Most studies suggest they do to at least a moderate extent - particularly vegetables. But, two studies reported together in the November 1, 2000 issue of the Journal of the National Cancer Institute find no benefit at all. The Nurses’ Health Study and the Health Professionals Study together followed about 135,000 women and men over a period of ten to sixteen years during which time 1,181 cases of bowel cancer occurred. So, it is a very large study. The investigators found absolutely no benefit from even as much as six or more servings of fruits and vegetables in the daily diet.
The authors acknowledge that between two-thirds and three-quarters of other studies do show reduced risk from eating more fruits and vegetables as a group or from specific vegetables, but they argue theirs are particularly good studies that are supported by other good studies.
They conclude "high consumption of fruit and vegetables did not appear to be protective against cancers of the colon or rectum in our large U.S. cohort".
Commentary: These are well-conducted studies by good investigators. Does this mean that including six to eight or more portions of fruits and vegetables in your daily diet is definitely useless in helping prevent bowel cancer? The answer is absolutely not.
Many other studies do find evidence that vegetables (more than fruits) reduce the risk of bowel cancer. Additionally, these investigators have published the results of a study on the same subjects saying folic acid reduces bowel cancer risk. Fruits and vegetables are a very good source of folic acid. To get around that apparent contradiction, the authors say that other sources of folic acid, including vitamin supplements, are more important sources than fruits and vegetables. They may well be correct, but the apparent contradiction with their own previously published study is still a bit disturbing.
At present, in part because of the weakness of the Food and Drug Administration, the public is going to be deluged by claims of health promotion-disease prevention for specific foods, particularly fruits and vegetables. Most of those claims are exaggerations and are inadequately documented.
This article, among others, tells us we had better be cautious about claims relating to health benefits of fruits and vegetables. For example, the claim is made that one-third of the overall cancer (and one-half or more of the risk for bowel, breast, and prostate cancer) risk can be ascribed to diet. That claim is, at present, inadequately documented.
Nevertheless, fruits and vegetables, if taken in good amounts (at least four to six servings of fruits and vegetables combined each day) do appear to offer modest protection against some cancers, coronary heart disease, and stroke.
Multiple servings a day of fruits and vegetables should be part of the prudent diet; whether they specifically reduce the risk of bowel cancer is not clear.