What is dietary fiber?
Where is fiber found?
What does fiber look like?
How much fiber?
Dietary fiber content of foods
Soluble and insoluble
The digestion of dietary
How to change from a low-fiber to a
Dietary fiber was once called
‘roughage’ and was assumed to be indigestible fibrous material that
went in one end of the body and eventually emerged from the other.
Dietary fiber is much more complicated than originally thought and
undergoes important changes in the intestine. It has a part to play in
many areas of health and longevity including the health of the
intestine, in diabetes or fluctuating blood-sugar levels, heart
disease and cancer. However, not every kind of fiber has equal value
in each of these areas.
What is dietary fiber?
Just as there are many different
vitamins, each with separate actions to perform in the body, so there
are different types of dietary fiber with varied roles. The old term
“roughage” was measured as “crude fiber”, and referred mainly to
cellulose - one of the types of dietary fiber. Roughage ignored the pectins, gums, hemi-celluloses and the saponins that all have a
bearing on our health. These different types of fibers are found in
different foods and you cannot assume your needs are being met just
by eating, say, unprocessed bran or an apple a day.
Where is fiber found?
Dietary fiber occurs only in plant
foods, including grains, cereals (and foods such as breads and pasta),
fruits, vegetables, legumes, seeds and nuts. Meat, fish and dairy
products have other important nutrients, but they have no dietary
fiber. To get the full range of the different types, you need to
select a wide variety of plant foods in your daily meals and your
What does fiber look like?
You can see strings of fiber in
asparagus or spinach stalks, and the grainy fibers in some breads and
cereal products. However, the gluey types present in oats and barley
do not appear fibrous and if you have ever added pectin to jam to help
it set, you will know it is a fine white powder with no obvious
fibers. Some foods that look fibrous, such as celery, have very little
fiber while others which have no obvious stringiness, such as bananas
or potatoes, are good sources of fiber.
The average daily diet has about 15
grams of dietary fiber. By contrast, people in some countries, and
many who are vegetarians, have fiber intakes which may be three to
four times this level. In general, 30-40 g of fiber a day is
Dietary fibers that form a “gel”,
when mixed with water or with digestive juices in the intestine, are
classified as soluble fibers. This includes “gummy” fibers and
hemi-celluloses as well as pectins in fruits. Soluble fibers occur in
barley, oats, apples, cabbage and some other vegetables, and in
legumes. They can help lower cholesterol, regulate blood sugar levels
and help in the prevention of bowel cancer.
Insoluble fiber is found in
whole-wheat products (whole meal bread, wholegrain cereals, wheat
bran) and in vegetables. It is valuable to prevent constipation and
may alter the bacteria in the bowel so that some substances implicated
in causing breast cancer are removed from the body.
For good health, try to have a
mixture - some soluble and some insoluble fiber.
content of foods
av. serve of any Beans, kidney or baked,
1 cup cooked
Peas, average serve
Sweet-corn kernels, 1 cup
Fruit, dried, 50 g
seeds, 30 g
Coconut, fresh, 75 g
Peanut butter, 30 g
white, 2 slices
multigrain, 2 slices
whole meal, 2 slices
rye, 2 slices
bran, mixed cereal,
bran, processed wheat
cornflakes, rice bubbles
mixed cereals (flakes and
porridge (rolled oats)
Wheat biscuits, 2
Wheat germ, 1 tbsp
cooked, 1 cup
white, 2 cups
Rice, cooked, 1 cup
Wheat, cracked, cooked,
digestion of dietary fiber
Dietary fiber is not broken down by
the enzymes which digest proteins, fats and carbohydrates in the small
intestine. Rather, most types of fibers are digested by bacteria in
the large intestine. Soluble fibers are 100 per-cent digested, while
insoluble ones are digested to varying degrees. Only one type of
fiber, lignin, is not digested at all, although it may help remove
some substances from the body.
While they are digesting dietary
fiber, bacteria produce special acids, called short chain fatty acids.
These provide a direct source of energy for the cells in the
intestine. One acid causes an electrical stimulation in the bowel wall
that helps the muscle wall propel food wastes along the intestine.
Another has been shown to stop the action of an enzyme, which
bowel-cancer cells need in order to multiply.
Constipation is more common in women
than in men, possibly because many women do not eat enough
high-fiber foods. There may also be sex differences in the blood flow
to the intestine and in the production of special gut hormones that
help move foods along the intestine. A lack of water makes
Regular bowel movements are important
but, in fact, it is the consistency of the stools which is more
important than frequency. Small, hard stools constitute constipation.
Many people take laxatives. Those
containing anthroquinones (including some herbal laxatives) can damage
the nerves in the bowel wall. The bowel is a muscular wall, so
regular exercise in moving food along its length is important.
Laxatives can destroy muscle tone and should not be used over long
periods. A low fiber intake also makes the intestinal muscle walls
Increasing dietary fiber to 30-40
grams a day and drinking six to eight glasses of water will prevent or
cure constipation for most people. Those who have resorted to
laxatives over long periods may find they also need the help of a mild
faecal softener until their improved diet becomes effective.
Soluble and insoluble fibers work in
different ways to increase stool bulk.
Insoluble fibers provide bulk and
also absorb water to contribute to the stools. Soluble fibers cause
useful bacteria to multiply
by the million and their dead bodies
are then excreted.
About 70 per cent of the weight of
stools represents their water content; the other 30 per cent represents
the dead bodies of bacteria which have digested soluble fiber plus
some undigested soluble and insoluble fiber.
As bacteria digest fiber, they
produce gases. This is normal and when you eat more fiber, you produce
more gas. However, if you increase your fiber intake gradually, you
will have fewer problems with excessive gas production.
Foods which produce the most gas
include legumes, certain vegetables (cabbage, cauliflower, broccoli,
Brussels sprouts) and apple juice. Soaking legumes and then discarding
the soaking water helps. Eating the clear, outer husk on legumes also
helps reduce gas because this coating contains substances which can
bind the cause of some of the gas.
Gas and abdominal distension with
pain may occur in people who cannot digest milk sugar, or lactose. The
natural content of fructose and sorbitol in apple and pear juice also
causes excessive ‘wind’ in some people.
from a low-fiber to a high-fiber diet
White toast with honey
oats, wheat biscuits or bran cereal
Whole meal toast with marmalade
|Coffee and biscuits
|Coffee and whole meal fruit
/ white bread
|Chicken and salad
whole meal bread
Apple, with peel
2 or 3 vegetables