MONDAY, Aug. 15 (HealthDay News) -- The higher a person's vitamin D levels, the higher the risk of non-melanoma skin cancer, such as basal cell carcinoma and squamous cell carcinoma, finds new research.

But the study, appearing in the Aug. 15 issue of the Archives of Dermatology, stops short of saying that high vitamin D levels might actually cause these types of cancer, the most common malignancies in the United States.

And because ultraviolet (UV) radiation exposure is necessary for vitamin D production in the body, it might simply mean that people with more sun exposure tend to develop more non-melanoma skin cancers. It's unclear whether it's the damage from UV rays that accounts for the risk, or rising vitamin D levels that accompany exposure to the rays.

"This adds to the murky water [surrounding the relationship between vitamin D and skin cancer]," said Dr. Vijay Trisal, assistant professor of surgical oncology at City of Hope Cancer Center in Duarte, Calif. "Is it vitamin D or sun exposure? The two go hand-in-hand."

Other scientists have investigated a possible relationship between vitamin D and skin cancer, but so far the results have been limited and conflicting.

One study suggested that higher vitamin D levels might actually protect against skin cancer. This could be because vitamin D may inhibit a pathway involved in cancer, said Dr. Melody Eide, a dermatologist with Henry Ford Hospital in Detroit, and lead author of the current study.

But two other studies had results suggesting the opposite.

Eide and colleagues based their findings on 3,223 mostly female, white patients in a Detroit health maintenance organization who had visited a doctor either because they had osteoporosis or low bone density.

Many more patients (2,257) had too-low levels of vitamin D than had adequate levels (966).

Over a follow-up period of almost 10 years, 163 participants developed basal cell carcinoma, 49 developed squamous cell carcinoma, and 28 developed both.

Those with vitamin D levels above a certain threshold had a 70 percent greater risk of developing one of these cancers. (That threshold was 15 nanograms per milliliter; people with less than that were considered deficient in vitamin D.)

People with higher vitamin D levels also tended to develop their skin cancer on parts of the body not typically exposed to sunlight, like the arms and legs, but that finding was not statistically significant, the researchers reported.

At this research stage, it's difficult to untangle the possible mechanisms behind this.

"It's a triangular relationship between UV light with the production of Vitamin D and the induction of skin cancer," Eide said. "That makes it difficult to know."

The study didn't take into account lifetime sun exposure, family history of skin cancer, vitamin D supplementation, exercise, smoking or several other factors that might have influenced the outcome of the study. In addition, the study authors noted that it was "highly likely" that the participants' exposure to sunlight might have skewed the results.

"We need some measure of lifetime cumulative UV exposure, which is very difficult to measure," Eide said. "We tend to move around a lot; people go on vacations. There could be critical windows during our life."

More information

The Skin Cancer Foundation has more on all forms of skin cancer.