New Doubts Cast on Fiber in Averting Cancer
Fiber supplements--long touted as a way to ward off colon cancer--may do nothing to help prevent polyps of the colon and the rectum that can lead to cancer and may even increase the risk of developing new ones, a European study has found.
The finding, reported in Saturday’s Lancet, is the latest in a succession of studies that pour cold water on the use of fiber as a colon cancer prevention strategy. The study tested a soluble fiber found in certain laxatives in people who had previously had polyps removed.
In their three-year study of several hundred people, the researchers found that those taking fiber supplements actually had a slightly higher recurrence rate of polyps than those taking either placebos or a calcium supplement.
The case for dietary fiber preventing colon cancer in people who have not yet developed abnormalities of the colon is also weakening, experts say.
Yet these findings are certainly no reason to turn away from a plant-rich diet that is low in animal fats and added sugars, nutrition experts stress. Although scientists don’t know the exact reason, that kind of diet is linked to strong health benefits, including a lowered colon cancer risk.
And fiber--even if it turns out to be a bust for preventing colon cancer--has strong effects on lowering the risk of heart disease and diabetes.
“People shouldn’t just say ‘Gee, that study didn’t work. I might as well go have that cheeseburger and onion rings,’ ” said Dr. David Heber, professor of medicine and director of the UCLA Center for Human Nutrition.
In the study, an international research team led by Dr. Claire Bonithon-Kopp of the University of Bourgogne in France enrolled 665 people (from nine European countries and Israel). All had previously had a colon polyp called a colorectal adenoma removed.
Each day for three years, one-third of the group drank an orange-flavored beverage containing 3.5 grams of a soluble fiber called ispaghula husk, which is an ingredient in some laxatives.
The other two groups received either 2 grams of calcium or a placebo.
Three years later, researchers examined the colons of 552 of the patients--and found at least one polyp had grown in 20.2% of the patients who had gotten the placebo and somewhat more--29.3%--of the patients who had gotten the fiber drink.
Only 15.9% of the patients who took calcium had developed a new polyp, in line with another 1999 study linking calcium supplements to a reduction in polyp recurrence.
This is not the first or even the largest study with disappointing results for fiber.
On Wednesday, in fact, the U.S. Food and Drug Administration announced its decision to disallow health claims linking supplements of dietary fiber to colon cancer prevention. “The results of studies about dietary fiber consistently showed a lack of relationship between dietary fiber supplements and the risk of colorectal cancer,” the FDA stated.
Earlier this year, two larger studies showed that supplementing diets with wheat bran fiber or upping the intake of fruits and vegetables and lowering fat for several years didn’t help prevent recurrence of polyps either.
However, these studies and the Lancet report focused on adults whose colons had already begun developing polyps. The effects of fiber on young, unblemished colons might be different--but many scientists are beginning to doubt that too.
Originally, the hope came largely from comparisons of colon cancer rates in cultures that eat different amounts of fiber--and from animal studies in which the cancers were artificially induced with chemicals, said Eric Rimm, associate professor of epidemiology and nutrition at the Harvard School of Public Health.
It made theoretical sense as well, Rimm said: bulking out the stools with fiber would hasten movement of fecal matter--including possible carcinogens--through the colon.
But more recent studies tracking the diets and diseases of large groups of people over many years, as well as controlled studies like the Lancet one, all came up negative.
“The evidence of high fiber diets protecting against colorectal cancer has become much weaker over the past several years,” said Dr. Michael Thun, head of epidemiological research for the American Cancer Society.
But is fiber actually harmful? Some researchers say the increased risk seen in this most recent study--from a fiber that is not part of a regular diet--might be a statistical blip.
“I would hate to assume from one study that something is doing harm without more evidence,” said Dr. Bernard Levin, vice president for cancer prevention and professor of medicine at the M.D. Anderson Cancer Center in Houston.
According to the American Cancer Society, more than 130,000 Americans will be diagnosed with colorectal cancer this year, and more than 56,000 people will die from it.
The American Cancer Society recommends that people age 50 or over begin colon cancer screening--because if polyps are caught and removed, much of colon cancer can be prevented.
Exercising, cutting down on red meat and therapies with aspirin-like drugs that fight bowel inflammation may also be fruitful ways to fight colon cancer.
Dr. David Alberts, associate dean for research at the University of Arizona College of Medicine in Tucson, said laxatives help ward off hemorrhoids and can be extremely beneficial for people who have chronic constipation--which can lead to life-threatening medical conditions.
