May 1, 2003 -- There's no question that a high-fiber diet is good for you. The question is, how good is it at preventing colon cancer?
While there's convincing evidence that dietary fiber helps prevent heart disease, diabetes, diverticulosis, and other diseases, studies have produced mixed results in its role in reducing risk of the second most common cancer in the U.S. Some find that a high-fiber diet lowers cancer risk, while others indicate it offers no protection. A few even suggest that dietary fiberincreases cancer risk.
What's more consistent is the two newest findings, both published in the May 3 issue of The Lancet. Although done by different research teams, both produced remarkably similar results, offering renewed hope for fiber's role against colon cancer.
In one, researchers at the National Cancer Institute looked at the diet of 34,000 people participating in a study of early detection for various cancers. All were free of precancerous colorectal polyps when the trial began. The researchers found that those eating a high-fiber diet -- upwards of 36 grams of fiber each day -- were 25% less likely to develop polyps than those eating fewer than 12 grams.
The other study, conducted on 520,000 people in 10 European countries and called the largest study ever, also initially found a 25% reduced rate in colorectal cancer in those eating high-fiber diets of about 35 grams daily compared with those eating less than 15. But upon further analysis, researchers said that low-fiber eaters who doubled their intake could reduce risk by 40% -- and the protective effect was greatest on the left side of the colon, where most cancers originate.
"And in our study, the people eating the most fiber were consuming as much dietary fat as those eating the lowest amounts," lead researcher of the British study, Sheila A. Bingham, PhD, tells WebMD. "And they still got the protective effects."
Why is that important? Because it's just one of the many issues that have contributed to the fiber-for-colon cancer debate and mixed study results, which began in the 1970s when a missionary doctor compared colon cancer rates between America and Africa. He attributed the high cancer incidence in affluent western countries to diets high in animal fats, and the low cancer rates in poorer countries such as Africa to the diets high in plant-based fiber.
"The way I see it, we have observed that diets high in fiber are associated with a lower colon cancer risk," says Marji McCullough, ScD, nutritional epidemiologist at the American Cancer Society and a fiber-for-cancer investigator. "But pinpointing the exact reason has been challenging. It's hard to exactly determine what is in red meat -- whether the fat or something else -- that seems to increase colon cancer risk."
Fiber is believed to help reduce risk in two ways: It keeps you regular to remove toxins from the intestines, and bacteria living in the gut feeds on it -- producing beneficial byproducts to keep the colon healthy.
So why so many conflicting results over the years?
Most of the trials that show no benefit to a high-fiber diet -- at least four well-publicized studies in 1999 and 2000 -- looked at either cereal fibers or fiber supplements such as Metamucil. "Bran fibers may be not be in the same proportions or even be the same type of fiber that might occur in a natural diet," McCullough tells WebMD. "And it may be that supplemental fiber is not helpful at all." In fact, one study published in The Lancet in October 2000 found that these fiber supplements actually increased the risk of colon polyps. Meanwhile, most studies that show a protective benefit investigated fiber that comes from fruits and vegetables, as well as whole grains.
And while the "bottom line" findings may differ, there are similarities between these new Lancet results and even some studies to indicate no benefit from fiber: Measurable protection in the 30 grams-plus range.
"Actually, we found that there was a 35% reduced lower risk in people who ate something like 33 grams a day vs. those who had 14," says Edward Giovannucci, MD, of Harvard Medical School, a researcher on the ongoing Health Professionals and Nurses Health studies, whose findings suggested no benefit from a high-fiber diet. "But we statistically controlled for other factors, including smoking, aspirin use, red meat, alcohol. And when those factors were taken into account, the benefit from fiber went entirely away. That's why our studies suggested no benefit."
So what should you do? As these new studies suggest -- both done by well-respected researchers and carefully controlled, says McCullough -- increase your fiber intake, as long as it's mostly from fruits, vegetables, and whole grains. McCullough, Giovannucci, and Bingham advise against fiber supplements.
"Foods high in fiber have lot of other nutrients that benefit health in many ways, and possibly phytochemicals we haven't identified yet," says McCullough. "Most of the studies that indicate a reduced risk of colon cancer find the benefit at 30 to 35 grams, so I would recommend you aim at that level. Even if fiber isn't proven to prevent colon cancer, you can't go wrong with a plant-based diet that's high in fruits, vegetables, and whole grains."