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All About Fiber

Facts
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 fiber
Constipation
Flatulence
How to change from a low-fiber to a high-fiber diet

Facts

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.
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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.
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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 snacks.
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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.
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How much 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 recommended.
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Soluble and Insoluble

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.
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Dietary fiber content of foods

Food

Fiber (g)

Vegetables, av. serve of any Beans, kidney or baked,  
1 cup cooked
Peas, average serve
Sweet-corn kernels, 1 cup

Fruit, average piece
Fruit, dried, 50 g

Nuts or seeds, 30 g
Coconut, fresh, 75 g
Peanut butter, 30 g

Bread
  white, 2 slices
  multigrain, 2 slices
  whole meal, 2 slices
  rye, 2 slices

Cereals (average bowl)
  bran, mixed cereal,
  bran, processed wheat
  bran flakes
  cornflakes, rice bubbles
  mixed cereals (flakes and
      fruit)
  muesli, natural
  porridge (rolled oats)
Bran, unprocessed, 2 tbsp
Wheat biscuits, 2
Wheat germ, 1 tbsp

Barley, cooked, 1 cup
Pasta, cooked
white, 2 cups
whole meal
Rice, cooked, 1 cup
  white  
  brown
Wheat, cracked, cooked, 
  1 cup

1

18
5
7

3
9

3
10
3


2
3
5
3


11
9
7
1

3
5
4
6
4
2

4

4
9

2
3

5

   
   
The 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.
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Constipation

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 constipation worse.

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 become slack.

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.
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Flatulence

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.
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How to change from a low-fiber to a high-fiber diet

Low-fiber choices High-fiber choices
Breakfast
Cornflakes
White toast with honey  
Rolled oats, wheat biscuits or bran cereal  
Whole meal toast with marmalade
Morning tea
Coffee and biscuits   Coffee and whole meal fruit loaf
Lunch
Chicken sandwich / white bread
Apple, peeled  
Chicken and salad sandwich / whole meal bread
Apple, with peel

Afternoon snack

Chocolate bar   Banana  

Dinner

Grilled steak
Chips
Green salad
Ice-cream  
Grilled steak
Jacket potato
2 or 3 vegetables
Fruit salad  
Total = 11 g fiber Total = 41 g fiber

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