Dr. Barnett Kramer, deputy director in the office of disease prevention at the National Institutes of Health, said: "Over time, the messages on diet and cancer have been ratcheted up until they are almost co-equal with the smoking messages. I think a lot of the public is completely unaware that the strength of the message is not matched by the strength of the evidence."
Fat in the diet, the studies found, made no difference for breast cancer. "For fat and breast cancer, almost all of the prospective studies were null," Dr. Schatzkin said.
Fiber, in the form of fruits and vegetables, seemed to have a weak effect or no effect on colon cancer.
The more definitive randomized controlled trials were disappointing, too, with one exception. A study reported in May found that women with early stage breast cancer who followed a low-fat diet had a 20 percent lower risk of recurrence.
Even so, the effects were just marginally statistically significant. The study's principal investigator, Dr. Rowan Chlebowski of the Harbor-U.C.L.A. Medical Center, said it needed to be repeated before scientists would be convinced.
[A] four-year study that asked whether beta carotene, with or without vitamins C and E, could protect against colon polyps, from which most colon cancers start, found no effect. People who took either beta carotene, vitamin C, vitamin E or all three had virtually identical rates of new polyps compared to participants taking dummy pills.
Another study, of 22,000 doctors randomly assigned to take beta carotene or a placebo, looked for an effect on any and all cancers. It found nothing. Two more, involving current and former smokers, found that those taking beta carotene actually had slightly higher lung cancer rates than those taking placebos.
This said having more fish, fruits and vegetables probably doesn't hurt and hypotheses are moving towards volume or type of food.
Specific food can affect general health, he added, but as for a major role in cancer, he doubts it. He now believes that it is the amount of food people eat, not specific foods or types of foods, that may make a difference. "I think the truth may be that particular food choices are not as important as I thought they were," Dr. Byers said.