MONDAY, June 27 (HealthDay News) -- Certain women at risk for developing melanoma, the most severe form of skin cancer, may cut the likelihood in half by taking vitamin D/calcium supplements, a new study suggests.

"It looks like there is some promising evidence for vitamin D and calcium for prevention of melanoma in a high-risk group," said lead researcher Dr. Jean Tang, an assistant professor of dermatology at Stanford University School of Medicine.

The women most at risk of developing the life-threatening cancer are those who have had a previous non-melanoma form of skin cancer, such as basal cell or squamous cell cancer, the researchers said.

Vitamin D and calcium are well-known for their roles in bone growth, but they also affect other cells in the body. Some studies have shown that vitamin D and calcium are associated with lower risk of colon, breast, prostate and other cancers, the researchers said.

Tang speculated that cancer cells lurking in the skin of women who have had a previous skin cancer may be waiting to develop into melanoma. "But if they take calcium and vitamin D that reduces the risk of developing an actual tumor," she said.

As little as 400 international units (IU) of vitamin D daily may be protective, Tang said. The U.S. Institute of Medicine now recommends 600 IU of vitamin D daily, she added.

Calcium has also been shown to reduce tumor growth in patients with colon cancer, Tang said. "So maybe calcium has a role, too," she said. "I can't say whether it was the calcium or the vitamin D that was important."

But the combination seemed to convey a benefit, she added.

Whether these results would be seen in men or young women isn't known, Tang noted. But an earlier study led by Tang found a benefit from vitamin D in reducing the risk of melanoma among older men.

"More studies need to be done, because we want to make sure these results are true in other communities," Tang said.

The report was published in the June 27 online edition of the Journal of Clinical Oncology.

For the study, Tang's team collected data on 36,282 postmenopausal women, 50 to 79 years old, who took part in the Women's Health Initiative study. As part of a test to see if calcium plus vitamin D had any effect on hip fractures or colon cancer, the women were randomly assigned to take supplements or placebo.

The supplements were 1,000 milligrams of calcium and 400 IU of vitamin D daily.

Over about seven years of follow-up, the women taking the supplements who had had previous non-melanoma skin cancer reduced their risk of developing melanoma by 57 percent, compared with similar women not taking the supplements.

The melanoma risk reduction was not seen among women who had not had an earlier non-melanoma skin cancer, the study authors noted.

Overall, only 176 cases of melanoma developed, said the researchers.

In the United States, more than 68,000 cases of melanoma are diagnosed in adults each year, according to the U.S. National Cancer Institute.

Hoping to uncover why vitamin D and/or calcium may be beneficial, Tang said the team next intends to test the compounds directly on cancer cells.

Commenting on the study, Dr. Michael Holick, professor of medicine, physiology and biophysics at Boston University School of Medicine, said a lot of sun exposure early in life increases the risk for non-melanoma skin cancer, but may actually lower the risk of developing melanoma. Sunlight is a source of vitamin D.

"Melanoma is a different story. Being exposed to sunlight, making some vitamin D may very well be protective of melanoma," Holick said. "The thinking is, improving your vitamin D status, whether by supplements or by exposure to sunlight, you are providing your skin cells with a mechanism to prevent them from becoming malignant," he said.

What role calcium may play is unknown, Holick said. "We don't know whether vitamin D can have its effect in the absence of calcium or vice versa; there's rationale for both," he said.

Holick said he thinks the finding would be the same for men and other groups.

People can get their vitamin D from diet, sun exposure and supplements. Fatty fish and fortified dairy products are two dietary sources of vitamin D.

Holick said he recommends that children take 1,000 IU of vitamin D a day and adults, 2,000 IU.

More information

For more information on skin cancer, visit the U.S. National Library of Medicine.