Dietary fiber and colon cancer: The pendulum swings (again)

Content provided by the Faculty of the Harvard Medical School

Dietary fiber and colon cancer: The pendulum swings (again)

Colon cancer takes the lives of more than 28,540 American men each year, almost as many as prostate cancer. And it kills nearly as many women, placing it second only to lung cancer on the list of cancer killers. It’s a grim statistic, but there is a silver lining. Colon cancer deaths have been steadily declining, in large part due to major efforts at early detection.

Early detection and improved treatment certainly help, but prevention would be even better. Rapidly advancing research into the genetics of the disease holds great promise, but it is still far from clinical deployment. But you don’t have to wait for breakthroughs in molecular biology to reduce your risk; act now.

The same lifestyle that promotes good general health has been associated with a reduced risk of many malignancies. In the case of colon cancer, people who exercise regularly, even if it’s just walking 30 minutes a day, are about 50% less likely to get colon cancer than couch potatoes. And diet may be even more important. For one thing, an appropriately low caloric intake, along with exercise, will fight obesity, which has been linked to a sharp increase in the risk of colon cancer (see Table 1). And although the studies are not conclusive, other measures may also help, including a high consumption of calcium and vitamin D (low-fat dairy products, multivitamins, calcium supplements), a high consumption of folic acid (vegetables, multivitamins), and eating less animal fat and protein (red meat). Studies are supporting the long-held belief that low-dose aspirin therapy can also reduce the risk of colon cancer.

Scientists have been debating the true value of these protective factors for years. But the greatest (and oldest) controversy centers on the nutrient that new studies identify as the most beneficial of all: dietary fiber.

Humble beginnings

The first observations on dietary fiber and intestinal health didn’t rely on computer-assisted epidemiology but were made by simple observations in the health clinics of colonial Africa. Working as medical missionaries in Kampala, Uganda, some 50 years ago, Dr. Denis Burkitt and Dr. H.C. Trowell noticed that various “Western” diseases were rare in their African patients. The list included metabolic disorders (obesity and diabetes), cardiovascular diseases (heart attacks and high blood pressure), and intestinal problems (constipation, diverticulosis, appendicitis, hemorrhoids, hernias, and colon cancer).

Although they were keenly aware of the many genetic, environmental, and lifestyle differences between their African and English patients, the good doctors singled out the roughage in the African diet as the major protective factor. Dr. Burkitt stated the “fiber hypothesis” most simply (and explicitly) by noting that he could predict the number of a patient’s hospital visits from the size and frequency of bowel movements, bulky stools indicating less illness.

Modern studies have confirmed the role of dietary fiber in reducing the risk of diabetes, obesity, heart disease, and nonmalignant intestinal disorders. And a Harvard study has found protection against gallbladder disease. But the hope that dietary fiber could reduce the risk of colon cancer has gone up and down.

Negative studies

The first major blows to the fiber hypothesis came from Harvard. In 1999, the Nurses’ Health Study reported no link between dietary fiber and the prevalence of colon cancer. It was a large and careful study, but all 88,757 subjects were women. Harvard scientists also reported similar results for men, however. The Health Professionals Follow-up Study of 47,949 men agreed that dietary fiber had no clear link to the risk of colon cancer, though it did have some protection against adenomas, the benign polyps that are the precursors of nearly all colon cancers.

The Harvard studies were joined by other negative studies from the United States, Finland, and Sweden. Fortunately, however, scientists on both sides of the Atlantic have continued to examine the question, and their latest results, as of summer 2005, provide support for fiber.

The PLCO study

The Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial is a major American project that got started in 1993. Among other things, it’s one of two studies that should tell us whether or not PSA screening for prostate cancer saves lives. As part of its investigation into colon cancer, the PLCO trial evaluated 33,971 volunteers with an average age of 63. None of them had been diagnosed with colon polyps or cancer when the study began. Each subject filled out a 137-item food frequency questionnaire. Over the next seven years, all the subjects underwent sigmoidoscopies; 90% of them were normal, but roughly 10% had one or more adenomas.

In the PLCO study, dietary fiber was strongly associated with protection against adenomas. The people who ate the most fiber were 27% less likely to have adenomas than the folks who ate the least fiber. Grain cereals and fruits provided the greatest benefit.

If dietary fiber reduces the risk of adenomas, it should also protect against colon cancer, since nearly all colonic malignancies arise from these benign polyps. Indeed, another important study supports that possibility.

The EPIC study

The European Prospective Investigation of Cancer and Nutrition (EPIC) Study evaluated diet and colon cancer in 519,978 people living in 10 countries with a broad range of dietary habits. The volunteers, who ranged in age from 25 to 70, were tracked for six years. Compared with the people who ate the least fiber, those who ate the most enjoyed a 42% reduction in the risk of colon cancer. No particular source of fiber was more protective than others; the study did not evaluate fiber supplements.

Why the difference?

These two studies support a protective role for fiber; the older studies did not. What’s the difference — and what does it mean for you?

It’s not a question of care and competence; all the studies were meticulous and all used the latest epidemiological techniques. Researchers are not sure why their results differ, but several possibilities came to mind. The participants in the studies may have eaten different types of fiber. In the Nurses’ Health Study, for example, the women in the highest fiber group averaged less than 5 grams of cereal fiber a day, yet the PLCO study identified cereal and grains (along with fruits) as particularly protective. More important is the range of fiber consumption in each study. In the Nurses’ Health Study, the average daily consumption ranged from 9.8 grams in the lowest group to 24.9 grams in the highest, while in EPIC, the range was from 12.6 to 33.1 grams. That means 30% of the EPIC participants averaged substantially more fiber than the highest group of nurses. To put it in perspective, the average American diet contains just 16 grams of fiber a day, while Dr. Burkitt’s Africans averaged over 50 grams.

The Institutes of Medicine have set new daily fiber intake targets for men at 38 g/day for those under 50, and 30 g/day for men over 50. That’s more than twice the amount in the average diet, but if the EPIC scientists are right, it could reduce the risk of colon cancer by 40%.

Fiber and health

The Harvard studies did not find that fiber reduces the risk of colon cancer, perhaps because even the high-fiber participants did not eat enough. But they have confirmed many other benefits of fiber, even in moderate amounts. For example, the Health Professionals Study of 43,757 men found that fiber reduces the risk of heart disease. Over six years, the men who ate the most fiber (averaging 28.9 grams a day) enjoyed a 41% reduction in heart attacks compared to the men who ate the least (12.4 grams a day). All in all, for each 10-gram increase, a man’s risk of heart disease drops by 19% — and just one bowl of high-fiber cereal can provide those 10 grams of protection.

The Health Professionals’ Study also linked a high-fiber diet to a 42% decrease in the risk of diverticulosis. It’s a big benefit, since half of all Americans over 60 years of age have this condition, which can cause intestinal pain, bleeding, or infection.

By reducing constipation and straining, dietary fiber also helps prevent hemorrhoids and hernias. Dr. Burkitt was right — and so were the moms of America, who advocated lots of “roughage” for their families before fast foods changed our eating habits.

As American’s menu changed in the second half of the 20th century, diabetes, obesity, and hypertension increased to near epidemic proportions. The reasons are complex, but our shift away from whole grains, fruits, and vegetables to refined and processed foods certainly played a role. Indeed, studies from Harvard and many other institutions tell us that dietary fiber can benefit the metabolism as well as the intestines.

Fiber is good for your health.

How does it work?

It’s easy to see how fiber helps the gastrointestinal tract. Because humans cannot digest fiber, it is not absorbed into the body. But the fiber that stays in the intestinal tract slows the emptying of the stomach, producing a feeling of fullness that may help prevent overeating. It draws water into the intestinal tract, making the stools bulkier and easier to pass. That’s the main way it helps prevent diverticulosis, hernias, and hemorrhoids, and the more rapid and complete emptying of the colon may contribute to the benefit against colon cancer by clearing it of irritating substances. Other possible mechanisms include fiber’s ability to change the metabolism of intestinal bacteria and to neutralize the toxins produced by these bacteria and by the body itself. Since the rectum is normally empty, it is not affected by this benefit. The EPIC study found that the incidence of rectal cancer does not vary with fiber consumption, but a 2004 study of 2,157 residents of Utah and California linked a high consumption of fiber to a 46% reduction in the risk of rectal cancer.

Intestinal benefits are easy to understand, but how can fiber help the metabolism and heart? Several ways seem likely. Fiber slows the absorption of carbohydrates, reducing insulin levels. It also binds bile acids, lowering blood levels of LDL (“bad”) cholesterol. And the Health Professionals’ Study found that when a high-fiber diet includes lots of fruit and vegetables, it appears to lower blood pressure.

You stand to gain a lot from dietary fiber, but to get all these benefits, you need to eat a good mix of high-fiber foods. That’s because there are many types of fiber, each with its virtues. In general, insoluble fiber produces the most intestinal benefit, while soluble fiber does the most for your metabolism. Table 2 shows some of the important types of fiber and the foods that contain various forms of these very large, complex carbohydrate molecules that cannot be digested by humans.

Where’s the fiber?

In plants. Animal foods do not contain any fiber, but it’s abundant in vegetable products such as the bran of whole grains, the leaves and stems of plants, and nuts, seeds, fruits, and vegetables. Pick and choose among the foods you like best, but be sure you eat enough. Table 3 lists some sources of dietary fiber and indicates the best sources of soluble fiber, which is particularly useful for lowering cholesterol and fighting diabetes (yes, oat bran really works).

How much fiber is enough? Doctors don’t know for sure, but the results of the EPIC study suggest that the American Gastroenterological Association is right in recommending 30 grams a day — or more.

Can you get too much fiber? Probably not; remember that Dr. Burkitt’s African patients consumed more than 50 grams a day. But fiber is an acquired taste, so build up your “dose” slowly and be sure to drink plenty of water. Make sure you get enough minerals and vitamins as well. Although doctors now know that fiber is good to treat, as well as prevent, diverticulosis, most still advise patients with the ailment to avoid seeds.

Skeptics sometimes dismiss fiber as “rabbit food.” Perhaps. But the PLCO and EPIC studies add to a large body of evidence that a high-fiber diet will have you hopping to health.

Last updated:	August 21, 2006