Loading up on fruits and vegetables led to a "very small" effect on cancer risk, according to an analysis of data on the eating habits and health of almost 500,000 people in Western Europe.
Most of those marginal benefits were limited to women.
An increase in fruits and vegetable consumption of about seven ounces a day reduced cancer risk by 3 percent during a median follow-up of almost nine years. A 3.5 ounce per day increase in total vegetable intake reduced cancer risk by 2 percent.
The findings suggest that authorities should be a bit more circumspect about promoting fruit and vegetable intake to reduce cancer risk, according to an article in the April 21 issue of the Journal of the National Cancer Institute.
"Our study supports the notion of a modest cancer preventive effect of high intake of fruits and vegetables and we can exclude chance as a likely factor," Dr. Paolo Boffetta of Mount Sinai Medical Center in New York and colleagues wrote.
"Nevertheless, the observed association of cancer risk overall with vegetable and fruit intake was very weak, and we cannot entirely rule out the possibility of residual confounding by these or other factors," they added.
"Given the small magnitude of the observed associations, caution should be applied in interpretation of the results."
Multiple health organizations worldwide recommend consumption of five servings of fruit and vegetables daily -- about 14 ounces total -- to ward off heart disease, cancer, and other conditions.
However, no study has demonstrated conclusively that a high intake of fruit and vegetables reduces the risk of cancer, the authors wrote.
On the basis of data from more than dozen studies, the international Agency for Research on Cancer has classified evidence of a chemopreventive effect as limited.
Most studies have focused on fruit and vegetables' chemopreventive effects on specific types of cancer, the authors continued. The few studies that have examined overall cancer incidence produced mixed results.
In an effort to clarify the issue, Boffetta and coauthors reviewed data from the European Prospective Investigation into Cancer and Nutrition (EPIC).
The study examined associations between diet and cancer in 10 Western European countries with widely varying intake of fruit and vegetables.
The analysis included about 480,000 adults ages 25 to 75, about two-thirds of them women. During a follow-up, the researchers found that 9,604 men and 21,000 women had cancer diagnoses. They also found that fruit and vegetable intake and total vegetable intake had little effects on cancer risk across the entire study population.
Increasing fruit and vegetable intake by seven ounces a day was associated with 3 percent reduction in the risk of developing any type of cancer, while an increase of 3.5 ounces per day decreased cancer risk by 2 percent. Total fruit intake did not have a significant impact on cancer risk.