Dietary fiber includes a variety of plant-derived compounds, i.e.,
non-starch polysaccharides and lignin, that are not digested by endogenous
intestinal enzymes. Fiber is composed of a variety of materials that are
often categorized as insoluble or soluble in boiling water. Insoluble
polysaccharides include cellulose and some forms of hemicellulose. Lignin
is part of insoluble fiber that is not a polysaccharide but a polymer of
phenylpropane molecules. The soluble fibers include pectins, gums, and
mucilages.
Fiber is neither digested nor absorbed in the small intestine but is
fermented by bacteria in the colon. On a typical American diet, about 75%
of ingested fiber is fermented. While there is no absolute requirement for
dietary fiber, it is almost certainly needed for normal bowel function.
Adequate fiber clearly prevents constipation, diverticular disease of the
colon, and hemorrhoids. Many other physiologic effects have been
attributed to fiber and consumption of diets high in fiber may prevent a
variety of conditions including obesity, diabetes, gallstones, coronary
artery disease, or colon cancer. Epidemiological studies show fairly
clearly that diets high in fruits, vegetables and cereal grains are
associated with decreased risk of developing these diseases.
Diet recommendations: Current average fiber intake is
estimated at about 12 g/day. Many health organizations recommend 20-35
g/day but there is no level set in the current Recommended Dietary
Allowances.
Food sources: Foods that are rich in dietary fiber
include whole grains (particularly the bran portions), legumes, and some
fruits and vegetables. Most foods contain mixtures of both insoluble and
soluble fiber. Examples of foods rich in insoluble fiber are wheat bran,
nuts and barley; soluble fiber is found in oat bran, melons, and dried
fruits. Beans are good sources of both fiber types. Cooking does not
decrease the amount of fiber in food but changes its three-dimensional
structure.
Potential hazards: There are some concerns that
phytate, which accompanies dietary fiber, will decrease the absorption of
certain minerals like iron, calcium, magnesium, and manganese. However,
most long term studies have not confirmed this danger. Very high intakes
of isolated sources of dietary fiber can prevent adequate intake of other
nutrients and may, in rare instances, lead to intestinal obstruction or
volvulus of the colon. Most untoward effects have been traced to
inadequate fluid intake with consumption of large amounts of fiber,
usually in a purified form. This indicates the desirability of obtaining
fiber from food sources, rather than from supplements.
Recent research: Fermentation products of fiber
produced by bacteria in the colon include propionic and butyric acid. Some
experiments have shown these to lower serum cholesterol or cause cancer
cells to differentiate.
