Sun-exposed white men are less likely to get prostate cancer than their less tanned brethren, a new study shows.

That's no reason for men to recklessly sunbathe. The greater a person's lifetime sun exposure, the greater a person's risk of skin cancer. But the finding does indicate that vitamin D -- which humans can get from sun exposure -- protects against prostate cancer.

Also protective are genes that let some people's bodies use vitamin D more efficiently, find Esther M. John, PhD, of the Northern California Cancer Center; Gary G. Schwartz, PhD, of Wake Forest University, and colleagues.

"It's a pretty impressive finding," Schwartz tells WebMD. "Men with high solar exposure had their risk of prostate cancer cut in half. This leaves us with even greater confidence that vitamin D deficiency really does increase a man's risk of prostate cancer."

The findings appear in the June 15 issue of Cancer Research.

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Vitamin D Detective Finds Prostate Cancer Clue

Schwartz first proposed a link between prostate cancer and vitamin D in 1990. That's when he noticed that the populations most likely to get too little vitamin D are the same populations most likely to get prostate cancer.

"Prostate cancer is more common in northern latitudes, in blacks, and in the elderly. That resembles essentially the same people who most often got what used to be called rickets, a bone-deforming disease linked to lack of exposure to sunlight," Schwartz says. "So I argued that if vitamin D deficiency causes one disease -- rickets -- there is no reason why it cannot cause another disease -- prostate cancer -- later in life."

People living in the north and elderly people tend to get less time in the sun than young people living in southern climes. Unlike other vitamins, a person's main source of vitamin D isn't food; it's sunshine. The body makes its own vitamin D, but only when it's exposed to the sun.

Vitamin D is made from sunlight acting on the skin," Schwartz says. "Eighty percent to ninety percent of vitamin D in the body is derived from sunlight, not from diet."

Additional evidence of a vitamin D-prostate cancer link came from laboratory studies. Schwartz and colleagues found that prostate cancer cells are less likely to behave like cancer cells when exposed to vitamin D.

Moreover, Schwartz notes that prostate cells are able to process vitamin D. In fact, the surface of a prostate cell bears a molecule called a vitamin D receptor or VDR. When vitamin D plugs into one of these receptors, it sets off a complex chain of events thought to protect the cell against cancer.

But when other researchers looked at whether sun exposure and vitamin D levels were linked to prostate cancer, they got mixed results. Some found a link. Others did not. Clearly, something else is going on.

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Sun Exposure and Prostate Cancer

John, Brooks, and colleagues decided not to take a one-time measure of vitamin D levels or to rely on people's estimates of sun exposure. They used a device to compare skin pigmentation on a person's exposed skin to pigmentation on that person's unexposed skin. They then calculated that person's sun exposure.

The researchers studied 450 non-Hispanic white Americans with prostate cancer and compared them with 455 similar men without prostate cancer.

The bottom line: Those with high sun exposure were 49 percent less likely to have prostate cancer.

The researchers also got blood samples from study participants. They looked for vitamin D receptor genes.

People who carried genes for particularly effective vitamin D receptors were 54 percent to 33 percent less likely to have prostate cancer, depending on the gene involved.

All this is very interesting to cancer researcher Jay Brooks, MD, chief of hematology/oncology at the Ochsner Clinic in Baton Rouge, La.

"These are very interesting and intriguing observations," Brooks tells WebMD. "One interesting thing is the vitamin D aspect. Another thing is the cancer risk reduction with certain genes of an individual. We are beginning to look at genetic markers that predict a risk of certain cancers. In terms of prostate cancer, hopefully we will soon have improved medicines to reduce the risk of cancer in these individuals."

Brooks warns that the current study does not prove vitamin D deficiency causes prostate cancer. It's too soon, he says, for people to start taking vitamin D in hopes of cutting their risk.

"Ten years ago we thought higher levels of vitamin A would help prevent cancer, but now we know it ups the risk of lung cancer," Brooks says. "This is interesting science, but until a randomized clinical trial is done, it will not change our recommendations to patients."

Schwartz says scientists are only beginning to recognize the importance of vitamin D.

"My feeling is we are getting a better sense of vitamin D-deficiency diseases," he says. "Prostate cancer does seem to resemble rickets in this regard. We prevented rickets by putting vitamin D in milk. There is hope we could prevent prostate cancer using the same kind of approach."

Read WebMD's "New Methods for Targeting Cancer"

By Daniel J. DeNoon, reviewed by Brunilda Nazario, MD

SOURCES: John, E.M. Cancer Research, June 15, 2005; vol 65: pp 1-10. Gary G. Schwartz, PhD, associate professor of cancer biology and public health science; and director, prostate cancer center of excellence, Wake Forest University, Winston-Salem, N.C. Jay Brooks, MD, chief of hematology/oncology, Ochsner Clinic, Baton Rouge, La.