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

The word fiber (North American) can also be spelled fibre (British). It comes from the Latin word fibra, meaning fiber, thread, string, filament, entrails. Dietary fiber refers to nutrients in the diet that are not digested by gastrointestinal enzymes.

This Medical News Today information article provides details on the types of dietary fiber (soluble and insoluble), a list of some fiber-rich foods, the function and benefits of insoluble fiber, and the function and benefits of soluble fiber.

The article also highlights how much fiber people should consume, why fiber is important, and what to do if you suffer from food allergies and find including fiber in your diet difficult.

Two types of fiber - soluble and insoluble

Legumes are rich in soluble
and insoluble fiber

Fiber is made up of non-starch polysaccharides, such as cellulose, dextrins, inulin, lignin, chitins, pectins, beta-glucans, waxes and oligosaccharides. The word fiber is misleading, because many types of dietary fibers are not fibers at all.

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.

Dietary fiber foods are generally divided into predominantly soluble or insoluble.

Both types of fiber are present in all plant foods, but rarely in equal proportions.

Fiber-rich foods

A good diet involves watching your calorie count, including food rich in nutrients and vitamins, avoiding saturated fats, as well as paying particular attention to all sources of fiber, according to Warren Enker, M.D.1, F.A.C.S. Vice Chairman, Department of Surgery, Beth Israel Medical Center.

Diary products, white bread, and other starches are low in fiber.

Below is a list of some foods that contain high amounts of fiber.

Cereal grains - ½ cup cooked Soluble Insoluble
Barley 1g 4g
Oatmeal 1g 2g
Oatbran 1g 3g
Seeds Soluble Insoluble
Psyllium seeds ground (1 Tbsp) 5g 6g
Fruit (1 medium fruit) Soluble Insoluble
Apple 1g 4g
Banana 1g 3g
Blackberries (½ cup) 1g 4g
Citrus Fruit (orange, grapefruit) 2g 2-3g
Nectarine 1g 2g
Peach 1g 2g
Pear 2g 4g
Plum 1g 1.5g
Prunes (¼ cup) 1.5g 3g
Legumes (½ cup cooked) Soluble Insoluble
Black Beans 2g 5.5g
Kidney Beans 3g 6g
Lima Beans 3.5g 6.5g
Navy Beans 2g 6g
Northern Beans 1.5g 5.5g
Pinto Beans 2g 7g
Lentils (yellow, green, orange) 1g 8g
Chick Peas 1g 6g
Black eyed Peas 1g 5.5g
Vegetables (½ cup cooked) Soluble Insoluble
Broccoli 1g 1.5g
Brussels Sprouts 3g 4.5g
Carrots 1g 2.5g

What are the functions and benefits of insoluble fiber?

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 toxic 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.

What are the functions and benefits of soluble fiber?

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

Benefits of soluble fiber:

  • It reduces cholesterol, especially levels of LDL (bad cholesterol)
  • It regulates sugar intake, this is especially useful for people with diabetes and metabolic syndrome

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

How much insoluble and soluble fiber should I eat?

According to the Academy of Nutrition and Dietetics2, the recommended daily amount of fiber for women is 25 grams and for men its 38 grams. However, after the age of 50 it is recommended that women drop their intake to 21 grams and men to 30 grams.

Most dietitians say your ratio of insoluble vs. soluble fiber should be 75% to 25%, or 3 parts insoluble to every 1 part soluble. As most high-fiber containing foods usually have both types, it should not be necessary to be too careful about dividing them up.

Oat, oat brans, psyllium husk and flax seed are rich in both types of fibers. In other words, your focus should be on fiber intake in general, rather than what type of fiber.

If you consume 25g of fiber each day you should meet your daily requirements. Ideally, you should consume 5 servings of fruit and vegetables, as well as some servings of whole grain products, each day.

The American fiber intake gap

The illustration below shows (in gray) how much fiber people should be eating and (in black) how much they actually consume in the United States.

American Fiber Intake Gap

The USDA6 says people are not eating enough fiber.

Why fiber is good for you

Eating fiber has many benefits for your health. According to the University of Maryland Medical Center3, the consumption of soluble fiber has been shown to protect you from developing heart disease by reducing your cholesterol levels. The consumption of insoluble fiber may also reduce your risk of developing constipation, colitis, colon cancer, and hemorrhoids.

People with diabetes who consume a lot of fiber tend to need less insulin than those whose fiber intake is low.

A high-fiber intake can significantly contribute towards your body-weight control. Fiber fills you up without adding calories (fiber calories are not absorbed by your body) - this can help treat or prevent overweight/obesity. 

Most foods which are high in fiber are also very good for you for other reasons. Take, for example, fruit and vegetables and whole grains, they are high in fiber but also rich in vitamins and other essential nutrients. In other words, if you seek a high-fiber diet, not only will you be protecting your health because of your fiber intake, but also because you will consume other essential nutrients.

Recent developments on why fiber is good for you from MNT news

High fiber diet protects you from stroke. Researchers from the University of Leeds' School of Food Science & Nutrition in Leeds, United Kingdom, reported an association in the journal Stroke4 between eating more fiber and a lower risk of first time stroke.

Fiber intake level key in warding off death. A study conducted by researchers from the National Cancer Institute5 found that those who ate a high-fiber diet decreased their risk of dying over a nine year period compared to those who ate less fiber. A total of 219,123 men and 168,999 women ages 50 to 71 years of age were included in the study.

Food allergies

If you suffer from food allergies, which often seem to be high-fiber foods, getting the right amount of fiber can be a challenge. With such a wide variety of fiber containing foods around, you should be able to find some that you are not allergic to. When you do find the foods you can tolerate, you may have to forward plan more than other people who do not have food allergies. Pharmacies sell fiber supplements, which can help you bridge the gap.

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

  • Apples
  • Pears
  • Melons(fresh)
  • Carrots
  • Potatoes
  • Swede
  • Broccoli
  • Green beans
  • Pumpkin
  • Zucchini (courgette)
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