WASHINGTON (AP) â€” A diet rich in fruits and vegetables helps protect against heart disease and diabetes, but it has no effect against colon and rectal cancer, according to a new study.
Harvard researchers report in the Journal of the National Cancer Institute that studies involving more than 136,000 health professionals who were repeatedly interviewed over 16 years found that eating fruits and vegetables had virtually no effect on the incidence of colon and rectal cancer.
This finding, to be published on Wednesday, is the opposite of dozens of studies over the last 20 years that reported some colorectal cancer protection from fruits and vegetables.
Karin B. Michels, a researcher at the Harvard Medical School and first author of the new study, said she was surprised at the results because she, too, thought that fruits and vegetables protected against colon cancer.
``When I reviewed the evidence (from past studies), I found it was not as positive as we had all thought,'' Michels said. ``When I did this study, I had assumed I would find a protective effect. Part of my expectation was based on these previous studies.''
Instead, she found that people who ate lots of fruits and vegetables were just as likely to get colon or rectal cancer as anybody else.
Michels said a close look at the earlier studies show that they were either based on the diet recollections of cancer patients, or found only a small suggestion of protection against colon or rectal cancer.
``If you look carefully, most of them didn't find much (protection),'' she said. The studies that did find some benefit, Michels said, were linked to only one type of food, such as garlic.
``If they found one food as protective, then the whole study was interpreted as being positive,'' she said. ``I think it has been overinterpreted.''
Other experts were quick to denounce the findings.
``It would be a mistake to interpret these results as anything but what they are: a single set of findings on an important topic that's attracting more and more scientific attention each day,'' Dr. John Potter, head of the Cancer Prevention Research Program at the Fred Hutchinson Cancer Research Center in Seattle, said in a statement.
Potter added: ``The public needs to keep in mind that this finding contradicts the bulk of available evidence on the link between high consumption of fruits and vegetables and lower risks of cancer.''
Michels said that the new study dealt only with colon and rectal cancer. She said fruits and vegetables have been proven to protect against heart disease, diabetes and some other forms of cancer, but not for colon or rectal cancer.
In the new study, researchers analyzed the dietary habits of 88,764 women in the Nurses Health Study, and 47,325 men in the Health Professionals Follow-up study.
These studies began collecting dietary and lifestyle data in 1980 and conducted follow-on diet questions periodically for 16 years. At the end of that time, there were 937 cases of colon cancer and 244 cases of rectum cancer.
The researchers then related the cancers to dietary habits and found that fruits and vegetables conferred no protection, Michels said.
All the people in the studies, she said, were health professionals and ``more aware of the healthy lifestyle than the average population,'' Michels said. ``As a consequence, their diet is somewhat healthier than the American population in general.''
If there is protection from fruits and vegetables, she said, then it should have showed up in this population. Yet, the rate of colorectal cancer in the study group, Michels said, is about the same as for the general U.S. population.
Earlier published evaluations of the same two health care professional studies, Michels said, showed that fruits and vegetables offered protection against heart disease, stroke, diabetes and possibly some other cancers. She said the two studies also showed that a diet rich in red meat was linked to higher rates of colorectal cancer, while aspirin and multivitamins offered some protection.
Michels emphasized that the Harvard study is not the final answer on fruits and vegetables and colon cancer. She said the findings applied only to Americans between the ages of 30 and 75, the age group in the studies.
``It may be possible that diet factors in childhood and in the 20s may be important,'' Michels said.
Also, she said that the experience in other countries, where less meat is consumed, may be different. A large European diet and cancer study is now underway, Michels said.