"Until recently, researchers studying colon cancer have concentrated on the potential protective effect of diets high in fiber," he said. "Today we see more and more investigations into the harmful effect of diets high in meat."
Many prior studies had found that a high fiber intake lowered the risk of colon cancer, but three separate trials in 2000 -- two published in the New England Journal of Medicine and one in The Lancet -- found that fiber intake and the risk of colon polyps, often a precursor to colon cancer, had no significant association.
"Clinical trials are ideal for studying how to treat diseases like cancer, not how to prevent them," Prince said. "Those trials are important, but they by no means end the debate."
With the fiber theory dealt a serious blow, studies have shifted their attention to other potential dietary factors such as fat, lack of folate, sedentary lifestyle, obesity, and the intake of red and processed meat.
Several large cohort studies -- studies that track diet and disease rates of large groups of people over several years -- have compounded the association between red meat and colon cancer. A January 2005 study of 148,610 adults between the ages of 50 and 74, published in the Journal of the American Medical Association, found those with a long-term history of high levels of meat consumption were at a 50-percent higher risk for distal colon cancer than those who ate little meat, even when weight, exercise, fiber intake and vitamin supplement use were taken into account.
Researchers have come up with several theories as to the connection between meat and colon cancer, among them a red meat blood pigment called heme iron has been found to cause cancer tumors in a laboratory a study on blacks -- a group that tends to consume more meat than the national average -- by Dr. Stephen O'Keefe of the University of Pittsburgh suggests that certain gut bacteria may give off carcinogenic byproducts when meat consumption is high.
Prince noted that, although more information will be released concerning a high-red-meat diet's role in colon cancer risk -- including presentations and posters at the two-day AICR International Research Conference of Food, Nutrition and Cancer in Washington -- information on the protective effect of fiber will continue to be presented.
Since the three studies denouncing the connection between fiber and colon cancer were published in 2000, other studies continue to suggest that fiber does protect against colon cancer.
"The crux issue here may be the shape of the overall diet," Prince said, adding that a diet higher in vegetables, fruit and whole grains than the current American average, and subsequently lower in red meat, will probably prove to be effective in protecting against colon cancer.
However, all of the studies miss a significant point, according to Mike Adams, a nutrition expert and author of "Grocery Warning: How to Recognize and Avoid the Groceries that Cause Cancer, Diabetes, Heart Disease, High Cholesterol, High Blood Pressure and Other Common Diseases."
"What consumers and scientists don't seem to understand about the link between meat consumption and colon cancer is that the real danger stems from the chemicals found in the meat, not necessarily the meat itself," he said. "Most processed meats contain sodium nitrite, a chemical that directly promotes cancer. Even fresh meats from non-organic animals contain numerous cancer-causing contaminants such as PCBs, pesticides and heavy metals."