Although previous studies have produced conflicting findings about the effectiveness of such a diet, the new research found a benefit.
"We found that eating a largely plant-based diet with higher intakes of fruits, vegetables, whole grains, nuts, seeds, vegetable oils, and low-fat dairy in women and fish in men was associated with a reduced risk of colorectal cancer," says Paige Miller, PhD, a researcher at Pennsylvania State University.
Eating in this healthful way reduced the risk of colon cancer by 65% in women and by 62% in men, she says. "Why fish was a part of the protective dietary pattern only in men and low-fat diary only in women is not known at this time," Miller tells WebMD.
The study is published in the Journal of Nutrition.
About 147,000 new cases of colorectal cancer were diagnosed in 2009 in the U.S., according to the American Cancer Society.
Diet and Colorectal Cancer: The Study
Miller and her team evaluated the diets of 431 men and women with colorectal cancer and the diets of 726 healthy men and women who didn't have colon cancer.
They categorized the participants into a fruits-and-vegetables diet pattern and a meat-potatoes-refined grains pattern. In men, a third pattern -- a diet rich in alcohol and sweetened beverages -- was found.
They also looked at how well participants followed the 2005 Dietary Guidelines for Americans and the MyPyramid recommendations, which suggest a diet rich in fruits, vegetables, and whole grains.
In addition to finding the reduced risk of colorectal cancer for people eating the diet heavy in fruits and vegetables -- 62% reduced risk for men and 65% for women -- Miller found that the more closely men and women adhered to the Dietary Guidelines, the lower the cancer risk.
Men and women with higher adherence to the guidelines had a lower risk of colorectal cancer, reducing it by 44% (men) and 56% (women).
Miller's advice? "Rather than focusing on a single food, nutrient, or other dietary component, focus on eating an overall plant-based diet that emphasizes fruits, vegetables, whole grains, nuts, seeds, and vegetable oil," she says.
The diet pattern associated with higher cancer risk in her study included greater intakes of red and processed meat, poultry, fried and white potatoes, high-fat dairy, sweets, salty snacks, butter, mayonnaise, gravy, pizza, and refined grains.
Diet and Colorectal Cancer: Second Opinion
"This is more data that a low-fat, low-meat, high-fiber diet is protective," says Donald David, MD, chief of the division of gastroenterology at the City of Hope Cancer Center, Duarte, Calif., who reviewed the study for WebMD. "More work needs to be done to find out which particular foods are protective."
"I think the take-home message for now is, adopting a diet [that is] good for us anyway is going to protect us from colon cancer to some extent."
Experts speculate that a diet emphasizing fiber-rich fruits, vegetables, and whole grains moves waste through the colon more quickly, giving harmful substances less time to damage cells there.
David calls the overall up to 65% reduction in colon cancer risk found in the new study "fairly significant protection."
Diet, he says, is just one factor that affects risk for colon cancer, of course. A history of bowel disease such as ulcerative colitis increases the risk of colorectal cancer, as does family history of the disease, according to the American Cancer Society.
Smoking, being very overweight, and excess alcohol use have also been linked to a higher colorectal cancer risk.
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