The beaches will be packed this Fourth of July weekend, and dermatologists can only hope that people will wear sunscreen and avoid the sunburns that increase the risk of skin cancer later in life.

While the ultraviolet radiation in sunlight can harm skin, it also stimulates the production of vitamin D, and some researchers are now investigating whether vitamin D may have a role in preventing or treating cancer.

The hypothesis that vitamin D may reduce cancer risk is based in part on studies showing that vitamin D deficiency is associated with an increased prevalence of cancer, and on evidence that vitamin D, when converted into a hormone, promotes the normal growth of cells and has anticancer properties.

"Many pharmaceutical companies are interested in developing the vitamin D hormone or analogues for use in cancer treatment," says Dr. Anthony Norman of the University of California, Riverside, who is a leader of the Vitamin D Workshop, a group of researchers that meets every 3 years.

The group cosponsored a 3-day meeting on vitamin D and cancer at NCI last November, where participants stressed the need for a new analysis of all of the studies, both positive and negative, on vitamin D and cancer risk.

A summary paper on the NCI meeting to be published this fall in the Journal of Steroid Biochemistry and Molecular Biology reports that there was general agreement that vitamin D deficiency is associated with a higher prevalence of cancer (and several other diseases), and that the vitamin D hormone can decrease the proliferation of cells and control malignant cell growth.

Most important, perhaps, the participants saw a need for larger studies that could begin to address the many unresolved questions for the field, and they said "academia, public funding agencies, and industry should urgently design appropriate studies to better define the causal relationship between vitamin D nutrition and cancer."

The current information about vitamin D can be confusing. The skin manufactures vitamin D after exposure to ultraviolet radiation, yet dermatologists say that no amount of sun exposure is safe because ultraviolet radiation can cause skin cancer. The amount of vitamin D that might help prevent cancer, if any, is not known.

Nor is it known just how much or how little sunlight is needed to replenish stores of vitamin D because the answer varies depending on the season, a person's skin color (the process takes longer for people with darker skin), a person's distance from the equator, and clothing.

"If you want to increase your vitamin D levels, the safest way is to take supplements," says Dr. Sue Ingles of the University of Southern California, who co-authored a new study on sunlight and advanced prostate cancer among men in San Francisco.

The study found that the risk of prostate cancer was reduced by 50 percent in men who had high levels of sun exposure during their lifetimes, compared with men who had low lifetime levels. The study, in the June 15 Cancer Research, included 450 white men with advanced prostate cancer and 455 healthy white men.

"A risk reduction of 50 percent is quite large, given how little is known about what men can do to prevent prostate cancer," says co-author Dr. Esther John of the Northern California Cancer Center. "It will be important to see if other studies replicate these results."

Most of the risk factors for prostate cancer are things that cannot be changed, such as growing older, being African American, and having a family history of the disease. A vitamin D deficiency, on the other hand, may be a modifiable risk factor.

"It would be profoundly important for public health if vitamin D could prevent some prostate cancers," says co-author Dr. Gary G. Schwartz of the Wake Forest University Comprehensive Cancer Center in North Carolina. "Vitamin D is safe, inexpensive, and available, and there's no need to get it from sunlight."

The researchers detected an even greater risk reduction among men in the high-exposure group who had certain forms of a gene involved in regulating vitamin D.

"What's interesting here is that it was the combination of sun exposure and genetic variants that made the difference," says Dr. John. "The field is moving toward trying to understand the combined effects of genetic and nongenetic factors, but you need large studies to detect these effects."

The researchers expect to publish results from larger studies now underway involving more Caucasians, as well as African Americans and Hispanics, in northern and southern California.

In October, the Journal of Steroid Biochemistry and Molecular Biology is slated to publish 25 papers representing the proceedings of the vitamin D/cancer meeting held in November 2004.

By Edward R. Winstead