What Is Dietary Fiber and How Much Is Needed?

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Fiber is found in the cell walls of plants, including the plants you eat. Fiber functions as a skeleton for the plants to help maintain plant shape and structure. Fiber is good for people, too, but not because it contains any nutrients - in fact, human digestive enzymes cannot break fiber down like carbohydrates, fats, and proteins.

Since fiber can't be digested, it can't be absorbed like other nutrients, so it passes through the small intestine into the colon.

 That's good because it adds bulk to the stool, which makes elimination easier and helps keep the colon healthy. Some disorders -- like diverticulitis, constipation, and irregularity -- may be associated with inadequate fiber intake.

Following a high-fiber diet may help you lose weight. Probably because the fiber in high-fiber foods helps you feel fuller longer.

Fiber is also good for overall digestive health because the friendly bacteria that live in your colon ferment some types of fiber, creating beneficial short-chain fatty acids that help keep the intestinal walls healthy. (Unfortunately, it also causes the formation of intestinal gas - but there are ways to deal with that).

Classifying Fiber - Soluble and Insoluble

One way to classify fiber is by how easily it dissolves in water. Soluble fiber dissolves in water, which helps soften stools, making them easier to eliminate from the body.

Soluble fiber is found in oats, citrus fruits, apples, barley, psyllium, flax seeds and beans. 

Insoluble fiber is found in whole grains, nuts, wheat bran and vegetables. This fiber doesn't dissolve in water, so it helps move material through the colon faster by increasing the bulk of the stool. That can be helpful to people who suffer from constipation or irregularity.

Diets high in insoluble fiber may also decrease the risk of diabetes.

What Makes a Fiber?

Dietary fiber is made up of some combination of cellulose, hemicellulose, lignin, pectin, chitin, gums, beta-glucan, and resistant starches. Here's a look at each component:

Cellulose and Hemicellulose

Cellulose is an insoluble dietary fiber and may also be used as a functional fiber. Celluloses are long straight chains of glucose molecules and are found as the central component in cell walls of plants.

The bacteria in your intestinal tract cannot ferment cellulose well either, so the primary function of cellulose is to increase stool bulk and decrease the time it takes for the fecal material to pass through the colon. Foods that contain significant amounts of cellulose include bran, legumes, nuts, peas, roots, cabbage and apple skins.

Hemicellulose is found in bran, nuts, legumes and whole grains. Rather than just long straight chains (like cellulose), hemicellulose may have side chains and branches. Because of these variations, some hemicelluloses are soluble in water, and some are insoluble, plus some forms are fermented by bacteria while others are not.

Lignin

Lignin has lots of branches of chemicals called phenols rather than sugar molecules. Phenols are currently being studied for a variety of health-related effects including antioxidant actions. Lignin is insoluble in water and indigestible by the friendly bacteria. Food sources include root vegetables, wheat, and berry seeds.

Pectin

If you've ever made jams or jellies at home, you've probably used pectin to help your fruit gel. Pectin is another water-soluble fiber found in the cell walls of plants. But it doesn't make a good stool bulking agent because it is a favorite fiber for the friendly bacteria in your gut to ferment, so that very little passes through the colon. Pectin is found in apples, legumes, nuts and citrus fruits.

Chitin

Chitin is similar to cellulose because it's insoluble in water, and made up of glucose chains. But it also has amino acids attached, similar to proteins. Chitin is found not only in plants but also in the exoskeletons of insects and shells of crustaceans.

Gums

Gums are soluble in water secreted by plants when they're damaged. Gums are used in food manufacturing as thickening and gelling agents. Examples of gums include guar gum, carob gum, gum Arabic and xanthan gum.

Beta-glucan

Beta-glucan is a water-soluble dietary fiber found in oats and barley, and it's often used as a functional fiber and added to foods. Beta-glucans have been shown to reduce cholesterol levels and help control blood-sugar levels.

Resistant Starches

Resistant starch really is a starch, but it's considered to be a fiber because amylase -- the enzyme that breaks starch into individual glucose units -- doesn't work on this type of starch. Resistant starch can occur as starch trapped in cell walls of plants or may be formed while cooking or food processing.

Okay - So How Much Fiber Do I Need?

According to the Institute of Medicine:

The recommended intake for total fiber for adults 50 years and younger is set at 38 grams for men and 25 grams for women, while for men and women over 50 it is 30 and 21 grams per day, respectively, due to decreased food consumption.

Do you need to worry about getting the different types of fiber? Probably not. As long as you eat a variety of high-fiber foods such as grains, nuts, legumes, fruits and vegetables, you will get plenty of soluble and insoluble fibers.

Oh and remember I said that some fiber may cause gas? People who currently have low fiber diets may want to increase their daily intake of high-fiber foods slowly because some fiber may increase gas and bloat. Over time, your body adjusts to the increased fiber intake and the gas and bloat will decrease.

Sources:

"Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids." Institute of Medicine of the National Academies. September 05, 2002. Accessed May 26, 2016. https://iom.nationalacademies.org/Reports/2002/Dietary-Reference-Intakes-for-Energy-Carbohydrate-Fiber-Fat-Fatty-Acids-Cholesterol-Protein-and-Amino-Acids.aspx.

Gropper SS, Smith JL, Groff JL. "Advanced Nutrition and Human Metabolism." Sixth Edition. Belmont, CA. Wadsworth Publishing Company, 2013. Accessed May 26, 2016.

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