Low vitamin D levels may increase the risk of bladder cancer, researchers report.

Five of seven studies the researchers reviewed linked low vitamin D levels with an higher risk of bladder cancer.

In experiments with the cells that line the bladder, the researchers found that the cells activate and respond to vitamin D. This, in turn, can stimulate an immune response, the study authors said.

The finding is important because the immune system may help prevent cancer by identifying and destroying abnormal cells before they develop into cancer, according to lead author Rosemary Bland, an honorary associate professor at the University of Warwick in England.

"More clinical studies are required to test this association, but our work suggests that low levels of vitamin D in the blood may prevent the cells within the bladder from stimulating an adequate response to abnormal cells," Bland said in a university news release.

But, the study did not prove that vitamin D causes bladder cancer risk to drop.

"As vitamin D is cheap and safe, its potential use in cancer prevention is exciting and could potentially impact on the lives of many people," she said.

The body produces vitamin D through exposure to sunshine. It's also obtained from foods such as fatty fish and egg yolks. Previous studies have linked vitamin D deficiency with heart disease, mental impairment, autoimmune conditions and cancer, according to background information with the study.

The study was presented Monday at the annual meeting of the Society for Endocrinology, in Brighton, England. Research presented at meetings should be considered preliminary until published in a peer-reviewed journal.