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Fruits and Veggies Help Just a Little in Decreasing Cancer Risk

A large, eight-year study in Europe shows that eating the recommended servings of fruits and vegetables made people only slightly less likely to be diagnosed with cancer
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ISTOCKPHOTO/HANNAMARIAH

Despite decades of entreaties from the World Health Organization (WHO) and mothers alike to eat more fruits and vegetables, a new study has found that these dietary additions appear to do little to decrease the overall likelihood of getting cancer.

The recommendation that people eat at least five servings (about 400 grams) of fruits and veggies each day, espoused by the WHO since 1990, was based on studies that found a link between higher intakes of these foods and lower risks for cancer and other diseases.

Since the 1990s, however, evidence from large studies has been mounting that the protective effects of these foods against cancer in particular might be modest—if it exists at all. (Other research has continued to show that diets high in fruits and vegetables are important for preventing conditions such as obesity and cardiovascular disease.) A new report, analyzing cancer incidence in 478,478 men and women ages 25 to 70 over more than eight years in 10 European countries, found "a very small inverse association between the intake of total fruits and vegetables and cancer risk," the researchers concluded.

Among those who have had the highest relative benefit from high fruit and veggie intake were  smokers and heavy drinkers. They actually had a lower risk of getting the types of cancers they are prone to, such as respiratory and digestive ones, although their risk was still greater than their non-smoking and non-heavy-drinking counterparts. And although the men in the study seemed to have the lowest risk if they ate both fruits and vegetables, women appeared to also decrease their risk even if they only ate large amounts of vegetables. The researchers caution, however, that because the decreases in cancer rates, while statistically significant, were so small, the results should be used carefully.

About 6 percent of the people in the study were diagnosed with cancer during the follow-up period. The researchers concluded that if the results of the analysis can be broadly applied, upping daily fruit and vegetable consumption by about 150 grams (equivalent to about one cup of cherry tomatoes or 1.5 medium bananas), from most dietary levels, could prevent about 2.5 percent of all cancers in most populations. The results appeared online April 6 in the Journal of the National Cancer Institute.

The results are in line with other findings both in the U.S. and abroad that suggest the protective effect of fruits and vegetables is "much smaller than had been believed 10 years ago," Harvard School of Public Health's Walter Willett, who wrote an editorial accompanying the study, noted in an e-mail to ScientificAmerican.com. People who eat more fresh fruits and vegetables are also more likely to make other healthful lifestyle choices, such as exercising more and drinking and smoking less, which the researchers noted "may have contributed to a lower cancer risk" overall.

Counting cancers
To obtain their large sample, the researchers analyzed data collected for the European Prospective Investigation into Cancer and Nutrition (EPIC) project. Although the study was conducted throughout Europe, where residents of countries such as Greece and Italy are thought to have healthier diets to begin with, the researchers say that results would probably be similar if the analysis had been done in the U.S. "There is no reason to expect a different effect in the U.S. vs. Europe for a comparable level of consumption," Paolo Boffetta, of the Mount Sinai School of Medicine in New York and lead researcher on the study, wrote in an e-mail to ScientificAmerican.com. Those in the study reported that they ate a median of 335 grams of fruits and vegetables each day. Bnd if people in the U.S. ate fewer fruits and vegetables overall, he notes, "the magnitude of the effect would be seemingly lower."

Large cancer risk studies have increasingly been designed as so-called prospective studies, like this one. In a prospective study, participants are followed over time (e.g. across five years), and baseline information is compared with later follow-ups. Much of the older research—including studies that found more of a protective effect of fruits and vegetables—was done using a case-control method, in which one group (the "cases") already has a condition and is polled about past behaviors and compared with a population that does not have the condition (the "controls"). Many researchers fault case-control work as being a weak method for showing direct causality, especially because it is subject to substantial recall bias.

As Willett noted in his editorial, "This study strongly confirms the findings from other prospective studies that the results of case-control studies were overly optimistic and that any association of fruits and vegetables with risk of cancer is weak at best."

One of the downsides of the new analysis, however, is that researchers only obtained dietary information at the start of the study, with cancer information being gathered through direct patient follow up or official records. This means that if individuals changed their diet much between 1992 and 2000—even if they started eating loads of fruits and vegetables the month after the study started—those changes are not reflected in the final analysis. Boffetta explains that this aspect of the study design could "be an explanation of the weak effect we found. If we had been able to measure properly changes in diet, the measured protective effect might have been stronger."

Growing fruit and vegetable consumption
Despite the modest cancer protection that fruit and vegetable consumption showed in this study, Boffetta is not convinced that these foods should be overlooked in the fight against cancer. Fruit and vegetable consumption earlier in life might play a role in establishing overall lifetime cancer risk, he notes. "Ideally, one would like to measure diet at different points in time: during adolescence, young adulthood, etc. This would allow a more valid estimate of the association between different dietary factors (not only fruits and vegetables) and cancer risk," he explains. Such long-term, detailed studies, however, are difficult to execute given the need for substantial follow-up times.

Willett agrees that longer-term research is needed. "We know from other aspects of cancer epidemiology that many risk factors act most importantly during childhood and early adult life," he notes. "We are only beginning to explore these dimensions of the relation of diet to risk of cancers."

Although the new study's findings cast yet more doubt on zealous claims that fruits and veggies can stave off cancer, experts reiterate the importance of the foods to help prevent other ills, such as cardiovascular disease. Boffetta does not think the study results should prompt any changes in daily fruit and vegetable intake suggestions, saying that, "recommendations should take all health aspects into account, not just cancer risk."

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