Why is fiber good for your body? Let’s go back a few years. Fiber was big in the mid-1980s, when President Ronald Reagan was diagnosed with colon cancer and Kellogg ran TV commercials saying that high-fiber foods like All-Bran could “reduce the risk of some cancers.”
But the fiber boomlet was soon eclipsed by the (much bigger) oat bran craze, followed by the low-carb bubble, and the whole-grain movement (with scattered mini-fads in between).
Now things have come full circle. Fiber is back. Fiber is showing up in foods because, well, companies have figured out how to put it there, and they know that if they pump up the fiber, people will pull out their pocketbooks.
Foods that never had any (yogurt, ice cream, water, juice) sometimes have some, and foods that always had some (cereals, breads, pasta) often have more. But before we ask why fiber is good for your body, we should ask if fiber is good for your body.
Is fiber good for you? It may not prevent colon polyps and cancer as suggested.
In the mid-1980s, the evidence made it seem like a slam-dunk that fiber could prevent colon cancer. Then in April 2000, two large studies released unexpected news.
In a three-year trial, roughly 700 people who were told to eat 13 grams of wheat bran a day had no fewer new precancerous colon polyps than 700 others who were told to eat only 2 grams a day. And in a four-year trial, roughly 1,000 people who were told to eat a lower-fat diet rich in fiber (36 grams a day) from fruits, vegetables, and whole grains had no fewer polyps than 1,000 others who were told to eat their usual diet.
“The trials were not ambiguous,” said John Baron, a professor of medicine at the Geisel School of Medicine at Dartmouth who has conducted trials testing calcium and folic acid on the risk of precancerous colon polyps. “They showed no effect.”
Despite the two disappointing trials, the World Cancer Research Fund and the American Institute for Cancer Research concluded in 2011 that fiber-rich foods reduce the risk of colorectal cancer. Its evidence: studies like the European Investigation into Cancer and Nutrition (EPIC), which tracked more than 500,000 people in 10 countries for five years. EPIC found a 40 percent lower risk of colon cancer in people who reported eating more fiber-rich foods.
So why did the two trials strike out? Maybe it’s not fiber, but something else about fiber eaters that cuts their risk of cancer. Or maybe the trials found nothing because they looked at polyps, not cancers. If fiber keeps polyps from progressing to colon cancer but has no effect at earlier stages, you may not see a connection between fiber and polyps in these trials.
The bottom line: The jury is still out for fiber lowering the risk of colorectal cancer. The evidence is stronger for fiber preventing coronary heart disease and diabetes than it is for cancer.
This doesn’t mean fiber is out. But why is fiber good for your body?
“There’s moderately strong evidence that fiber is linked to a reduced risk of diabetes, and it’s based on whole foods like vegetables, fruits, and whole grains,” says JoAnn Manson, professor of medicine at Harvard edical School and chief of preventive medicine at Brigham and Women’s Hospital in Boston.
In two studies—on roughly 65,000 women and 43,000 men—those who reported eating the most fiber from grains (8 grams a day) had about a 30 percent lower risk of diabetes than those who reported eating the least fiber from grains (3 grams a day). Still, it could always be something else about fiber eaters, or something other than the fiber in grains, that matters. “We don’t have large-scale trials showing that fiber prevents diabetes,” cautions Manson.
But in short-term clinical studies, the gummy soluble fibers (in foods like oats and barley) keep a lid on blood sugar.
“There’s good evidence that fiber slows the absorption of the carbohydrate in foods, which leads to a less marked increase in blood sugar and less demand for insulin,” explains Manson. If those studies had lasted longer, researchers might have found that insoluble fiber also lowered blood sugar.
When insulin-resistant women were fed a cereal high in insoluble fiber for a year, nothing happened for six months. Then the bacteria in the colon started to change, and the women became more sensitive to insulin.
How? Bacteria in the colon affect gut hormones, which could keep beta-cells in the pancreas from dying. Beta-cells produce insulin. Much of that evidence is preliminary, however. What fiber does to bacteria in the colon is still pretty much a black box.
Sources: Engl. J. Med. 342: 1156, 2000; N. Engl. J. Med. 342: 1149, 2000; Lancet 361: 1496, 2003; JAMA 277: 472, 1997; Diabetes Care 20: 545, 1997.