Are Foods with High Fiber Really That Good for You?
Do foods with high fiber deserve all the health-benefits accolades they are given?
Most people know they should eat more foods with high fiber. But they don’t know why. (Hint: It’s not to lower the risk of colon cancer.)
And many people assume that all foods with high fiber are the same. In fact, some fibers lower cholesterol, some lower blood sugar, and some help with regularity.
Those differences didn’t matter so much when all of our fiber came—intact and unprocessed— from foods with high fiber like whole grains, beans, fruits, and vegetables, since each usually has a mix of fibers.
But now companies are adding isolated fibers—mostly purified powders—to ice creams, yogurts, juices, drinks, and other foods that have always been fiber-free.
What are those fibers good for? It’s not clear that anyone knows.
How much fiber do you need
How much fiber do you need? According to food labels, 25 grams is a day’s worth. That’s right for women 50 and under, but men of the same age need 38 grams, says the National Academy of Sciences. And the targets drop to 21 grams for women and 30 grams for men over 50.
It’s not that people need fiber less as they get older. “The advice is to get 14 grams of fiber per 1,000 calories, and older people need fewer calories,” explains Thomas Wolever, a fiber researcher and professor of nutritional sciences at the University of Toronto.
Most Americans consume half the recommended levels. A typical woman gets about 13 grams of fiber a day, while the average man hovers around 17 grams.
What’s the harm in falling short of the target? Here’s a rundown of some of the links between fiber and health.
Fiber and heart disease
The daily fiber targets “are based on data that fiber prevents cardiovascular disease,” notes Joanne Slavin, a University of Minnesota researcher who served on the National Academy of Sciences Panel on the Definition of Dietary Fiber.
The NAS relied heavily on studies that found a lower risk of heart disease in people who reported eating the most foods with high fiber (about 29 grams a day for men and 23 grams a day for women). In each of those studies, the fiber that seemed to protect the heart came from cereals, breads, and other grains, not from fruits or vegetables.
But it was never absolutely clear that it was the fiber that mattered. Several inconsistencies have always troubled scientists.
What matters about fiber
Fiber or whole grains? It’s hard for researchers to know if it’s the fiber, or something else in whole grains, that matters.
“Whole grains also have phytoestrogens, antioxidants, lignans, vitamins, and minerals, so a lot comes along with the fiber package,” says Slavin.
Soluble or insoluble? The kind of fiber that’s linked to a lower risk of heart disease isn’t the kind that lowers cholesterol.
Although all fruits, vegetables, and grains have both soluble and insoluble fiber, most grains, like wheat, are richer in insoluble fiber, which is not broken down by digestive enzymes or by bacteria in the gut.
In contrast, a few grains (oats and barley, for example) are richer in viscous (gummy) soluble fibers, which the bacteria in the lower intestinal tract can break down.
Fiber and Diabetes
“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 Medical School and chief of preventive medicine at Brigham and Women’s Hospital in Boston.
The evidence that fiber prevents diabetes parallels the evidence that it prevents heart disease, as do the inconsistencies.
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).
Fiber and GI tract disorders
A few studies have suggested that people who eat more insoluble fiber have a lower risk of diverticular disease, which is caused by small pouches that bulge out through weak spots in the large intestine, sometimes becoming inflamed or infected.
But fiber’s starring role in the GI tract is in the stool department. Insoluble fiber tends to help “laxation” by adding bulk to stool.
“We know that insoluble fibers like wheat bran are good for stool weight and laxation and soluble fibers like pectin aren’t,” explains the University of Minnesota’s Joanne Slavin.
After dozens of studies, researchers have even estimated how much you can expect stool weight to increase for each gram of fiber you eat. (That’s 5.4 grams for wheat- bran fiber, 4.9 grams for fruit and vegetable fiber, 3 grams for isolated cellulose, and 1.3 grams for isolated pectin, in case you were wondering.)
Insoluble fiber from bran helps prevent constipation because it bulks up the stool. “The bran fiber is still there at the end of the GI tract, where it binds water, so it’s going to increase stool weight,” explains Slavin.
In contrast, most soluble fibers, like the pectin in fruits and vegetables, are digested by bacteria in the gut, “so there’s nothing left at the end of the GI tract.”
But it’s not just a question of insoluble vs. soluble. For example, psyllium, the (mostly) soluble fiber in Metamucil, is a laxative. And wheat bran has a bigger impact than cellulose—its purified cousin— even though both are insoluble.
The bottom line on foods with high fiber
- Whole-grain breads and cereals, which are naturally rich in fiber, are linked to a lower risk of heart disease and diabetes.
- Foods rich in insoluble fibers, like wheat bran, help prevent constipation and possibly diverticular disease.
- Isolated oat fiber and soy fiber are insoluble, so they may help keep you regular. Polydextrose may also help, but inulin and maltodextrin don’t seem to.
Sources: JAMA 275: 447, 1996. JAMA 281: 1998, 1999. JAMA 277: 472, 1997. Diabetes Care 20: 545, 1997. Nutr. 128: 714, 1998. Spiller, Ga, ed., CRC Handbook of Dietary Fiber in Human Nutrition (Boca Raton, FL: CRC Press, 1993), 263-349.