Sept.25, 2007 -- Eating a diet with plenty of fruits and vegetables doesn't appear to strongly reduce colon cancer risk, but it may help protect slightly against getting one type of colon cancer, according to a new analysis.
Researchers pooled the results of 14 studies that included more than 750,000 men and women followed for six to 20 years, hoping to resolve the ongoing debate of whether fruits and veggies significantly reduce the risk of colon cancer. Studies have produced conflicting findings.
"Overall, a higher intake of fruits and vegetables does not strongly reduce your risk of colon cancer," says Anita Koushik, PhD, a research scientist at the University of Montreal in Canada, and a study author. She conducted the research while a research fellow at the Harvard School of Public Health in Boston. "But we did see some suggestion that the lowest intake of fruits and vegetables might be associated with an elevation in risk."
And when the researchers looked at colon cancer by site, some benefit in risk reduction was found in those who ate a diet high in fruits and vegetables, she says. "What we found was there was a statistically significant reduction in distal colon cancer risk with an increased intake of fruits and vegetables." The distal colon is the left-hand side of the colon, including the rectum, sigmoid colon, and descending colon.
"Eating fruits and vegetables does not appear to be as protective as researchers thought," says Marji McCullough, RD, ScD, strategic director of nutritional epidemiology for the American Cancer Society in Atlanta, and a study co-author. But, she adds, "there might still be something going on. It looks weakly related."
The researchers classified fruit and vegetable intake into five categories, from lowest -- those who averaged less than 200 grams or about two servings a day -- to highest -- those who ate 800 grams or more per day, or about eight or more servings.
During the follow-up periods of the 14 studies, 5,838 men and women were diagnosed with colon cancer. When the researchers looked at the fruit and vegetables intakes, they found only weak links to colon cancer risk reduction.
"People in the highest vs. lowest intake of fruits and vegetables had a 9% lower risk of colon cancer overall, but it did not reach statistical significance," McCullough says.
When the researchers evaluated the association by site of the cancer, they found those who ate the highest amounts had a 26% reduced risk of distal colon cancers. "For proximal colon cancer (the rest of the colon) there was no association," Koushik says. "Even though we saw this [positive] association with distal colon cancer [risk], the differences between distal and proximal were not statistically significant."
The study is published in the Oct. 3 issue of the Journal of the National Cancer Institute.
The analysis of the 14 studies may have some inherent flaws, says Mansi Shah, RD, a clinical dietitian at the Outpatient Cancer Center at the Samuel Oschin Comprehensive Cancer Institute at Cedars-Sinai Medical Center, Los Angeles, who reviewed the findings for WebMD.
The diet was self-reported. "People can forget about what they eat, and they can overestimate," Shah says.
Although the link between fruit and vegetable intake and colon cancer was weak, Shah says a diet rich in fruits and vegetables is still recommended for a number of other reasons. Eating a lot of fruits and vegetables, she says, may be associated with a healthier diet overall.
"Most people who are consciously eating healthy, eating a lot of fruits and vegetables, are the ones who say they try to limit their intake of fat and refined carbohydrates," she says.
What to Do?
McCullough and Koushik agree. Eating plenty of fruits and vegetables might help people avoid other unhealthy foods known to raise colon cancer risk, says McCullough. Such as? "Red and processed meats are consistently linked with an elevated risk of colon cancer," she says.
In 2007, about 112,340 new cases of colon cancer and 41,420 new cases of rectal cancer are expected in the U.S., according to the American Cancer Society. An estimated 52,180 deaths from colorectal cancer are expected this year.