The researchers aren't saying the fruits and vegetables have no effect. "Fruits and vegetables are likely to be protective, although the effect is not likely to be large," says study author Paolo Boffetta, MD, MPH, deputy director of The Tisch Cancer Institute at Mount Sinai School of Medicine in New York.
But he hastens to add that eating plenty of fruits and vegetables is good for a number of other reasons, such as reducing the risk of cardiovascular disease. His study looked at the big picture, he tells WebMD, and so it's still possible that specific fruits and vegetables, or substances in them, could be more cancer-protective.
Fruits, Vegetables, and Cancer Risk: The Study
''In the past, there was a strong belief that fruits and vegetables were strongly protective against cancer," Boffetta says. "In the last 10 or 15 years there have been a number of studies that did not confirm this relationship."
So Boffetta and his colleagues analyzed data from the large EPIC study (European Prospective Investigation Into Cancer and Nutrition). It included more than 142,000 men and 335,000 women from 10 Western European countries, evaluated between 1992 and 2000.
The researchers looked at detailed dietary information and asked about alcohol intake, smoking, and other lifestyle habits.
After a median follow-up of nearly nine years, Boffetta's team looked at the association between fruit and vegetables and cancer risk and focused on the effect of increasing intake.
More than 30,000 participants learned they had cancer during the follow-up period.
Fruits, Vegetables, and Cancer Risk: Results
The median intake of fruits and vegetables was 335 grams a day, the equivalent of about two and a quarter apples. "One apple or one fruit is about 150 grams," Boffetta says.
They looked at the effect of increasing intake. "The more you eat, the more protective," Boffetta tells WebMD, "but the magnitude of this effect is very weak."
For instance, he says, those who increased their intake by about 200 grams a day, or about 1.5 servings a day, had a 3% or 4% reduced risk of getting cancer.
The effect was weaker for fruits when considered alone than for vegetables, he says.
Boffetta is not certain whether the findings hold for a U.S. population, but he speculates they probably do.
It's also possible that certain substances in specific fruits or vegetables may be more protective, Boffetta says. Lycopene, a substance found in tomatoes, for instance, has been found to reduce prostate cancer risk.
"Our purpose was really to look at the big picture," he says. "It's a little diluted when you look at the big picture."
Fruits, Vegetables, and Cancer Risk: More Opinions
Enthusiasm for fruits and vegetables to protect against cancer swelled in the 1990s, when some experts expected a diet rich in fruits and vegetables would reduce cancer risk as much as 50%, says Walter Willett, MD, DrPH, chairman of the department of nutrition at the Harvard School of Public Health, in an editorial accompanying the study.
The National Cancer Institute's 5-a-Day program was then developed in 1991.
But later studies didn't confirm such a strong link, he says.
And the new study findings, Willett says, "add further evidence that a broad effort to increase consumption of fruits and vegetables will not have a major effect on cancer incidence."
Even so, he says, a diet rich in fruits and vegetables will reduce the risk of cardiovascular disease, and ''a small benefit for cancer remains possible."
It is still possible, he agrees, that certain substances in fruits and vegetables or specific fruits and vegetables will be found to have a stronger protective effect against cancer.
The study findings are no reason to cut back on fruits and vegetables, says Connie Diekman, RD, director of university nutrition at Washington University in St. Louis and past president of the American Dietetic Association.
"While we may not understand what all fruits and vegetables can do to help prevent disease and promote health, inclusion of more fruits and vegetables can aid satiety [feeling full and satisfied], help reduce calorie intake, and certainly boost overall nutrition," she says.