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The fact is that not all fiber has the same components.

Some classes are extremely beneficial, while others can create digestive problems in some people.

This article describes everything you need to know about the various types of fiber.

How Fiber Classified

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“Fiber” refers to several collections of carbohydrates that humans can not digest.

We lack the digestive enzymes needed to break them down, so they pass through most of the digestive system unchanged.

The prescribed intake is 38 grams for men and 25 grams for women. Most people are only consuming approximately half of that or 15-17 grams per day.

Fiber is often seen in plant foods, including fruits, vegetables, legumes, whole grains, nuts, and seeds (for more details, here is a list of high-fiber foods).

There is really a huge variety of many fibers discovered in foods.

The difficulty is that they are usually classified in different ways, which can be highly confusing.

Fiber is classified into two main types.

  • Dietary fiber: Fiber discovered naturally in foods.
  • Functional fiber: Fiber that is extracted and separated from whole foods, then added to processed foods.

The major difficulty in classifying fiber is because it tells nothing about its health effects.

However, a common alternative approach is to classify fiber based on its solubility (soluble and insoluble), fermentability (fermentable and non-fermentable), viscosity (viscous and non-viscous).

Then there is still another class of nutrients termed resistant starches, usually classified as dietary fibers.

Soluble and Insoluble Fiber

The solubility of fiber refers to its capacity to dissolve in water.

It has often been categorized as either soluble or insoluble:

  • Soluble fiber blends with water in the gut, forming a gel-like substance. It can decrease blood sugar spikes and has various metabolic health benefits.
  • Insoluble fiber does not mix with the water and passes through the digestive system mostly intact. It works mostly as a “bulking” agent and may help speed the passage of food and waste through your gut.

Soluble fibers include gums, pectins, psyllium, beta-glucans, and others. Insoluble fibers include lignin and cellulose.

Fermentable Fiber

An approximated 100 trillion live bacteria reside in the human gut, mainly in the large intestine.

These bacteria are crucial for optimal health in humans. They play many roles linked to mental health, weight management, blood sugar control, immunity, brain function.

They are so great that they are usually referred to as the “forgotten organ.”

Because humans can’t digest fiber, it ends up reaching the large intestine mostly unchanged.

This is where fermentable fiber comes into play. These are fibers that the friendly gut bacteria can digest (ferment) and use as fuel.

This improves the number and balance of friendly gut bacteria, producing short-chain fatty acids with powerful health benefits.

Most fermentable fibers are soluble, but some insoluble fibers can function in this way.

Fermentable fibers include pectins, guar gum, beta-glucans, inulin, and oligofructose.

You fermentable fibers in beans and legumes. A 1-cup serving often provides up to half of the suggested daily intake of fiber.

All that being said, one of the by-products of fiber fermentation is gas. This is why foods high in fermentable fiber can cause flatulence and stomach discomfort, especially if people are not used to eating much fiber.

Viscous Fiber

The vicious fiber is soluble fibers that form a thick gel when blended with water.

When you eat viscous fiber, it forms a gel-like substance that “sits” in the gut.

This slows down the digestion and absorption of nutrients, resulting in a prolonged feeling of fullness and reduced appetite.

A review of 44 studies on fiber treatments found that only viscous fibers reduced food intake and caused weight loss.

Viscous fibers include glucomannan, beta-glucans, psyllium, pectins, guar gum. Good whole-food sources include legumes, asparagus, Brussels sprouts, oats, and flax seeds.

Resistant Starch

Starches are the main kinds of carbohydrates in the diet.

However, they are long chains of glucose molecules, seen in potatoes, grains, and many other foods.

Some starch is really resistant to digestion, so that it passes through the digestive system unchanged.

This type of starch is called resistant starch, and it functions like soluble, fermentable fiber in the gut.

Resistant starch has many powerful health benefits. It promotes digestive health, improves insulin sensitivity, lowers blood sugar levels, and significantly reduces appetite.

There are several excellent food sources of resistant starch, including green bananas, various legumes, cashews, and raw oats.

Additionally, certain starchy foods tend to form large amounts of resistant starch if they are cooled down after cooking. This includes white potatoes and white rice.

Raw potato starch is also very high in resistant starch, and some people eat it as a supplement.

A fructan is a term used to explain a small chain of fructose molecules.

Oligofructose and inulin are the two main fructan varieties in the diet. They can feed the beneficial bacteria in the gut and have been shown to help treat certain types of diarrhea.

Nevertheless, fructans are also classified as FODMAPs, which are known to cause digestive issues in many people.

The biggest source of fructans in the modern diet is wheat.


Glucomannan is a viscous fiber that is commonly marketed as a weight-loss supplement.

Numerous studies have shown that glucomannan can cause modest weight loss, fight constipation, and increase risk factors for heart disease.


These fibers have a special molecular structure that makes them highly viscous in the gut.

Beta-glucans can improve insulin sensitivity and lower blood sugar levels. They can also significantly decrease cholesterol levels and increase feelings of fullness.

The main food sources of beta-glucans are oats and barley.