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".
