Proving that fruits (and vegetables) help prevent cancer has been particularly difficult—and not for want of trying. You may be surprised to know that there isn’t actually much evidence that produce protects against cancer overall, and when studies do find benefits, they tend to be minimal.
It takes many years for cancer to develop, and this makes it hard to identify causal or contributing factors, including diet. In retrospective diet studies, for example, people often do not accurately remember and report what they ate years ago. Moreover, cancer is not a single disease but rather a catchall term for more than 100 distinct conditions, and its various forms may respond differently (or not at all) to dietary factors.
Consider also that fruits and vegetables are not equivalent in their anti-cancer effects; some may be more protective than others, and specifically for certain cancers. Thus, observational studies that look at overall produce intake are likely to find little or no influence on cancer risk, because any effects from protective fruits and vegetables would be obscured by those that are not protective. Another complication: Genetic factors may modify the association between fruit and vegetable intake and cancer. In addition, what you ate when you were young may play an outsized role in preventing or promoting cancer.
What to do: The process that sets the stage for the development and progression of cancer is complex, and it is still likely that diet plays some role, at least for certain cancers in certain people. Inconsistent study results should not dissuade you from eating a variety of fruits and vegetables. At the very least, a high intake of produce will crowd out foods that may increase the risk of cancer (such as processed meats), while also helping to prevent cardiovascular disease and other conditions.