Taking vitamin D may help protect women who have already had non-melanoma skin cancers against a much deadlier form of the disease, suggests a new study.
But researchers caution that the results need to be confirmed with further studies, given that the number of women in their study who got melanoma - the most dangerous type of skin cancer - was low to begin with.
"We're not recommending super high doses" of vitamin D, study author Dr. Jean Tang, from Stanford University School of Medicine in Redwood City, California, told Reuters Health.
However, she added, "I feel good about saying if you've already had a non-melanoma skin cancer, you're already at risk for developing melanoma in the future, (and) taking a little bit of calcium and vitamin D, while other studies need to be done, seems reasonable and not harmful."
Some evidence has suggested that vitamin D might help protect against skin cancer and other cancers by influencing cell growth, and perhaps stopping cells from turning into cancer cells, researchers explained.
Tang and her colleagues wanted to see if that might be the case in a group of women participating in the Women's Health Initiative trial, a study whose main focus was to look at the effects of diet and hormone therapy on disease risks.
The researchers looked back at data that had been collected on about 36,000 women between the ages of 50 and 79. Half of those women took supplements with 1,000 milligrams of calcium and 400 international units of vitamin D3 each day, while the other half took an inactive placebo supplement.
Using questionnaires and reports from doctors' visits, the researchers were able to track how many women got skin cancer over the next 7 years, on average.
They found no difference in how frequently women in either group were diagnosed with non-melanoma skin cancers or with melanoma, according to the findings published in the Journal of Clinical Oncology.
In all, about 1,700 women in each group were diagnosed with non-melanoma skin cancer, while 82 women taking calcium and vitamin D and 94 in the placebo group got melanoma.
However, women who reported previously having non-melanoma skin cancer - which would mean they were at higher risk for getting melanoma later - were less likely to get melanoma if they were taking the extra calcium and vitamin D.
The overall numbers were small - 10 women out of about 1,100 with a history of non-melanoma skin cancer got melanoma in the supplement group, compared to 24 out of a similarly-sized placebo group.
Tang said she expects any link between the supplement combo and skin cancer would probably be a result of vitamin D.
The Institute of Medicine recommends that most adults get 1,000 to 1,200 mg of calcium per day and 600 to 800 IU of vitamin D. It sets a recommended upper limit at 2,000 mg of calcium and 4,000 IU of vitamin D.
Getting too much vitamin D is rare, but has been linked to kidney stones and an increased risk of other liver and kidney conditions.
Despite the limited nature of this particular study, "I think there is no reason for women not to increase their vitamin D intake," said Dr. Michael Holick, who studies vitamin D and calcium at Boston University and was not involved in the new study.
Holick told Reuters Health that vitamin D's role in reducing the risk of colorectal and breast cancer is "pretty compelling," and that it could potentially protect against other diseases - such as type 2 diabetes and infectious diseases - as well. There is also good evidence that both calcium and vitamin D can help prevent osteoporosis.
The link between vitamin D and cancer is a controversial one, said endocrinologist Dr. Joan Lappe, of Creighton University School of Medicine in Omaha, Nebraska - and especially for skin cancer, there just isn't enough evidence to make a claim about its benefits, she said.
"I think we're going to see that vitamin D does have an effect on cancer, but the short answer is there are not enough rigorous trials out there," Lappe, also not linked to the new research, told Reuters Health.
And because the Women's Health Initiative study used relatively low doses of vitamin D, it's still possible that higher doses will make a difference for skin cancer risk in a wider population, she said.
Tang said that her team is now recruiting women for another study on the link between vitamin D and skin cancer using higher vitamin doses to see if the connection holds.
Until more evidence is available, Lappe concluded, "I think everyone should be aware that vitamin D is important for your health and either take some supplements or talk to their doctor about it."