Harvard Magazine (current issue)

Harvard Magazine
Main Menu · Search ·Current Issue ·Contact ·Archives ·Centennial ·Letters to the Editor ·FAQs

Harvard Health


The Fiber Facts
Looking beyond fat to heart health.
Photograph by Carl Tremblay

As America's infatuation with low-fat diets has matured into a full-fledged love affair, less and less attention has been paid to the importance of eating enough dietary fiber. Although many people recall the heyday of granola, or the oat-bran craze of the late 1980s, they're less likely to reach for these foodstus than for heavily promoted items like nonfat cookies and "guiltless" ice cream.

Earlier this year a major study of diet and heart disease put fiber back in the spotlight. The results of this investigation, conducted at the Harvard School of Public Health, confirm what mothers have always said: Start the day with a bowl of cereal and eat lots of fruits and vegetables.

According to a report in the February 14, 1996, issue of the Journal of the American Medical Association, the fiber in these foods, when eaten in sufficient quantities, apparently reduced heart-disease risk among more than 43,000 male health professionals who were studied for six years. Men who ate more than 25 grams of fiber per day had a 36 percent lower risk of developing heart disease than those who consumed fewer than 15 grams daily.

Every 10 grams of fiber added to the diet--whether it was from bananas, bran flakes, or baked potatoes--lowered the risk of heart attack by 19 percent. Upon closer inspection, it appeared that cereal fibers were even more beneficial to the heart: risk fell by 29 percent for every 10-gram increase in daily intake. This probably means that several different biological mechanisms are at play and that their effects are additive.

Fibers are comprised of complex carbohydrates and natural polymers such as lignin. They give shape, structure, and strength to plant materials-much as threads give body to cloth. Soluble fiber breaks down as it passes through the watery gastrointestinal tract, forming a gel that traps some substances related to high cholesterol levels and others associated with cancer. Oats, kidney beans, citrus fruits, apples, and potatoes are rich in this type. Instead of dissolving in water, insoluble fiber travels through the small intestine, makes stools heavier, and speeds their passage through the gut. Wheat bran and whole grains, as well as the skins of many fruits and vegetables, are rich sources of this type. In the large bowel, bacteria turn both soluble and insoluble fiber into short-chain fatty acids, which provide energy for the body and may help protect against cancer.

Fiber's benefits to the heart are so great that in the recent Harvard study a high-fiber diet not only worked for men with laudable health habits, but also reduced the coronary heart-disease risk for those men who were sedentary and overweight. It even provided some benefit to smokers.

Scientific evidence for a link between dietary fiber consumption and coronary heart disease began surfacing about 20 years ago. In 1977, a British study of 337 male bankers and bus drivers indicated that men who ate more brown bread and breakfast cereal were less likely to have a heart attack, over a 10- to 20-year period, than those who favored white bread and pastries. Although it's clear that fiber helps reduce the risk for heart disease, there are still unanswered questions about how it accomplishes its good works.

Soluble fiber is at least a partially solved mystery. Epidemiologic studies show that people on high-fiber diets have lower total cholesterol levels and may be less likely to form harmful blood clots than those who consume less of this indigestible material. Experiments confirm that soluble fiber can slow the liver's manufacture of cholesterol and of clotting proteins such as factor VII and plasminogen activator inhibitor type 1.

How does this work? One theory is that gut bacteria deserve the credit, because they ferment fiber into products that then act on the liver, altering its activity. Although the cholesterol-lowering ability of soluble fiber is certainly a plus, it isn't sufficient to explain the degree of risk reduction seen in people on fiber-rich diets. When results of the recent Harvard study were analyzed, researchers found that 60 to 80 percent of the fiber the men consumed was insoluble and this form appeared to offer the most powerful protection against heart attack.

One reason insoluble fiber might be a lifesaver is that people who consume a lot of it probably eat less food than other folks. Because the digestive tract can handle only so much bulk at once, a bowl of raisin bran (which is full of insoluble fiber) is more filling than a plate of ham and eggs. There is also some evidence that insoluble fiber may interfere with the absorption of dietary fat.

Because fiber slows the digestion of other foods as well, it can help blunt the sudden spikes in blood glucose level that may occur after a low-fiber meal. Each of these peaks stimulates the pancreas to pump out insulin. Some researchers believe that a lifetime of them could contribute to Type II (non-insulin dependent) diabetes-which typically strikes after age 40 and more than doubles the risk for stroke or heart disease.

Get It Where You Can

Current guidelines recommend that an adult consume 25 grams of fiber daily, obtained from a diet rich in cereals, fruits, and vegetables. An additional benefit for people who follow this advice is that they may simultaneously fulfill their need for vitamins and minerals and should have little difficulty limiting fat intake to less than 30 percent of total calories. Twenty-five grams of fiber may seem like a lot, but this can be met by following the recommendations of the U.S. Department of Agriculture's food pyramid: eat 2-4 daily servings of fruits, 3-5 servings of vegetables, and 6-11 servings of cereal and grain. Keep in mind that these portions aren't huge: for example, a half-cup of 100 percent bran provides 8.4 grams of fiber and two-thirds of a cup of shredded wheat 2.6 grams; a medium-size apple provides 3.5 grams, an orange 2.6 grams, a banana 2.4 grams; a medium potato (with skin) 2.5 grams, and half a cup of kidney beans 7.3 grams.

Although oat bran has been back in the news of late, most nutrition experts don't believe that any one fiber source is better than the others. Companies that make oat cereals are mounting massive advertising campaigns now that the Food and Drug Administration has allowed them to state on product labels that oatmeal or oat-bran cereals may reduce the risk of heart disease as part of a diet low in fat and cholesterol.

A 1992 review of 20 studies examining the connection between oats and health, published in the Journal of the American Medical Association, concluded that these cereals can cause a modest reduction in blood cholesterol level. But it's worth remembering that many cereal grains contain fiber and some types, such as wheat bran, actually contain larger amounts.

The payoff to eating a fiber-rich diet may be enormous: as the evidence mounts, it appears that consuming the right foods could prove every bit as important to the health of Americans as eliminating cigarette smoking.

Reprinted from The Harvard Health Letter. Copyright " 1996 President and Fellows of Harvard College. All rights reserved. For subscription information, write Harvard Health Letter, P.O. Box 420300, Palm Coast, Florida 32142-0300, or contact "pbibbins@warren.med.harvard.edu".

Main Menu · Search ·Current Issue ·Contact ·Archives ·Centennial ·Letters to the Editor ·FAQs
Harvard Magazine

Harvard Magazine (current issue)