Louise Chang, MD
Suffering from diarrhea? Conventional wisdom says choose foods high in
fiber. Constipated? Eat a high-fiber diet, experts suggest.
That’s right, as contradictory as it may seem, the same remedy can work for
diarrhea and constipation.
“Part of the answer is that fiber helps normalize transit time, or the rate
at which food moves through the digestive tract,” says Joanne L. Slavin, PhD,
RD, professor of food science and nutrition at the University of Minnesota.
Constipation happens when food moves too slowly through the large
intestines, often resulting in hard stool that is difficult to pass. Eating
fiber-rich foods helps move the contents of the large intestine along more
quickly. Fiber also absorbs water, softening stools so that they pass more
Diarrhea occurs when undigested food moves too fast, before the intestines
can absorb water, resulting in loose stools. Fiber’s ability to absorb water
helps make stools more solid. And by slowing transit time, fiber gives the
large intestines a chance to absorb additional water. Fiber also helps bulk up
the contents of the large intestines, binding indigestible food together.
“Having something left at the end of digestion and absorption turns out to
be necessary to form a normal stool,” says Slavin, a leading expert on fiber
Fiber is the indigestible part of carbohydrates found mostly in plants.
Recent research reveals that there are many forms of fiber, each with a unique
effect on nutrition and health. Two important categories are soluble and
Soluble fiber partly dissolves in water, creating a gel-like substance that
helps lower cholesterol. Sources of soluble fiber include oats, barley, rye,
dried beans, oranges, and apples.
Insoluble fiber remains more intact as it passes through the digestive
system. That makes insoluble fiber especially helpful in preventing or easing
constipation. Insoluble fiber may also help with weight loss, by making meals
seem more filling without adding calories. Sources of insoluble fiber include
wheat, brown rice, celery, carrots, nuts, and seeds.
Foods can contain both soluble and insoluble fiber.
Fiber can also be distinguished on the basis of its source. Research
suggests that cereal fibers have an edge in aiding digestion, as well as
proving beneficial in protecting against coronary artery disease, type 2
diabetes, and metabolic syndrome.
“Cereal grains are typically higher in total fiber than fruits and
vegetables, so that may explain why they stand out in research studies,” Slavin
says. Cereal fiber comes from oats, wheat, barley, and other grains.
Certain sources of fiber may be especially helpful for treating particular
conditions. If your goal is to lower blood cholesterol levels, for instance,
Slavin recommends helping yourself to lots of oats, barley, and beans, which
are rich in soluble fiber. To boost levels of the friendly bacteria that
inhabit the intestines and help digest food, she recommends wheat, onions,
artichokes, and chicory. These foods are loaded with fructo-oligosaccharides,
components in some forms of fiber that encourage the growth of these helpful
Before we start worrying about which fibers to choose, most of us would do
well to choose any form of fiber. Surveys show that the average American
consumes only half the fiber needed for optimal health, including a healthy
digestive system. Experts agree, the best way to keep digestion on track is to
increase consumption of fiber in all its many forms. Eat a varied diet that
includes whole grains, fruits, vegetables, nuts, seeds, and legumes.
SOURCES:Joanne L. Slavin, PhD, RD, professor of food science and nutrition,
University of Minnesota.Grabitske, H., Critical Reviews in Food Science and Nutrition, vol
49: pp 327-360.Grabitske, H., Journal of the American Dietetic Association, October
2008; pp 1677-1681.Grabitske, H., Nutrition Today, September/October 2008; pp
193-197.Jones, J., Phys Sportsmed, December
2008; vol 36(1): pp 18-33.
The Health News section does not provide medical advice, diagnosis or treatment. See additional information.