Low levels of circulating vitamin D are associated with a greater risk of colorectal cancer, European researchers say.

A case-control study found that people in the top fifth of vitamin D levels had 40 percent lower risk for colorectal cancer than those in the bottom fifth, according to Mazda Jenab of the International Agency for Research on Cancer in Lyon, France, and colleagues.

But more research is needed to see if increasing circulating vitamin D can effectively reduce the risk of the disease, they concluded online in the British Medical Journal.

The main role of vitamin D is to maintain calcium levels in cells and bone metabolism, the researchers noted in the journal. But there is some evidence that it may also play a role in cancer control by modulating cell growth and death and by reducing the development of blood vessels to support tumor tissue.

Unfortunately, they said, epidemiological evidence -- mostly based on counting people's dietary intake of vitamin D rather than taking blood circulating levels -- is inconclusive.

To fill the gap, they turned to the European Prospective Investigation into Cancer and Nutrition (EPIC) study, which is prospectively following 520,000 volunteers from 10 European countries.

Jenab and colleagues compared 1,248 people who developed colorectal cancer after enrollment with 1,248 controls who were disease-free.

Dietary and lifestyle data were obtained from questionnaires and researchers measured circulating vitamin D concentrations.

The researchers found:

There was a linear trend for colorectal cancer. Those in the lower fifths of vitamin D levels significantly more likely to get the disease than those in higher fifths.

The same trend was seen for colon cancer but not rectal cancer.

Compared with volunteers with a mid-level concentrations, those in the lowest fifth had a 32 percent increased risk of colorectal cancer.

Higher concentrations were associated with lower risk.

The vitamin D concentration was associated with lower colorectal risk in a dose-response manner, the researchers reported.

The study's strengths include its large size and prospective (following a group of people before any got sick) design, the researchers said. Also levels of circulating vitamin D before a diagnosis of cancer were available for all participants.

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