a variety of high fiber foods including berries, nuts, seeds, fruits, vegetables, and whole grains

Fiber is a type of carbohydrate that the body can’t digest. Though most carbohydrates are broken down into sugar molecules called glucose, fiber cannot be broken down into sugar molecules, and instead it passes through the body undigested. Fiber helps regulate the body’s use of sugars, helping to keep hunger and blood sugar in check.

Children and adults need at least 25 to 35 grams of fiber per day for good health, but most Americans get only about 15 grams a day. Great sources are whole grains, whole fruits and vegetables, legumes, and nuts.

Types of Fiber

Fiber comes in two varieties, both beneficial to health:

Soluble fiber, which dissolves in water, can help lower glucose levels as well as help lower blood cholesterol. Foods with soluble fiber include oatmeal, chia seeds, nuts, beans, lentils, apples, and blueberries.

Insoluble fiber, which does not dissolve in water, can help food move through your digestive system, promoting regularity and helping prevent constipation. Foods with insoluble fibers include whole wheat products (especially wheat bran), quinoa, brown rice, legumes, leafy greens like kale, almonds, walnuts, seeds, and fruits with edible skins like pears and apples.

Further defining fiber

Under the umbrella terms of insoluble and soluble fibers, you may see fiber described in other ways. It can be viscous with a gel-like quality, or fermentable because it acts as food for gut bacteria that break down and ferment it. Fibers that are not broken down by bacteria, called nonfermentable, travel intact to the colon and can add bulk and weight to stool so it is easier to pass. These properties offer health benefits such as slowing down digestion, delaying blood sugar rises after meals, promoting healthy colonies of bacteria, or having a laxative effect. In addition, there are many subtypes of soluble and insoluble fibers, some of which occur naturally in plant foods and others that are synthetically made.

The National Academy of Medicine defines fiber as: 1) dietary fibers (nondigestible carbohydrates and lignans) that occur naturally in plants, and 2) functional fibers that are extracted from plants or synthetically made and are nondigestible with a beneficial health effect in humans. [1] Some types of fiber fall into both categories, such as oligosaccharides and resistant starches that may be naturally occurring or synthetically made.

Naturally occurring plant fibers:

  • Cellulose, hemicellulose – Insoluble fiber found in cereal grains and the cell walls of many fruits and vegetables. It absorbs water and adds bulk to stool, which can have a laxative effect.
  • Lignins – Insoluble fiber found in wheat and corn bran, nuts, flaxseeds, vegetables, and unripe bananas that triggers mucus secretion in the colon and adds bulk to stools. Has laxative effect.
  • Beta-glucans – Soluble highly fermentable fiber found in oats and barley that is metabolized and fermented in the small intestine. Acts as a prebiotic. Can add bulk to stool but does not have a laxative effect. May help to normalize blood glucose and cholesterol levels.
  • Guar gum – Soluble fermentable fiber isolated from seeds. Has a viscous gel texture and is often added to foods as a thickener. It is metabolized and fermented in the small intestine. Does not have a laxative effect. May help to normalize blood sugar and cholesterol levels.
  • Inulin, oligofructose, oligosaccharides, fructooligosaccharides – Soluble fermentable fibers found in onions, chicory root, asparagus, and Jerusalem artichokes. May help to bulk stool with a laxative effect, normalize blood glucose, and act as a prebiotic. People with irritable bowel syndrome may be sensitive to these fibers that can cause bloating or stomach upset.
  • Pectins – Soluble highly fermentable fiber found in apples, berries, and other fruits. Minimal bulking or laxative effect. Due to its gelling properties, it may slow digestion and help normalize blood sugar and cholesterol levels.
  • Resistant starch – Soluble fermentable fiber found in legumes, unripe bananas, cooked and cooled pasta, and potatoes that acts as a prebiotic. Adds bulk to stools but has minimal laxative effect. May help to normalize blood sugar and cholesterol levels.

Manufactured functional fibers, some of which are extracted and modified from natural plants:

  • Psyllium – Soluble viscous nonfermentable fiber extracted from psyllium seeds that holds onto water and softens and bulks stools. Has laxative effect and is an ingredient in over-the-counter laxatives and high-fiber cereals. May help to normalize blood sugar and cholesterol levels.
  • Polydextrose and polyols – Soluble fiber made of glucose and sorbitol, a sugar alcohol. It can increase stool bulk and have a mild laxative effect. Minimal effect on blood sugar or cholesterol levels. It is a food additive used as a sweetener, to improve texture, maintain moisture, or to increase fiber content.
  • Inulin, oligosaccharides, pectins, resistant starch, gums – Soluble fibers derived from plant foods as listed above, but are isolated or modified into a concentrated form that is added to foods or fiber supplements.

Fiber and Disease

Fiber appears to lower the risk of developing various conditions, including heart disease, diabetes, diverticular disease, and constipation. Fiber’s beneficial role in the gut microbiome may produce anti-inflammatory effects that alleviate the chronic inflammation associated with these conditions. [2]

a person picking up an almond from a plate with nuts and berries

Should I avoid nuts and seeds with diverticulosis?

If you have diverticulosis, chances are you’ve heard that maybe you should avoid certain foods with small hard-to-digest particles: nuts, seeds, popcorn, corn, and fruits and vegetables with seeds like raspberries, strawberries, cucumber, or tomatoes. The reasoning is that these small undigested food particles might become trapped in the diverticular pouches and become inflamed from bacterial infection, causing the uncomfortable condition called diverticulitis. People who have experienced intense symptoms of diverticulitis often change their diets to avoid these foods in hopes of preventing a recurrence. However, evidence has shown this practice to be more of an urban legend than helping to reduce recurrences, and can deter people from eating foods that may actually help their condition in the future.

Although the role of diet with diverticular disease has long been debated, a high-fiber intake with a focus on whole grains, fruits, and vegetables has been found to have a strong association with decreased risk of diverticular disease and diverticulitis. [18] When it comes to nuts and popcorn, research following more than 47,000 men from the Health Professionals Follow-up Study actually found a lower risk of diverticulitis when eating these foods. [25] Including these foods also did not increase the risk of developing new diverticulosis or diverticular complications.


Bottom Line

There are many types of dietary fibers that come from a range of plant foods. It’s important to not hyperfocus on a particular fiber because of its specific proposed action, as each type offers some level of health benefit. Therefore, eating a wide variety of plant foods like fruits, vegetables, whole grains, legumes, nuts, and seeds to reach the fiber recommendation of 25-35 grams daily best ensures reaping those benefits.

Some tips for increasing fiber intake:

  • Eat whole fruits instead of drinking fruit juices.
  • Replace white rice, bread, and pasta with minimally processed brown rice and other whole grains like barley, millet, amaranth, farro, and
  • Add high-fiber foods to current meals: 1-2 tablespoons of almonds, ground flaxseeds, or chia seeds to cereals; diced vegetables to casseroles, stir-fried dishes, and soups.
  • For breakfast, choose cereals that have a whole grain as their first ingredient. Another tip is to look on the Nutrition Facts label and choose cereals with 20% or higher of the Daily Value (DV) for fiber.
  • Snack on crunchy raw vegetables or a handful of almonds instead of chips and crackers.
  • Substitute beans or legumes for meat two to three times a week in chili and soups.
  • If it is difficult to eat enough fiber through food, a fiber supplement such as psyllium or methylcellulose powders or wafers can be used. They can help bulk and soften stool so it is easier to pass. However, fiber supplements are not intended to completely replace high-fiber foods.

Last reviewed April 2022

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