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.

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 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

Read the full article on harvard Medical International