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Can Diet Prevent Colorectal Cancer?: A Dialogue
RussellColorectal cancer, or cancer of the rectum and large intestine (colon), is a major public health problem in every corner of the developed world. In the U.S. alone, approximately 120,000 Americans are diagnosed with colorectal cancer annually; each year, 55,000 Americans die from it. For too long, it has been the second most deadly form of cancer. Unfortunately, we are making frustratingly little progress in our fight against this killer.
Decades of advances in surgery, radiation therapy and chemotherapy that have conquered or tamed many once-deadly diseases have brought only modest declines in the death rate from colorectal cancer. Many are beginning to ask whether we ought to be expanding our efforts to prevent this cancer, as opposed to treating it.
Since colorectal cancer, of all the major cancers, seems to be the most intimately tied to diet, many researchers have been looking into into ways in which the cancer might be prevented through changing how, what and how much we eat.
Joel, you are an expert in the field of colorectal cancer prevention. Despite two recent, well-publicized studies that cast doubt on the health benefits of dietary fiber — which we'll discuss a little later — I think you'd agree that just about every expert believes that certain "healthy" diets appear to help protect against colorectal cancer. What we are talking about are diets that are high in fruits and vegetables, high in fiber, low in calories and low in animal fat.
The key components shared by these diets are significant amounts of calcium, vitamin D, folate or folic acid, vitamin E, fiber, selenium and garlic. Before getting into these components, I would like to ask you about the link between animal fat and red meat, and colorectal cancer. I think there is some fairly strong evidence that a diet high in saturated animal fat and red meat (beef, pork and lamb) can increase your risk.
My question is, how much animal fat are we talking about and how many servings of red meat are healthy? What guidelines should people follow on this issue?
MasonThere is a lot of scientific evidence that red meat and saturated animal fat, by themselves, increase your risk of getting colorectal cancer. And many people are unaware that the amount of red meat in your diet is a risk factor in itself, whether the meat contains a lot of fat or very little. Roughly speaking, a diet containing less than 40 grams of animal fat per day conveys one-half the risk of colon cancer of a diet containing 65 grams or more per day. Similarly, a diet containing 60 grams or less of red meat per day carries half the risk of a diet containing 130 grams or more per day.
Furthermore, heavily cooked or well-done meat, in which the surface is browned or blackened, contains substances called heterocyclic amines, which in a laboratory setting have been found to cause cancer. More studies need to be done, however, to prove that eating well-done meat raises your actual cancer risk.
Fiber: Is There Anything to It?
RussellExperts have long believed that eating a diet rich in fiber is good for cancer prevention and good health in general. However,two recent intervention studies have raised serious questions about this.(1)(2)
Joel, what do you think about the role of fiber in a healthy diet?
MasonWhat you are referring to are two papers presented in The New England Journal of Medicine (Spring 2000). Both of these dealt with the question of whether a high-fiber diet could prevent the growth of polyps. Polyps are small growths that can form inside the colon; even though these kinds of polyps are normally benign, people who develop them are more likely to develop colorectal cancer years down the line.
These papers looked at two polyp studies that appeared to show that the amount of fiber in a person's diet made little difference in their risk of developing polyps. From this, many have jumped to the conclusion that that these studies have proven that fiber does not prevent colorectal cancer. I disagree.
There are many problems with these studies but the most important is that they were conducted over a three- to four-year period. The natural history of colorectal cancer is much longer. The evolution from a tiny polyp to cancer takes decades, not years.
Given the huge body of evidence that fiber helps prevent colorectal cancer, I am not yet willing to close the door on that possibility. I continue to feel that it is reasonable to advise people who are concerned about a healthy diet to increase their fiber intake to 25 or 30 grams per day, emphasizing wheat bran. Of course, this should be done gradually, since the digestive system needs a little time to adjust to a drastic increase in fiber intake.
(2) Comments have been made