Fiber, also known as roughage, is the indigestible part of plant foods that travels through our digestive system, absorbing water along the way and easing bowel movements.

Dietary fiber refers to nutrients in the diet that are not digested by gastrointestinal enzymes but still fulfil an important role.

Mostly found in vegetables, fruits, whole grains, and legumes, fiber has a host of health benefits, including reducing the risk of heart disease and diabetes.

In this article, we will look at the different types of fiber, why they are important, and what foods contain high levels of fiber.

Fast facts on fiber

Here are some key points about fiber. More detail and supporting information is in the main article.

  • Fiber is often split into two types: soluble and insoluble.
  • Dairy products and white bread have little to no fiber.
  • Cereal grains, seeds, vegetables, and fruits are good sources of fiber.
  • Fiber helps speed up the elimination of toxic waste through the colon.
  • Oat cereals, Brussels sprouts, oranges, flax seeds, and beans, such as kidney, black, and pinto, are all good sources of soluble fiber.

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Fruits, vegetables, whole grains and legumes are all good sources of dietary fiber.

Eating fiber has many health benefits:

Protection against heart disease - according to the University of Maryland Medical Center, the consumption of soluble fiber has been shown to protect against heart disease by reducing cholesterol levels.

Gastrointestinal health - the consumption of fiber promotes regular bowel movements and prevents constipation. It may also reduce the risk of developing colitis and hemorrhoids. There is also mixed evidence that consuming fiber might help reduce the risk of colon cancer.

Diabetes - people with diabetes who consume a lot of fiber tend to need less insulin than those whose fiber intake is low. Fiber can help slow the absorption of sugar, helping to prevent spikes after meals.

Body weight - a high-fiber intake can significantly contribute toward body-weight control. Fiber produces a feeling of fullness without adding calories (fiber calories are not absorbed by the body) - this can help treat or prevent overweight/obesity.

Most foods that are high in fiber are also very healthy for other reasons. Take, for example, fruit, vegetables, and whole grains; they are high in fiber but also rich in vitamins and other essential nutrients. In other words, eating a high-fiber diet protects health through both the intake of fiber and other essential nutrients.

Fiber consists of non-starch polysaccharides, such as cellulose, dextrins, inulin, lignin, chitins, pectins, beta-glucans, waxes, and oligosaccharides.

There are two broad types of fiber: soluble and insoluble.

  • Soluble fiber dissolves in water. It changes as it goes through the digestive tract where it is fermented by bacteria. As it absorbs water, it becomes gelatinous.
  • Insoluble fiber does not dissolve in water. As it goes through the digestive tract, it does not change its form. It can also be fermented by bacteria in the colon.

Foods that contain dietary fiber are generally divided into predominantly soluble or insoluble; both types of fiber are present in all plant foods, but rarely in equal proportions.

Both forms of fiber have major health benefits.

Insoluble fibers have many functions, including moving bulk through the digestive tract and controlling pH (acidity) levels in the intestines.

Benefits of insoluble fiber:

  • Promotes regular bowel movements and prevents constipation.
  • Speeds up the elimination of waste through the colon.
  • By keeping an optimal pH in the intestines, insoluble fiber helps prevent microbes from producing substances which can lead to colorectal cancer.

Food sources of insoluble fiber include vegetables - especially dark green leafy ones, root vegetable skins, fruit skins, whole-wheat products, wheat bran, corn bran, nuts, and seeds.

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Legumes are rich in soluble and insoluble fiber.

Soluble fiber binds with fatty acids, it slows down the time it takes to empty the stomach and the rate of sugar absorption by the body.

Benefits of soluble fiber:

  • Reduces cholesterol, especially levels of low-density lipoproteins (LDL - bad cholesterol).
  • Regulates sugar intake, this is especially useful for people with diabetes and metabolic syndrome.
  • Soluble fiber is fermented by gut bacteria, improving immune, digestive, and overall health.

Good sources of soluble fiber include kidney beans, pinto beans, Brussels sprouts, broccoli, spinach, zucchini, apples, oranges, grapefruit, grapes, prunes, oatmeal, and whole-wheat bread.

According to the Academy of Nutrition and Dietetics, the recommended daily amount of fiber for women is 25 grams and, for men, it is 38 grams. After the age of 50, the recommended intake for women is 21 grams and men is 30 grams.

As most high-fiber containing foods usually have insoluble and soluble fiber, it is not necessary to be too careful about dividing them up.

In other words, the focus should be on fiber intake in general, rather than on the specific type of fiber.

Oat, oat brans, psyllium husk, and flaxseed are rich in both types of fibers.

Consuming 25 grams of fiber each day should be enough to meet daily requirements. Ideally, individuals should consume at least five servings of fruit and vegetables, as well as some servings of whole grain products, each day.

According to Dr. Warren Enker, at the Department of Surgery, Beth Israel Medical Center, MA, a good diet involves watching your calorie count, including food rich in nutrients and vitamins, avoiding saturated fats, and paying particular attention to all sources of fiber.

Below is a selection of foods that contain high amounts of fiber:

Cereal grains - ½ cup cookedSolubleInsoluble
Barley1 gram4 grams
Oatmeal1 gram2 grams
Oat bran1 gram3 grams
SeedsSolubleInsoluble
Psyllium seeds ground (1 Tablespoon)5 grams6 grams
Fruit (1 medium fruit)SolubleInsoluble
Apple1 gram4 grams
Banana1 gram3 grams
Blackberries (½ cup)1 gram4 grams
Citrus Fruit (orange, grapefruit)2 grams2-3 grams
Nectarine1 gram2 grams
Peach1 gram2 grams
Pear2 gram4 grams
Plum1 gram1.5 grams
Prunes (¼ cup)1.5 grams3 grams
Legumes (½ cup cooked)SolubleInsoluble
Black Beans2 grams5.5 grams
Kidney Beans3 grams6 grams
Lima Beans3.5 grams6.5 grams
Navy Beans2 grams6 grams
Northern Beans1.5 grams5.5 grams
Pinto Beans2 grams7 grams
Lentils (yellow, green, orange)1 gram8 grams
Chick Peas1 gram6 grams
Black-eyed Peas1 gram5.5 grams
Vegetables (½ cup cooked)SolubleInsoluble
Broccoli1 gram1.5 grams
Brussels Sprouts3 grams4.5 grams
Carrots1 gram2.5 grams

People who are allergic to high-fiber foods can find it difficult to get the right amount of fiber. However, with such a wide variety of fiber-containing foods around, it should be possible to find some that do not cause a reaction.

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Apples can be a good source of fiber for people who are allergic to other high fiber foods.

Also, pharmacies sell fiber supplements such as Metamucil, Citrucel, and FiberCon which can help bridge the gap. Although these products do not provide the same levels of vitamins and nutrients as natural fiber-containing foods, they can be useful for some people.

The following high-fiber foods are the least likely to be allergenic:

  • apples
  • pears
  • berries
  • artichokes
  • potatoes
  • swede and other root vegetables
  • broccoli
  • green beans
  • pumpkin
  • zucchini
  • peas
  • lentils
  • quinoa