the link between D and cancer and tackles how much to get.
The ABCs of D. Although you can get vitamin D from a few foods (see ?A Few Good Food Sources,? below right) and from supplements, we rely more on sunlight, which triggers the production of an inactive form of the nutrient called vitamin D3 (cholecalciferol) in the skin. This substance is ultimately converted into the active form of vitamin D called 1,25-dihydroxyvitamin D (calcitriol) in the liver.
One of the substances produced in this conversion process, 25-hydroxyvitamin D, is found in the blood and reflects the vitamin D we get from both diet and sunlight. It is the compound that’s measured in blood tests to assess vitamin D status. The current ?normal? range for vitamin D is 20 to 100 nanograms/milliliter, but most experts recommend aiming for at least 30.
Vitamin D is found in supplements both as D3 and as D2 (ergocalciferol). For years, both forms were considered interchangeable as good sources of vitamin D. However, studies have since found that D3 is the more potent form and has a greater effect on raising blood levels of 25-hydroxyvitamin D. Vitamin D2 is found in some plant foods like mushrooms. D3 is used most often to fortify milk. Both forms are used in multivitamins and single D supplements, so it behooves you to read labels to find one that specifies D3 or cholecalciferol as an ingredient.
The Cancer Connection. The theory that low vitamin D may increase cancer risk comes from population studies showing that people who live farther north (and so are exposed to less intense ultraviolet rays from the sun) and who spend more months indoors are more likely to get cancer. Plus, there is now a better understanding of D’s effect on the body’s cells. Uncontrolled cell growth is the hallmark of cancer, and vitamin D seems to keep cell growth in check. It may work by triggering the death of abnormal cells or by a process called anti-angiogenesis, which prevents cells from establishing a blood supply.
Cancer Data on D. A recent analysis of five studies found that people with the highest blood levels of vitamin D had only about half the risk of colorectal cancer compared to those with the lowest levels. The average blood level of D in the high group would probably require a daily intake of 1,000 to 2,000 International Units (IU) of vitamin D for someone with limited sun exposure, according to Edward Giovannucci, Sc.D., M.D., of Harvard’s School of Public Health, who was involved in the analysis.
Breast cancer rates may also be significantly reduced with elevated levels of D, says Cedric Garland, Dr.P.H., of the University of California at San Diego, who recently analyzed breast cancer studies and found that incidence declined as blood levels of vitamin D increased. Garland, who first posited the cancer-D connection, says the greatest benefit occurs at levels that exceed what you can get from diet alone. But he adds that the research to date suggests ?all women could substantially lower breast cancer risk by getting 2,000 IUs daily.?
D and Cancer Survival. Equally intriguing is early evidence suggesting that vitamin D may affect cancer survival as well as prevention. According to Giovannuci, one study out of Norway found that people diagnosed with colon, breast or prostate cancer during summer months, when vitamin D status is at its highest, were 30% less likely to die from cancer, even after 10 years, suggesting that D status at the time of diagnosis might influence prognosis.
Giovannucci has also been involved in studies suggesting that vitamin D may increase survival from lung and pancreatic cancer, two particularly deadly forms of the disease.
Check Your D Status. All our experts agree that it’s reasonable to ask your doctor to check your vitamin D blood level. Garland says it’s particularly important after age 40, when half of people may well be D deficient. Those at highest risk for low D levels include older adults, those with dark skin (who synthesize less D) and those who get little sun exposure.
The Bottom Line. When the Recommended Dietary Allowances for vitamin D are eventually revisited?years from now?experts agree they are likely to be much higher, so aim for far more than 100% of the Daily Value for D.
Maximize your vitamin D via several sources (see ?4 Ways to Beef Up Your D, above?), particularly if you?re in a high-risk group for deficiency.