Boston researchers report what they call the most convincing evidence yet that a diet rich in animal fats substantially increases the chance of developing colon cancer, and that substituting fish and chicken for red meats lowers the risk.
The study of 88,751 women “provides probably the strongest single piece of evidence relating animal fat and red meat to the risk of colon cancer,” said Dr. Walter C. Willett, a Harvard School of Public Health professor who led the research team.
The researchers at the Harvard Medical School and Brigham and Women’s Hospital found that women who ate beef, pork or lamb as a main dish every day had 2 1/2 times the risk of developing colon cancer as women who consumed these red meats once a month or less. According to Willett, “the majority of benefit (against colon cancer) is probably achieved” by cutting back red meat consumption to less than once a week. There is probably a small additional benefit of eliminating red meat from the diet.
The increased colon cancer risk posed by animal fat and red meat, however, is modest compared to some other cancer risks, such as the increased risk of cancer related to cigarette smoking.
Although men were not studied, the relations “are likely to be similar,” the researchers said.
The results, published in today’s New England Journal of Medicine, come from the ongoing federally funded Nurses Health Study. Since 1976, more than 120,000 female nurses, ages 30 to 55, from 11 states have been checked every two years to study risk factors for the development of cancer and other diseases. The current study covered colon cancers that developed between 1980 and 1986.
The project differs from other colon cancer studies in its large size and in that individuals were questioned both before and after they became ill, a technique that can minimize bias. The questions about the diet, a notoriously difficult subject to study, were detailed, and the researchers said their validity had been established in prior studies.
“We are on more solid ground” in relating animal fats to colon cancer, said Lawrence Garfinkel, an American Cancer Society consultant who is considered an expert in the field. “It is a well-done, well-designed and well-analyzed study.”
In recent years, researchers have focused on the interaction among three factors--genetics, diet and increasing age--to help explain why some people are at greater risk than others of developing colon cancer. There is evidence that the majority of people who develop colon cancer have an underlying genetic susceptibility, as well as evidence that individuals with multiple risk factors are at particularly high risk.
Colon cancer is the third most common malignancy in the United States, ranking behind lung cancer and breast cancer, according to the American Cancer Society. The society estimates that 110,000 cases were diagnosed this year and that 53,000 deaths will be recorded.
Two major dietary theories have been advanced about colon cancer--that dietary fat, particularly animal fat, increases the risk of cancer, and that dietary fiber decreases the risk.
These theories are based on epidemiological data, such as international comparisons of colon cancer rates, as well as animal and laboratory studies that have focused on the effects of fat and fiber on biochemicals and bacteria in the colon and the speed with which digested food moves through the intestines. For example, fiber, which helps to speed up the passage of food, may minimize the contact between potential cancer-causing substances and intestinal tissue.
During the six-year Boston study, 150 cases of colon cancer were documented. A disproportionate number occurred in the women with the highest consumption of animal fats. The strongest association was with beef, pork or lamb as a main dish.
On the other hand, the pattern of substituting fish and chicken for red meat, particularly chicken with the skin removed, was related to a lower risk of colon cancer. “This is an important practical finding,” Willett said. To achieve the benefit, people “don’t have to give up all animal products.”
Nutrition experts also emphasize the importance of low-fat cooking. Deep-fried fish, for example, contains far more fat than broiled fish.
The findings also suggest that people can further decrease their colon cancer risk by eating more fiber-containing fruits and vegetables, although the study cautions that these trends toward decreased risk were “not statistically significant.” Women with both the highest intake of animal fat and the lowest intake of fiber had 2 1/2 times the colon cancer risk of other women. These findings applied to all age groups.
The average person eats about 11 grams of fiber a day, compared to dietary recommendations that between 20 to 30 grams of fiber be consumed daily, according to Dr. Peter Greenwald, the director of cancer prevention and control at the National Cancer Institute. The recommended amount of fiber can be provided by three vegetable servings a day, two fruit servings and three servings of a whole grain, such as bread, cereal or pasta.
The average American gets 37% to 38% of his or her calories from fats of all types, including animal fats, Greenwald said. This compares to the federal recommendation that 30% or less of calories be consumed as fats.
In the Boston study, a vast majority of the nurses reported diets with more than 30% of the calories from fats. According to Greenwald, if more of the study participants had low-fat diets, the the risk of colon cancer would probably have been reduced more than 2 1/2 times.
Most cancers of both the colon and rectum appear to develop from pre-malignant abnormalities, called adenomatous polyps. These mushroom-like growths arise in the wall of the intestine and increase in frequency with advancing age.
Regular screening tests, such as rectal exams, stool blood tests and sigmoidoscopy, the examination of the colon with a lighted tube, are recommended by the American Cancer Society and most physicians for all adults starting between 40 and 50. Closer scrutiny is often recommended for close relatives of people with colon and rectal cancers or polyps.