Women who eat red meat every day are 2 1/2 times more likely to develop colon cancer than are women who eat it less than once a month, according to a six-year study of more than 88,000 nurses.
The study, published in today's issue of the New England Journal of Medicine, provides persuasive new evidence that eating animal fat, which is thought to be the chief culprit in red meat, increases the risk of cancer of the colon, or large intestine, the second most common fatal cancer in the United States.
That link had been suggested by previous epidemiological research, but the new study goes further to show a clear dose-response relationship. It also pinpoints a link to the fat in red meat but not to vegetable fats or the fats in dairy products, fish or poultry.
The more beef, lamb or pork the women in the study consumed, the higher their risk of developing the disease. Eating vegetable fat or fat from dairy foods did not appear to increase colon cancer risk, the study found. And eating either fish or chicken with its skin removed appeared to reduce the risk, but that may have been because women who frequently chose such foods were eating them instead of red meat.
About 110,000 Americans develop colon cancer each year, and about 53,300 die of it. The study's findings are likely to apply to men as well as women, and suggest that even a moderate change in the typical American diet could have a significant impact on those figures, said Meir J. Stampfer, an associate physician at Boston's Brigham and Women's Hospital and one of the report's authors.
"You don't have to adopt a totally vegetarian lifestyle to achieve the benefit," he said. By modestly cutting back on red meat, "one can substantially reduce one's risk without a whole lot of pain."
The 88,751 women in the study were participants in the Nurses' Health Study, a Boston-based project assessing the relationship between lifestyle factors and various diseases. In 1980 they filled out detailed dietary questionnaires, and they also weighed and recorded everything they ate or drank during four one-week periods. Researchers then monitored the women's health until 1986. Colon cancer developed in exactly 150 participants during the study period.
To assess the disease's possible link to diet, researchers divided all the women in the study into groups according to how much fat they ate and how many calories they consumed. They also analyzed their intake of various kinds of fat.
Stampfer said most women in the study ate a "typical American diet," with fat providing between 39 percent and 46 percent of daily calories. Only 290 participants were vegetarians.
Fat contributes about 38 percent of calories to the average American diet, and current government guidelines suggest reducing that proportion so that no more than 30 percent of calories comes from fat.
The study found that women eating the highest-fat diets were not more likely to get colon cancer than other women. But there was a correlation with animal fat consumption. When participants were divided into five groups according to how much animal fat they ate, women in the highest-fat-consuming group had almost twice the risk of colon cancer seen among women in the group with the lowest animal fat intake.
The link was even stronger when participants were compared according to their consumption of various kinds of meat. Women who ate beef, lamb or pork as a main dish once a day had 2 1/2 times the risk of those who ate those meats less than once a month.
Stampfer said there are several theories to explain why animal fat, especially when supplied by red meat, may increase the risk of intestinal cancer. High-fat diets lead to higher intestinal levels of bile acids, which are produced by the liver to aid in digestion of fats. It is thought that bile acids may promote the development of colon tumors. Another speculation is that red meats contain particular fat components or other substances that may trigger such cancers.
Contrary to other research, the study did not find convincing evidence that a high-fiber diet reduced the risk of colon cancer. The data did suggest that eating fiber from fruit might reduce risk, but that finding was not statistically significant.
Peter Greenwald, director of cancer prevention and control at the National Cancer Institute, said the study may have failed to detect a clear benefit from dietary fiber because few of the participants' diets were relatively high in fiber. He said most Americans eat fewer fruits and vegetables than dietary guidelines recommend.
"When we put together all the data we have, it comes through very clearly that eating a variety of vegetables, fruits and whole grains is beneficial" in preventing colon cancer, he said.