How Much Dietary Fiber Do You Need?

Fiber is found in the cell walls of plants, including the plants you eat: fruits, vegetables, legumes, and grains. Fiber functions as a skeleton for plants to help maintain their 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 in the same way as carbohydrates, fats, and proteins. Instead, fiber provides other benefits to the human body.

Benefits of Dietary Fiber

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.

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).

Requirements

The Dietary Guidelines for Americans (published by the U.S. Departments of Agriculture and Health and Human Services) includes recommendations for daily total fiber intake based on age and gender.

Most people (more than 90% of women and 97% of men!) do not get enough fiber. But if you currently have a low-fiber diet, you may want to increase your daily intake of high-fiber foods slowly because some fiber may cause gas and bloating. Over time, your body adjusts to the increased fiber intake and the gas and bloat will decrease.

Soluble vs. Insoluble Fiber

There are different types of fiber. One way to classify fiber is by how easily it dissolves in water. While it's helpful to know about the differences, you don't need to track how much of each you're getting. As long as you eat a variety of high-fiber foods such as grains, nuts, legumes, fruits, and vegetables, you will get plenty of both types.

Fiber Components

Dietary fiber is made up of a combination of cellulose, hemicellulose, lignin, pectin, chitin, gums, beta-glucan, and resistant starches.

Cellulose and Hemicellulose

Cellulose is an insoluble dietary 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, 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, and some forms are fermented by bacteria while others are not.

Lignin

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

Pectin

If you've ever made jam at home, you've probably used pectin to help the 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 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 and are 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.