Fiber free-for-all: not all fibers are equal

Nutrition Action Healthletter, July-August, 2008 by Bonnie Liebman

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Most people know that they should eat more fiber. But they don't know why. (Hint: It's not to lower the risk of colon cancer.)

And many people assume that all fiber is 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 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.

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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 the key links between fiber and health.

Heart Disease

The daily fiber targets "are based on data that fiber prevents cardiovascular disease," notes Jeanne 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 fiber (about 29 grams a day for men and 23 grams a day for women). (1,2) 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:

* 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 are broken down by bacteria in the gut.

"When researchers feed people viscous soluble fiber, it lowers cholesterol, but insoluble fiber doesn't," notes Wolever.

Yet in the large studies that the National Academy of Sciences relied on, a lower risk of heart disease was linked to foods rich in either kind of fiber, not just soluble. "It's a disconnect," says Wolever.

One possibility: even though the mostly insoluble fiber from grains doesn't lower cholesterol, it may protect the heart by reducing blood pressure or the risk of blood clots.

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"Insoluble fiber may prevent heart attacks by reducing inflammation," says Wolever. "We don't know how the heck it works."

* Fiber or fiber eaters? Researchers can't be sure if it's fiber, or something else about people who eat high-fiber diets, that lowers their risk of heart disease.

"We just don't have a lot of people who eat high-fiber, whole-grain diets and are out there smoking," says Slavin. "Eating fiber goes together with other healthy behaviors."

Researchers typically adjust for smoking, weight, exercise, education, alcohol, saturated fat, and other factors that influence heart disease risk, but they could still miss something.

Those inconsistencies didn't matter so much as long as people were getting their fiber from whole grains, fruits, and vegetables, rather than from purified fibers added to foods like ice cream.

"That's why we have always encouraged people to eat fiber from foods like whole grains," says Slavin. That way they're getting both soluble and insoluble fiber and the whole "fiber package."

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). (3,4)