Fiber is a type of carbohydrate in plant-based foods that your body cannot fully break down.

It plays a central role in digestive health and may offer protection against several conditions, including hemorrhoids, diverticulitis, and colorectal cancer (1).

However, many people are unsure how fiber may affect bowel movements and whether it can be used to help treat diarrhea.

This article takes a closer look at the different types of fiber and their effects on digestive health to determine whether fiber can help manage diarrhea.

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There are two different types of fiber — soluble and insoluble fiber.

Soluble fiber binds to water in your digestive tract, forming a thick, gel-like consistency as it moves through your body.

This type of fiber is also fermented by the beneficial bacteria in your gut, resulting in the production of short-chain fatty acids (2).

Insoluble fiber, on the other hand, is a type of fiber that doesn’t absorb water and passes through your digestive tract mostly intact. Unlike soluble fiber, insoluble fiber isn’t easily fermented by the bacteria in your gut (1).

Fiber-rich foods like fruits, vegetables, nuts, seeds, and legumes typically contain a combination of these two types of fiber in varying amounts. Many supplements also provide a mix of both types.

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Soluble fiber forms a gel-like consistency in your digestive tract and is fermented by bacteria in the gut. Insoluble fiber does not absorb water and passes through your body intact.

Soluble and insoluble fiber have different effects on digestive health and diarrhea.

Soluble fiber

Soluble fiber has been widely studied and promoted for its ability to ensure regularity and treat issues like constipation and diarrhea.

Psyllium, for example, is a type of soluble fiber supplement that has been shown to soften and ease the passage of hard stools to reduce symptoms of constipation (3).

Soluble fiber supplements like psyllium may not only soften stool but also improve the consistency of loose stool by soaking up extra water in the digestive tract to reduce urgency and bowel movement frequency in people with diarrhea (3).

Some studies also show that soluble fiber may improve symptoms of digestive conditions often associated with diarrhea, such as irritable bowel syndrome (IBS) (4).

One review of 22 studies associated soluble fiber with a significant improvement in symptoms of IBS, whereas insoluble fiber had no effect on IBS symptoms (5).

Insoluble fiber

Insoluble fiber may add bulk to the stool and act as a natural laxative (1).

It may also speed the movement of food through your digestive tract and promote peristalsis, which is the involuntary contraction of the muscles in your intestines (6).

For this reason, insoluble fiber supplements are sometimes used as a natural remedy to alleviate constipation (7, 8).

However, because insoluble fiber works as a laxative and increases bulk in the stool, it may not be ideal for the treatment of diarrhea.

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Soluble fiber may help treat both diarrhea and constipation. Insoluble fiber, on the other hand, promotes laxation and adds bulk to the stool, which may help prevent constipation.

Both soluble and insoluble fiber are found in a variety of foods, including fruits, vegetables, whole grains, and legumes.

Although many foods provide both types of fiber, certain foods may contain a higher amount of soluble or insoluble fiber.

Incorporating more foods that contain a higher amount of soluble fiber into your diet is a great way to promote regularity, reduce diarrhea, and improve your digestive health.

Some of the top sources of soluble fiber include:

  • Fruits: bananas, oranges, pears, grapefruit, blueberries, figs
  • Vegetables: broccoli, avocado, carrots, eggplant, tomatoes, Brussels sprouts
  • Legumes: kidney beans, lima beans, Great Northern beans, chickpeas
  • Whole grains: oats, barley, quinoa
  • Seeds: chia seeds, flaxseeds, psyllium seeds

Foods that are high in insoluble fiber include:

  • Whole grains: wheat bran, brown rice, whole wheat flour
  • Nuts: almonds, walnuts, pistachios, cashews, macadamia nuts, Brazil nuts
  • Seeds: sunflower seeds, sesame seeds,
  • Legumes: black beans, navy beans, pinto beans, lentils
  • Fruits: blackberries, strawberries, raspberries, unpeeled apples
  • Vegetables: cauliflower, green beans, carrots, potatoes
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Soluble and insoluble are found naturally in a variety of food sources, including fruits, vegetables, nuts, seeds, legumes, and whole grains.

In addition to eating more fiber-rich foods, fiber supplements are an easy way to boost your intake of fiber to support digestive health.

However, many types of fiber supplements are available, which can make it difficult to determine which help treat diarrhea.

Here are some of the most common ingredients found in soluble fiber supplements, which may be useful for the treatment of diarrhea:

  • psyllium
  • acacia fiber
  • chia seeds
  • flaxseeds
  • inulin

Keep in mind that fiber supplements, unlike high fiber foods, do not provide the essential vitamins, minerals, and antioxidants that your body needs.

While using fiber supplements may help treat diarrhea, it’s important not to use these supplements to replace nutrient-dense foods in your diet.

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Fiber supplements can be an easy and convenient way to increase your intake of soluble fiber. Ingredients like psyllium, acacia fiber, inulin, chia seeds, and flaxseeds are often found in fiber supplements.

Although consuming more soluble fiber may benefit digestive health and diarrhea, it’s important to increase your intake slowly over time.

This is important, as increasing your intake too quickly may cause side effects like gas, bloating, stomach pain, and cramps (9).

Because soluble fiber binds with water in your digestive tract, it’s also very important to stay hydrated and drink plenty of water.

Be sure to talk to your doctor before using fiber supplements, especially if you have any underlying health conditions or are taking medications.

Additionally, if you experience chronic or persistent diarrhea that doesn’t improve over time, you should consult a healthcare professional to determine the best course of treatment.

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Be sure to increase your intake of fiber slowly and stay hydrated. You should also talk to a doctor before using fiber supplements or if you experience symptoms that don’t improve over time.

Fiber can be classified as either soluble or insoluble. Each type of fiber is found in different sources and has unique effects on your digestive health.

Soluble fiber, in particular, binds with water in your digestive tract to form a gel-like substance and may help manage diarrhea and promote regularity.

Meanwhile, insoluble fiber adds bulk to the stool and may be a better option for those with constipation.