Fibers:  Which are Good and Bad For You?

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Fiber is an essential element of our diet and it can influence many aspects of health – from gut bacteria to weight loss.

Fiber considered a fundamental part of a healthy diet. Unfortunately, most people have a very basic understanding of fiber, and tend to lump it all into one category.

As we always say, no food is created equal. Same can be said about fibers. Some types are highly beneficial, while others can cause digestive problems in some people.

In this post, let’s find out what fibers really are and which ones are good for you, and which ones are not. Read on!

What is Fiber and How is it Classified?

Fiber refers to a diverse group of carbohydrates that humans can not digest because we lack the digestive enzymes required to break them down. As a result, they pass through most of the digestive system unchanged.

Fiber is mostly found in plant foods, including vegetables, fruits, legumes, whole grains, nuts and seeds (for more details, here is a list of 22 high-fiber foods).

There is actually a huge variety of different fibers found in foods, and they are often classified in different ways.

In 2001, fiber was formally classified into two main types (Source 1):

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

However, this way of fiber typology tells us absolutely nothing about their health effects. A popular alternative method is to classify fiber based on its solubility (soluble versus insoluble), viscosity (viscous versus non viscous) and fermentability (fermentable versus nonfermentable).

Also, worth noting is another class of nutrients called resistant starches, which are often classified as dietary fibers.

The recommended intake of fiber is 38 grams for men, and 25 grams for women. However, most people are only eating around half of that, or 15-17 grams per day.

Soluble vs Insoluble Fiber

The solubility of fiber refers to its ability to dissolve in water. Based on this, fiber has often been categorized as either soluble or insoluble:

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

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

Note that different plant foods have varying proportions of soluble and insoluble fibers.

Fermentable Fiber

Fermentable fibers are fibers that are friendly to gut bacteria. An estimated 100 trillion live bacteria reside in the human gut, mainly in the large intestine.

These bacteria play various roles related to weight management, blood sugar control, immunity, brain function and mental health (Source 1, Source 2, Source 3, Source 4, Source 5, Source 6). Due to their importance, they are called “the forgotten organ”.

So, how does fermentable fiber become friendly to gut bacteria? Here’s the story.

For starters, we 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 are able to digest (ferment) and use as fuel. This increases the number and balance of friendly gut bacteria, which also produce short-chain fatty acids with powerful health benefits.

Fermentable fibers include pectins, beta-glucans, guar gum, inulin and oligofructose. The best whole-food sources of fermentable fibers are beans and legumes. A 1-cup serving often provides up to half of the recommended daily intake of fiber.

A word of caution though, 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 a lot of fiber.

Viscous Fiber

Viscous fibers are types of soluble fibers that form a thick gel when they blend with water (viscosity of a fluid refers to its “thickness”).

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 (Source 1, Source 2, Source 3).

A review of 44 studies on fiber treatments found that only viscous fibers reduced food intake and caused weight loss. Impressive! How can they do that?

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

Resistant Starch

Starch is a type of carbohydrates found in our diet. They are long chains of glucose molecules, found in potatoes, grains and many other foods.

Resistant starches are starches that are actually resistant to digestion, so that they pass through the digestive system unchanged. They function like soluble, fermentable fiber in the gut.

Resistant starch can help improve digestive health, enhance insulin sensitivity, lower blood sugar levels and significantly reduce appetite (Source 1, Source 2, Source 3, Source 4, Source 5).

Resistant starch can be harnessed from green bananas, various legumes, cashews and raw oats.

Note: Certain starchy foods tend to form large amounts of resistant starch if they are cooled down after cooking such white potatoes and white rice.

Other Unique Fibers

Several fibers have specific health implications, and are worthy of highlighting.


A fructan is the term used to describe a small chain of fructose molecules. Oligofructose and inulin are the two main fructan varieties in the diet. They can feed the friendly bacteria in the gut, and have been shown to help treat certain types of diarrhea.

A word of caution,  fructans are also classified as FODMAPs (I hope you remember what they are), types of carbohydrates known to cause digestive issues to certain individuals (in fact, fructans and other FODMAPs trigger adverse symptoms in 3 out of 4 people with irritable bowel syndrome, a common digestive disorder). The biggest source of fructans in the modern diet is wheat (Source 1).


These fibers have a specific molecular structure that makes them highly viscous in the gut, and their health benefits have been extensively documented.

Beta-glucans can improve insulin sensitivity and lower blood sugar levels, and also significantly reduce cholesterol levels and increase feelings of fullness. Oats and barley are rich in beta-glucans.


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 improve risk factors for heart disease (Source 1, Source 2, Source 3, Source 4).

A Word to the Wise

Fibers are not created equal. So, mind what fiber you put in your stomach.
Fibers that are soluble, viscous and fermentable seem to be the healthiest, by far. Resistant starches are also incredibly healthy. Good sources of healthy fibers include vegetables, fruits, oats, legumes, nuts, dark chocolate, avocados, chia seeds and various other foods.


1 “Fiber: MedlinePlus Medical Encyclopedia.” 2006. 10 Mar. 2016 <>

“Importance of Fermentable Fiber | Gut Health & Wellness.” 2015. 10 Mar. 2016 <>

“What Is Viscous Fiber? | Healthy Eating | SF Gate.” 2013. 10 Mar. 2016 <>


5 “Fructan – Wikipedia, the free encyclopedia.” 2011. 10 Mar. 2016 <>

“GLUCOMANNAN: Uses, Side Effects, Interactions and …” 2012. 10 Mar. 2016 <>

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