Higher levels of vitamin D in the blood may help protect both men and women from cancers of the colon and rectum, confirm results of the largest study ever conducted on the topic.

Among more than 1200 people who developed colorectal cancer and an equal number who did not, researchers found that those with the highest levels of vitamin D in their blood had a nearly 40 percent reduced risk of developing colorectal cancer compared to those with the lowest levels.

The findings from the EPIC study - short for European Prospective Investigation into Cancer and Nutrition - confirm previous findings from smaller studies conducted largely among North American populations.

The EPIC findings “support a role for vitamin D” in the causes of colorectal cancer, EPIC investigator Dr. Mazda Jenab of the International Agency for Research on Cancer in Lyon, France, told Reuters Health.

“But this has to be balanced with caution regarding the potential toxic effects of too much vitamin D and the fact that very little is known about the association of vitamin D with either increased or reduced risk of other cancers,” Jenab said.

EPIC coordinator Dr. Elio Riboli of Imperial College, London, added: “There is consistent scientific evidence that low circulating vitamin D concentration is a marker of increased risk for developing colon cancer.”

Still, Riboli said, public health advocates should wait for more studies before recommending vitamin D supplements, as people can achieve average levels “with a balanced diet combined with regular and moderate exposure to outdoor sunlight.”

Ideally, these trials would shed light on whether vitamin D supplementation would cut the risk of colorectal cancer risk without inducing serious side events, Riboli explained.

The EPIC study enrolled more than half a million adults from 23 centers in 10 western European countries who were initially free of cancer. After several years of follow up, 1248 subjects developed colorectal cancer and these individuals were matched to 1248 study participants who did not develop colorectal cancer.

Prior to cancer diagnosis, the researchers collected detailed diet and lifestyle information and determined blood levels of 25-hydroxy vitamin D, which is a marker of the amount of the vitamin people get from diet, supplements and sun exposure.

Based on the design of the study, the authors did not calculate the risk of cancer for people with various levels of vitamin D. However, the overall rate of cancer in the group was .06 percent, or about one in 1,700 people, per year.

The researchers report in the British Medical Journal that people with the highest pre-cancer circulating vitamin D levels (100 nanomoles per liter of blood or higher) were about 40 percent less likely to develop colorectal cancer than those with the lowest levels (less than 25 nanomoles per liter).

There is currently no absolute consensus on optimal vitamin D levels. Some recent studies have suggested that, for colon cancer prevention, blood vitamin D levels should be maintained at a mid-level range - 50 nanomoles per liter or higher.

With this in mind, the EPIC researchers compared low and high blood vitamin D levels to a mid-level (50 to 75 nanomoles per liter). They found that while levels below the mid-level were associated with a higher risk, those above 75 nanomoles per liter were not associated with any additional reduction in colon cancer risk compared to the mid-level.

Jenab said this suggests that blood levels of vitamin D greater than 75 nanomoles per liter “may not be any more protective than the mid-level. This is important because the health effects of long term (greater than 1 year) exposure to high blood levels of vitamin D are currently largely unknown.”

A bigger issue, at the moment, may be inadequate vitamin D levels. “As more information emerges, it is becoming apparent that blood vitamin D levels in Europe and North America tend to be rather low with large proportions of the population likely falling into categories defined as deficient or insufficient,” Jenab said.

Until further study on the risks and benefits of vitamin D supplementation are available, Jenab said, “the key take home message for colorectal cancer prevention is: stop smoking, increase physical activity, reduce obesity and abdominal fatness, and limit intakes of alcohol and red and processed meats.”

SOURCE: British Medical Journal Online First, Jan 22, 2010.