The letter was sent to the lay press and to the editors of the medical journals that published the three studies, which looked at the role of diet in colon polyp recurrence, colon cancer and breast cancer.

The study by Schatzkin et al published in the April 20, 2000, issue of the New England Journal of Medicine looked at colon polyp recurrence in middle-aged subjects who had had polyps removed. Participants in the study agreed to eat a diet high in fruits and vegetables, but made those dietary changes just for the study, a four year period. The study found that subjects who with diets high in fruits and vegetables had just as much colon polyp recurrence as those who were not advised to make dietary changes. This short-term study looked at polyp recurrence, not cancer.

"I think everyone was pretty surprised by the polyp trial," says Nancy Potischman, PhD, a nutritional epidemiologist at the National Cancer Institute, and one of the researchers who signed the open letter after the AICR approached her. "It was very hard to find any major problems with the study. There were so many epidemiologists involved and it was so well done," she says. "But the people who choose to be involved in a long-term dietary study may have healthier lifestyles than the people who refuse to be in such studies. This would make it harder to find any differences between the dietary intervention group and the regular diet group," she adds.

A study by Michels et al in the Nov. 1, 2000, issue of the Journal of the National Cancer Institute asked participants over the course of several years to remember what foods they ate. Researchers found that those who said they ate the most fruits and vegetables did not experience significantly less colon cancer than other subjects.

The AICR notes in a press release that subjects asked to report diet from memory may be likely to over-report fruit and vegetable consumption, and under-report fat and sugar.

An analysis by Smith-Warner et al of eight previous studies published in the Feb. 14, 2001, issue of the Journal of the American Medical Association found no fewer cases of breast cancer among women who ate the most fruits and vegetables.

In the breast cancer study, Potischman says, the average number of servings consumed by participants was only three servings. The general recommendation from nutritionists is five servings a day -- "at least," she emphasizes.

The studies in question all had limitations, according to the letter. Though the research adds to the wealth of data on the diet-cancer link, because of the way the studies were conducted they cannot and do not determine how diets high in plant-based foods consumed over a period of four years or more affect cancer risk. The preponderance of evidence -- more than 200 previous studies -- still indicates a strong association between diets high in fruits and vegetables and lower cancer risk, states the letter.

Potischman says although the media did a good job in presenting these studies in context, because the studies got so much press, "I think there was a sense that the public may be getting the message that fruits and vegetables [may not reduce the risk of cancer.]"

Michael Thun, MD, Vice President for Epidemiology and Surveillance Research for the American Cancer Society, says he hopes those who follow nutrition and health will understand that the knowledge is gained incrementally, as each study brings more information to the mix.

"The field of nutrition and health is still young, and it?s natural for science to raise questions and test [the results], and from that to and fro comes progress," Thun says. "In general, you should never just go with the latest study. There?s certainly no reason to abandon a diet rich in fruits and vegetables."

For information on lowering cancer risk through diet, go to the Early Detection and Prevention pages on the ACS web site.