Food Nutrients - Dietary Fiber
What is it?
Dietary fibers are structural components of plants. The type and amount of fiber in plants
vary from species to species. A common misconception about fiber is that it is not
digested by enzymes in the body and therefore provides no calories or nutrients. But the
category "fiber" includes chemicals that are not fibrous, materials that can be
dissolved, and some substances that can be digested partially. We eat quite a complex
mixture of fibers.
Dietary fiber is a broad generic term; it includes the following chemicals, which form the
structural components of plants, including many of the plant foods we eat:
- cellulose
- hemicellulose
- lignin
- pectin's
- mucilage's
- gums
The first three are insoluble fibers which can absorb and hold water in the digestive
system. The others are soluble fibers, which are partially broken down in digestion to a
gel-like substance, which also retains water.
What does it do?
Fiber's ability to hold water and to bind minerals and cholesterol-like materials results
in a number of physiological effects which vary depending on the type of fiber and/or
where it is in the digestive tract.
- In the mouth, fiber stimulates the flow of saliva.
- In the stomach and small intestine, fiber dilutes the contents and delays the emptying
of food and the absorption of nutrients; this promotes a feeling of fullness.
- In the large intestine, fiber dilutes the contents and provides a place for bacterial
growth and digestion. The water-holding capacity of insoluble fiber in the lower intestine
softens the stool and increases stool size, so that the process of elimination is easier
and faster.
- In the large intestine, fiber also acts to bind certain chemicals. Different kinds of
fiber have different binding capacities: when fiber binds cholesterol-like compounds, it
lowers cholesterol, a healthy result; when fiber binds minerals, it decreases their
absorption, a less desirable result.
Because of these physiological effects, fiber is considered beneficial in preventing,
alleviating or curing a number of diseases and conditions, including:
- arteriosclerosis (hardening of the arteries)
- excess food intake
- diverticular disease
- irritable bowel syndrome
- Crohn's disease
- gallstone formation
- constipation
Where do you get it?
Recent recommendations suggest that we should be getting fiber from a variety of foods
high in different types of fibers, rather than from dietary supplements. A healthy diet
should provide a mixture of both soluble and insoluble fibers.
About eight grams of daily fiber intake should be in the form of soluble fibers, such as:
- fruits, especially apples and citrus
- vegetables, especially leafy green varieties
- oats
Major sources of insoluble fibers include:
- wheat bran
- whole grains
- legumes
- most fruits and vegetables
A good source of fiber should have at least three grams of fiber. High-fiber foods
provide five grams or more. The accompanying chart lists a variety of foods and their
fiber content.
Sources of fiber:
| Food |
Amount of fiber (in gms) in a 100g
(3.5 ounce) serving |
| Bread: |
|
| Bagel |
2.1 |
| Bran bread |
8.5 |
| Pita bread, white |
1.6 |
| Pita bread, whole white |
7.4 |
| White bread |
1.9 |
| Cereals: |
|
| Bran cereal |
35.3 |
| Bran flakes |
18.8 |
| Cornflakes |
2.0 |
| Oatmeal |
10.6 |
| Wheat flakes |
9.0 |
| Grains: |
|
| Barley, pearled |
15.6 |
| Cornmeal, whole grain |
11.0 |
| De-germed |
5.2 |
| Oatbran, raw |
6.6 |
| Rice, raw & brown |
3.5 |
| Rice, raw & white |
1.0-2.8 |
| Rice, raw & wild |
5.2 |
| Wheat bran |
15.0 |
| Fruits: |
|
| Apple, with skin |
2.8 |
| Apricots, dried |
7.8 |
| Figs, dried |
9.3 |
| Kiwifruit |
3.4 |
| Pears, raw |
2.6 |
| Prunes, dried |
7.2 |
| Prunes, stewed |
6.6 |
| Raisins |
5.3 |
| Vegetables: |
|
| Beans: |
|
|
|
7.7 |
|
|
5.4 |
|
|
7.2 |
| Broccoli, raw |
7.7 |
| Brussel sprouts, cooked |
2.6 |
| Cabbage, white & raw |
2.4 |
| Cauliflower, raw |
2.4 |
| Corn, sweet & cooked |
3.7 |
| Peas with edible pods, raw |
2.6 |
| Potatoes, white & baked, with skin |
5.5 |
| Sweet potato, cooked |
3.0 |
| Tomatoes, raw |
1.3 |
| Others: |
|
| Corn chips, toasted |
4.4 |
| Nuts: |
|
|
|
11.2 |
|
|
9.0 |
|
|
6.4 |
|
|
8.0 |
|
|
10.8 |
| Tahini |
9.3 |
| Tofu |
1.2 |
Source: Provisional table on the dietary fibre content of
selected foods (Washington, D.C.: U.S. Department of Agriculture,1988).
How much do we need?
A healthy adult should get 20-25 grams of fiber a day, based on the assumption that we
need 10-13 grams of fiber a day for every 1,000 calories consumed. Unfortunately, most
Americans consume only about 10 grams. Children ages 3-18 need less fiber than adults, and
they need different amounts at different ages. To calculate a child's daily fiber
requirements, add the child's age to the number five (for five grams). For example, a
four-year-old needs nine grams of fiber a day.
To get the appropriate amount of fiber, adults should include the following in their
diets:
- two to three servings of whole grains (as part of the 6-11 recommended daily servings)
- five servings of fruits and vegetables a day
- one or two servings of legumes every week
Is it safe?
Yes. However, increasing your fiber intake to recommended levels may cause some unpleasant
effects unless you do it gradually and drink plenty of water. This can help if you avoid:
- gas, distention and/or diarrhea resulting from increases in fiber intake
- colon obstruction caused by very large intakes of fiber
- interference with the absorption of some minerals (though this should not be a problem
if you eat a healthy diet.)
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