Vitamin D, cereal fiber shown to cut tumor risk; smoking strongly linked to higher risk
POTRLAND, Ore. – In one of the most comprehensive studies to date on colon-cancer risk, a team led by Veterans Affairs (VA), National Cancer Institute and Harvard researchers confirmed that proper intake of cereal fiber and vitamin D are associated with reduced risk of serious colon polyps that may lead to the disease. The study appears in the Dec. 10 Journal of the American Medical Association.
The study, which included more than 3,000 veterans at 13 VA medical centers, provides the most concrete evidence yet that vitamin D–the “sunshine vitamin–may play a role in the prevention of colorectal cancer.
“The finding that may surprise the scientific community is the vitamin D data,” said lead investigator David Lieberman, MD, chief of gastroenterology at the Portland VA Medical Center and Oregon Health and Science University. ” 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.”
In the study, men who consumed higher amounts of cereal fiber–more than about 4 grams per day–and vitamin D–more than 645 international units (IUs) per day–were significantly less likely to have serious colon polyps, or tumors, which are often the precursor to cancer. Another significant association with reduced risk was the use of non-steroidal anti-inflammatory drugs.
Men who took a daily aspirin were about two-thirds as likely to have a tumor. Lieberman said further study is needed before recommending this solely for protection against colon tumors, due to the potential for side effects over a lifetime of anti-inflammatory consumption.
Exercise, calcium, folic acid and multivitamins were shown in the study to be marginally beneficial in lowering risk.
In looking at factors that increase the risk of colon cancer, the research poses yet another reason to quit smoking: Smoking increased by nearly twofold the risk of having a tumor or benign polyps, which often become cancerous. Having a close relative with colorectal cancer also elevated risk, but not as much as smoking. The consumption of red meat and alcohol were associated with a slightly higher risk. Body weight and cholesterol proved unrelated to cancer risk in this study.
Cancer of the colon or rectum is the second deadliest form of cancer. About 55,000 Americans die from the disease each year. It is also considered one of the most preventable types of cancer, as there are several dietary factors that appear to play a protective role. Doctors recommend a diet rich in fruits, vegetables, and fiber from whole grains, and low in animal fat, especially from red meat.
Nutrients that appear key in this diet include calcium, vitamin D, folic acid–a B vitamin–and antioxidants such as vitamin E and selenium.
Sorting out the relative importance of these nutrients has remained a challenge for researchers. Lieberman said his new study is the largest ever to analyze a wide range of likely risk factors in patients with no history of colon disease. The study included 3,121 veterans, ages 50 to 75, who were symptom-free and underwent complete colonoscopies between 1994 and 1997.
Lieberman said the findings support current dietary guidelines for the prevention of colorectal cancer. “These data support relatively simple and safe recommendations that may reduce the risk of colon cancer,” said Lieberman. “Stop smoking, reduce alcohol and red meat consumption, take a multivitamin, exercise regularly, and consume vitamin D, calcium and cereal fiber in your diet.” He added that people with a family history of colon cancer should be “screened intensively” and should consult their physicians for advice.
Lieberman cautioned against over-consuming 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. A tablespoon of cod liver contains 1,360 IUs, and milk is fortified with 400 IUs per quart.
Exposure to sunlight triggers vitamin D production in the body, but researchers are unsure how this interacts with dietary intake to provide adequate levels for cancer protection. Lieberman said his findings should prompt research to determine how regular exposure to sunlight affects the risk of colon cancer.
The sunshine vitamin made headlines in May 2002 when scientists showed in an animal study that the nutrient works to prevent colon cancer by detoxifying the body’s own digestive products. But vitamin D has generally been overshadowed as a colon-cancer protector by calcium, which vitamin D helps the body use. In a 1999 study of 832 patients, calcium supplements had a modest protective effect against colorectal tumors. In Lieberman’s study, calcium alone showed only a minor beneficial effect.
Importantly, the new study validates the importance of early screening for colorectal cancer: More than 1 in 10 volunteers showed evidence of an advanced tumor, even though they had no history of colon trouble and no symptoms prior to the test. In a July 2000 article in the New England Journal of Medicine based on the same set of data, Lieberman’s team made the case for colonoscopy over sigmoidoscopy as the primary screening tool for colorectal cancer. In that analysis, sigmoidoscopy–a less extensive but frequently used test that examines only the lower part of the colon–missed a third of the serious lesions detected by colonoscopy.
Lieberman’s research was funded by VA’s Cooperative Studies Program, and involved collaborators from VA, the National Cancer Institute and Harvard School of Public Health.
David Lieberman, M.D., chief of gastroenterology at the Portland VA Medical Center, is the lead investigator for this study.