Fiber: Its Importance In Your Diet

What Is Fiber?

Fiber is a virtually indigestible substance that is found mainly in the outer layers of plants. Fiber is a special type of carbohydrate that passes through the human digestive system virtually unchanged, without being broken down into nutrients. Carbohydrates constitute the main source of energy for all body functions.

Food pyramid

Almost everyone hears about the need for enough fiber in the diet. But few people understand the importance of dietary fiber – or where to get it.

Fiber is important because it has an influence on the digestion process from start to finish:

  • Because it demands that food be more thoroughly chewed, fiber slows down the eating process and helps contribute to a feeling of being full, which in turn can help prevent obesity from overeating.
  • Fiber makes food more satisfying, probably because the contents of the stomach are bulkier and stay there longer.
  • Fiber slows digestion and absorption so that glucose (sugar) in food enters the bloodstream more slowly, which keeps blood sugar on a more even level.
  • Fiber is broken down in the colon (the main part of the large intestine) by bacteria (a process called fermentation), and the simple organic acids produced by this breakdown helps to nourish the lining of the colon.
  • These acids also provide fuel for the rest of the body, especially the liver, and may have an important role in metabolism.


Substantial amounts of fiber can be found in foods such as:

  • All-natural cereals
  • Whole-grain breads
  • Beans
  • Fruits
  • Vegetables
  • Nuts

Nice To Know:

Only plants produce fiber. No matter how chewy or “tough” animal products may be, they do not contain fiber – not even bones or eggshells.

There are two main types of fiber, and they have different effects on the body:

  • Insoluble fiber is mainly made up of plant cell walls, and it cannot be dissolved in water. It has a good laxative action.
  • Soluble fiber is made up of polysaccharides (carbohydrates that contain three or more molecules of simple carbohydrates), and it does dissolve in water. It has a beneficial effect on body chemistry, such as lowering blood cholesterol and blood sugar levels.

Nice To Know:

Dietary fiber is essentially the cell walls of plants. Cell walls provide the architecture or skeleton of a plant and serve several purposes:

  • They enclose and package the nutritious parts of the plant, especially the storage organs that are rich in starch, and the parts of the cells that contain sugars, vitamins, and minerals.
  • They provide a tough protective armor around the embryo of the future plant.

Nice To Know:

The understanding that fiber is good for you is relatively new. Until the 1970s, fiber was regarded, at best, as a nonentity – and at worst, as a hindrance to good nutrition. This attitude stemmed from years of food shortages and widespread undernutrition, when the aim was “getting the most out of food.”

Today, obesity is the most common form of malnutrition and is a factor in the two major causes of death – heart disease and cancers. So any food that helps people limit calories is desirable.

It was a naval doctor, T.L. Cleave (1906-83) who sparked the great re-think about fiber. He argued that refined or fiber-depleted carbohydrates are harmful in many ways. He was supported by a surgeon from East Africa, Denis Burkitt, who presented evidence that Western diseases are rare in Africa and other third-world countries where fiber intake is high.

Digestive tract

Facts about fiber

  • Fiber keeps stool soft and keeps the contents of the intestines moving.
  • Americans consume only about 10% of the fiber that they did 100 years ago.
  • A good diet should contain approximately 25 to 30 grams of fiber a day. The average American eats less than half of that.
  • The change in the way wheat was processed into flour at the turn of the century-from a crushing to a finer rolling process – accounts substantially for the depletion in dietary fiber.
  • Bran has the highest fiber content – about 25% to 45%.


What Are The Best Sources Of Fiber?

The following foods are good sources of fiber:

  • Whole grains (bran has the highest fiber content); this includes breads and cereals, whole-grain pastas, and brown rice
  • Nuts and seeds
  • Legumes (such as dried peas, beans, lentils)
  • Fruits
  • Vegetables
  • A dietary supplement of fiber products such as Citrucel orMetamucil, which are mixed with water and provide about 4 to 6 grams of fiber in each 8-ounce glass

When foods are processed, fiber is often removed. Foods made from white flour (bleached or unbleached) are poor sources of fiber, including white breads, pizza crusts, and regular pasta. In general, foods that are less processed are higher in fiber.

Some high-fiber foods – such as some breakfast cereals and convenience foods – are also high in sugar and salt, so take care to read the label before purchase.

Need To Know:

Q: Do I have to get my fiber from food? Is taking a fiber supplement enough?

A: Supplements provide only a very restricted type of fiber. Eating a diet of high-fiber foods usually incorporates various kinds of fiber, and that’s healthier. Fruits, vegetables, and oats have plenty of soluble fiber. Whole grains, bran, legumes, and many fruits and vegetables are full of insoluble fiber. Both soluble and insoluble fiber add bulk and softness to the stool. Insoluble fiber remains pretty much unchanged by the time it reaches the intestines, whereas soluble fiber acquires a soft, jelly-like texture. Both make stools easier to pass.


How Do I Get More Fiber In My Diet?

A good diet should contain approximately 25 to 30 grams of fiber a day. The average American eats less than half of that.

Getting more fiber in your diet doesn’t have to mean a drastic change. In fact, it’s best to start slowly, in order to avoid constipation from getting too much fiber all at once. Many fiber-depleted foods in the diet can be replaced by high-fiber alternatives.

Fruits and vegetables that are high in fiber include:

  • Apples
  • Beans
  • Berries
  • Broccoli
  • Brussels sprouts
  • Carrots
  • Cauliflower
  • Figs
  • Oranges
  • Pears
  • Peas
  • Prunes

Here are other good sources of fiber:

  • Bran muffins
  • Brown rice
  • Multi-grain cereals
  • Oatmeal
  • Popcorn
  • Whole-wheat bread

Need To Know:

It’s important to drink more fluids when you increase the amount of fiber you eat. You should drink at least eight glasses of water a day, especially when increasing your fiber intake.

Most everyday low-fiber foods have a higher-fiber alternative:

Some fiber-depleted foods

Corn flakes, crispy rice cereal

White bread


Cheese crackers

Fruit juice

Cakes, biscuits, sweets



Fiber-intact alternatives

Shredded wheat, puffed wheat

Whole-grain bread

Whole-grain muffins

Wheat crackers

Fresh fruit, stewed fruit

Dried fruit, nuts, raw carrots, celery

Fresh-fruit salad

Nut butters (cashew, almond, etc.)

Studies have shown that a high-fiber diet has widespread health benefits. And unlike many other treatments, fiber in the diet has no danger of adverse reactions, toxicity, or dangerous side effects.

How-To Information:

Here’s some advice on incorporating more fiber in your diet:

  • It’s best to start slowly, especially if you tend to become constipated. Introduce high-fiber foods gradually, over two to four weeks. Don’t start a high-fiber diet overnight!
  • Eat a wide variety of plant foods (foods that come from plants, as opposed to meats or dairy products). Different fibers do different jobs in the body.
  • Choose foods whose fiber content has not been depleted through processing.
  • Read food labels to learn how much fiber is contained in the various foods you eat.
  • Drink plenty of water – at least eight glasses a day.
  • Some medical conditions do not benefit from a high-fiber diet. If you are being treated for a health disorder, check with your doctor before adding fiber to your diet.
  • Raw bran increases the excretion into the stools of calcium, iron, and zinc. For most people eating a good balanced diet, this is of no consequence. But theoretically, it might lead to depletion of these minerals in pregnant and breast-feeding women, and in people with small appetites. Such people should take calcium supplements or extra milk or cheese if they are taking bran regularly.


How Does The Body Use Fiber?

Fiber has numerous effects in the large intestine:

  • Fiber is attacked and broken down by the huge population of bacteria that live in the colon.
  • The breakdown products are acids and gases. This process is called fermentation. Dietary fiber is only partly fermented, because some plant cell walls resist bacterial attack.
  • The simple organic acids produced by fermentation are mostly absorbed, and in doing so they nourish the lining of the colon. They also provide fuel for the rest of the body, especially the liver. This may have important consequences for metabolism; half the calories in fiber are made available to the body.
  • The gases arising from fermentation soften and enlarge the stool. They are also passed as wind (flatus) and can contribute to bloated feelings in some people.

Here is a summary of the actions of dietary fiber at different levels of the digestive system:


Makes work, slows ingestion, cleans teeth


Dilutes contents, distends, prolongs retention of food

Small intestine

Dilutes contents, distends, slows absorption

Right colon

Dilutes, distends, feeds bacteria and so acidifies contents

Left colon

Nourishes the colon, speeds up passage, lowers pressures

Rectum, anus

Softens and enlarges stool, prevents trauma to the body,

reduces strain of defecation

What Are The Health Benefits Of Fiber?

Fiber is helpful to the body in many ways:

Avoiding And Relieving Constipation

Fiber can absorb large amounts of water in the bowels, and this makes stools softer and easier to pass. Anyone starting a higher-fiber diet will notice the difference in stool bulk.

  • In almost all cases, increasing fiber in the diet will relieve constipation within hours or days.
  • Because stools are easier to pass, less straining is necessary, and this can help relieve hemorrhoids.

Need To Know:

Constipation can have other causes, however, so you should consult your doctor if it is not relieved by increased fiber.

Nice To Know:

On average, it takes 39 hours in women and 31 hours in men for food to pass through the colon and out of the body. This time varies a lot from person to person, depending on personality, state of mind, and fiber intake. Usually, the effect of fiber is to speed up this process.

Preventing Certain Diseases

Getting enough fiber in the diet can lower the risk of developing certain conditions:

  • Heart disease. Evidence is now growing to support the notion that foods containing soluble fiber (such as oats, rye barley, and beans) can have a positive influence on cholesterol, triglycerides, and other particles in the blood that affect the development of heart disease. Some fruits and vegetables (such as citrus fruits and carrots) have been shown to have the same effect.
  • Cancer. The passage of food through the body is speeded up when fiber is eaten. Some experts believe this may prevent harmful substances found in some foods from affecting the colon and may protect against colon cancer. (However, a recent study conducted by Harvard University concluded that eating high-fiber food did not appear to protect people from colon cancer.) Other types of cancer that are linked with overnutrition and may be prevented by a fiber-rich diet include breast cancer, ovarian cancer, and uterine cancer.
  • Diabetes. Adding fiber to the diet helps regulate blood sugar levels, which is important in avoiding diabetes. In addition, some people with diabetes can achieve a significant reduction in their blood sugar levels and may find they can reduce their medication.
  • Diverticular disease. Diverticular disease is a condition in which small pouches, called diverticula, develop in the wall of the colon. In a small percentage of people, these diverticula become inflamed or infected, a condition known as diverticulitis. Diverticular disease can cause pain, diarrhea, constipation, and other problems.
  • Gallstones and kidney stones. Rapid digestion leads to a rapid release of glucose (sugar) into the bloodstream. To cope with this, the body has to release large amounts of insulin into the bloodstream, and this can make a person more likely to develop gallstones and kidney stones (in addition to diabetes and high cholesterol).

For further information about diverticular disease, go to Diverticular Disease.

For further information about gallstones, go to Gallstones.

For further information about kidney stones, go to Kidney Stones.

Keeping Weight Under Control

Foods containing plenty of fiber have more bulk than low-fiber foods. If taken in the right form at the right time and at sufficient quantities, fiber can sometimes slow the onset of hunger.

Nice To Know:

To help control your weight with fiber:

  • Always try to take fiber in the natural form. For example, instead of sprinkling bran over your food, choose foods naturally high in fiber.
  • Avoid foods that have been made easier to eat and digest by removal of fiber, especially sugars (including fruit juices).
  • Choose foods that satisfy hunger without providing many calories, mainly vegetables and most fruits, which are rich in fiber.

Need To Know:

Some individuals claim that fiber alone can cause weight loss without the need to diet. But in fact, the only effective and safe way to lose weight is to:

  • Reduce calorie intake to a safe level
  • Get enough exercise to burn off excess calories

However, fiber can be a useful aid in reducing calorie intake.


Fiber: Frequently Asked Questions

Here are some frequently asked questions related to Fiber: Its Importance In Your Diet.

Q: Exactly how much fiber should I get in my diet?

A: The American Dietetic Association recommends 20 to 35 grams of fiber each day. Your doctor may also recommend drinking a fiber product such as Citrucel or Metamucil once a day. These products are mixed with water, and each 8-ounce glass provides about 4 to 6 grams of fiber.

Q: Can fiber really help me lose weight?

A: In a nutshell, fiber slows things down at the upper end and speeds them up at the lower end (in other words, “slow in, fast out”). It makes the entrance of food more difficult (by demanding that food be chewed), and it makes it easier to pass stools. In between, it makes food more satisfying, probably because the contents of the stomach are bulkier and stay longer. These actions can help you limit calories.

Q: Recently, when I tried eating more fiber, I had cramps and a bloated feeling. Is this normal?

A: Many people notice bloating, cramping, or gas when they begin to add more fiber to their diet. Adding fiber gradually to your diet over a period of time can help prevent this. It’s important to drink more fluids when you increase the amount of fiber you eat. The recommended amount of water is eight glasses a day.

Q: I drink lots of fruit juices. Does this count as extra fiber in my diet?

A: Fruit and vegetable juices usually contain practically no fiber, because the juice has been squeezed out of the plant material and the fiber is left behind. But freezing, drying, and normal cooking do not significantly change the fiber content of most foods. Fresh or cooked vegetables, as well as dried or canned fruit, all add fiber.

Fiber: Its Importance In Your Diet – Putting It All Together

Here is a summary of the important facts and information related to Fiber: Its Importance In Your Diet.

  • Fiber is a special type of carbohydrate that passes through the human digestive system virtually unchanged, without being broken down into nutrients.
  • Fiber has a positive influence on the digestion process from start to finish.
  • Fiber is found only in food that come from plants, including all-natural cereals, whole-grain breads, beans, fruits, vegetables, and nuts.
  • A good diet should contain approximately 25 to 30 grams of fiber a day, but the average American eats less than half of that.
  • When incorporating more fiber in your diet, it’s best to start slowly and to eat a wide variety of fiber-rich foods, since different types of fiber do different jobs in the body.
  • Fiber helps relieve constipation and hemorrhoids, can help keep weight under control, and can help prevent certain diseases such as heart disease, cancer, diabetes, diverticular disease, gallstones, and kidney stones.

Fiber: Glossary

Here are definitions of medical terms related to Fiber: Its Importance In Your Diet.

Carbohydrate: Carbohydrates constitute the main source of energy for all body functions; the most important being saccharides, starch, cellulose, and gum.

Colon: The main part of the large intestine, responsible for absorbing water and salts from the digested products of the small intestine, and passing the digested products into the rectum for removal from the body.

Diverticular disease: A range of conditions that develop from the presence of one of more small pouches that protrude out of the normally smooth wall of the colon; these pouches can become inflamed and cause symptoms that include abdominal pain, constipation or diarrhea, and bleeding from the rectum.

Fiber: The parts of plant cells that are undigested in the small intestine; fiber is important to the health of the digestive system.

Insoluble fiber: This type of fiber is mainly made up of plant cell walls, and it cannot be dissolved in water.

Soluble fiber: This type of fiber is made up of polysaccharides (carbohydrates that contain three or more molecules of simple carbohydrates), and it does dissolve in water.

Fiber: Additional Sources Of Information

Here are some reliable sources that can provide more information on Fiber: Its Importance In Your Diet.

The University of Maryland publishes a page on its Web site that contains a listing of fiber content in popular foods, good sources of fiber, and not-so-good sources. The page may be found at University Of Maryland Handouts.

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