Experts Debate Between Vitamin D and Cancer Risk

At a major scientific conference on diet and cancer, a panel of vitamin D experts weighed in, and presented evidence that sought to provide some clarity.

10/11/08 The association between vitamin D and cancer risk is one of the most studied, most complex and most controversial issues in nutrition science. At a major scientific conference on diet and cancer, a panel of vitamin D experts weighed in, and presented evidence that sought to provide some clarity.

The panel on vitamin D and cancer was part of the 2008 Annual Research Conference on Food, Nutrition, Physical Activity and Cancer, presented by the American Institute for Cancer Research (AICR) in Washington, DC.

Vitamin D is an essential nutrient, and vitamin D deficiency has been linked to chronic diseases, including some cancers. Humans acquire vitamin D in two ways - from exposure to sunlight, and from the diet. (Most of the vitamin D in American diets comes from foods that have been fortified with it, such as milk and cereal.).

How Much Sunlight?

Experts seeking to offer the public advice must weigh the fact that vitamin D is produced in skin exposed to sunlight against the clear risks of skin cancer caused by excessive exposure to UV rays, said Mary Frances Picciano, PhD, of the Office of Dietary Supplements at the National Institutes of Health. She noted that, as yet, no optimal level of sun exposure has been determined that poses no or little risk of skin cancer.

Some experts have proposed that the current definition of vitamin D deficiency is too low, and the US government is now considering raising the recommended intake for vitamin D. But Picciano noted that significant questions remain. More work, for example, must be done to ensure measurement of vitamin D blood levels is both accurate and consistent.

Vitamin D Deficiency Implicated in African American Cancer Rates

Lisa B. Signorello, ScD, of Vanderbilt University and the International Epidemiology Institute pointed out that many of the cancers linked to vitamin D deficiency are more prevalent among African Americans.

The high concentration of melanin in the skin of African Americans blocks the formation of vitamin D. Signorello also presented data suggesting that African Americans consume fewer foods that have been fortified with vitamin D. She suggested that this combination of lower vitamin D production and lower dietary intake may be a key factor in higher rates of some cancers among African Americans, and proposed that targeted efforts to increase vitamin D status may be an important means to prevent cancer in this group.

Two Clues to How and Why Vitamin D Seems Protective

The body produces a biologically active from of vitamin D that has been associated with anticancer effects in the lab and in epidemiological studies. John White, PhD, of McGill University presented data that may help explain why. His group found that the vitamin D receptor stimulates proteins that are involved with regulating the cell cycle and that also "turn on" tumor suppressor genes.

Another researcher presented evidence linking vitamin D to lower risk of skin cancer. Daniel Bikle, MD, PhD, and his colleagues at the University of California at San Francisco have demonstrated that animals that lack the receptor for vitamin D are more likely to develop skin cancer when exposed to UV rays.

He hypothesizes that the vitamin D receptor regulates two protective biochemical pathways in skin cells. When this receptor is not present, these pathways get disrupted in ways that make skin cancer more likely.

His findings suggest that very brief exposure to sunlight - 10 to 15 minutes, in his animal model - may stimulate the vitamin D receptor, activate those two pathways, and actually be protective against skin cancer, Dr. Bikle said. If borne out in human investigations, his research will represent a substantial contribution to the ongoing scientific discussion about optimal vitamin D levels and sunlight exposure.