Numerous early studies revealed what appeared to be a strong link between eating fruits and vegetables and protection against cancer. But because many of these were case-control studies, it is possible that the results may have been skewed by problems inherent in these types of studies, such as recall bias and selection bias. Data from cohort studies that follow large groups of initially healthy individuals for years have not consistently shown that a diet rich in fruits and vegetables prevents cancer in general. Data from the Nurses' Health Study and Health Professionals Follow-up Study support this finding. Over a 14-year period, men and women with the highest intake of fruits and vegetables (8+ servings a day) were just as likely to have developed cancer as those who ate the fewest daily servings (under 1.5).(2)

A more likely possibility is that fruits and vegetables may protect against certain cancers. The International Agency for Research on Cancer, which is part of the World Health Organization, recently completed a monumental review of the best research on fruits, vegetables, and cancer. Here's what this 387-page tome concludes about studies in humans: "There is limited evidence for a cancer-preventive effect of consumption of fruit and of vegetables for cancers of the mouth and pharynx, esophagus, stomach, colon-rectum, larynx, lung, ovary (vegetables only), bladder (fruit only), and kidney. There is inadequate evidence for a cancer-preventive effect of consumption of fruit and of vegetables for all other sites." (5) However, considering all evidence from human epidemiological, animal, and other types of studies, it appears that eating more fruit "probably lowers the risk of cancers of the esophagus, stomach and lung" and "possibly reduces the risk of cancers of the mouth, pharynx, colon-rectum, larynx, kidney, and urinary bladder." Eating more vegetables "probably lowers the risk of cancers of the esophagus and colon-rectum" and "possibly reduces the risk of cancers of the mouth, pharynx, stomach, larynx, lung, ovary and kidney."

Keep in mind that this is for total fruit and total vegetable consumption and that, as pointed out by the International Agency for Research on Cancer, specific fruits and vegetables may protect against specific types of cancer. For example, a line of research stemming from a finding from the Health Professionals Follow-up Study suggest that tomatoes may help protect men against prostate cancer, especially aggressive forms of it. (6-8) One of the pigments that give tomatoes their red hue - lycopene - could be involved in this protective effect. Although several studies other than the Health Professionals' study have also demonstrated a link between tomatoes or lycopene and prostate cancer, others have not or have found only a weak connection. Taken as a whole, however, these studies suggest that increased consumption of tomato-based products (especially cooked tomato products) and other lycopene-containing foods may reduce the occurrence or progression of prostate cancer. But more research is needed before we know the exact relationship between fruits and vegetables, carotenoids, and prostate cancer.(9)