Proper intake of vitamin D and cereal fiber can help reduce the risk of serious colon polyps that can progress to colon cancer, according to one of the most comprehensive studies yet done of risk factors for the disease.
At the same time, smoking, drinking moderate or heavy amounts of alcohol and having a close relative with colon cancer are the top risk factors identified in the study, published today in The Journal of the American Medical Association.
The study included more than 3,000 veterans at 13 VA medical centers around the country.
"The finding that may surprise the scientific community is the vitamin D data," said Dr. David Lieberman, lead investigator for the study. "Higher levels of vitamin D intake were associated with a lower risk of serious colon polyps. There have been some studies suggesting this, but our data are compelling." Lieberman is chief of gastroenterology at the Portland, Ore., VA Medical Center and the Oregon Health and Science University.
The study included veterans between the ages of 50 and 75 who were symptom-free when they underwent colonoscopies between 1994 and 1997. A total of 329 were found to have advance polyps or tumors, the rest had none. Specifically, men who ate higher amounts of cereal fiber - more than about 4 grams a day - and more than 645 international units a day of vitamin D were significantly less likely to develop colon polyps, registering a 5 percent to 6 percent lower risk.
Men who took a daily aspirin were about two-thirds less likely to have a tumor, but Lieberman said more study is needed before this strategy can be recommended, given the potential for side effects from a lifetime of anti-inflammatory consumption.
Smoking increased the risk of developing polyps by 85 percent, while having a close relative drove up the risk by 66 percent and moderate-to-heavy alcohol use increased risk by about 2 percent.
Researchers recommend a diet rich in fruits, vegetables and fiber from whole grains, and low in animal fat, especially from red meat. Nutrients that seem to be important in this diet, in addition to vitamin D, are calcium, folic acid and antioxidants such as vitamin E and selenium.
Lieberman said the findings support these dietary guidelines as a "low-risk strategy," including taking a daily multivitamin. But he cautioned against overconsumption of vitamin D, which can be toxic in high amounts and cause nausea, constipation, weakness and other symptoms. Among the best food sources are cod liver oil, salmon, mackerel, sardines and fortified milk.
Exposure to sunlight triggers vitamin D production in the body, but researchers are unsure how this source interacts with dietary intake of the vitamin to influence cancer risk. Lieberman said there needs to be more research to determine how regular exposure to sunlight affects the risk of colon cancer.