What Is Dietary Fiber? The Unedited Expert's Guide

So often, people believe fiber to be a simple part of the diet -- one usually thought to carry less significance than other dietary components such as carbs, fat, or protein. A glance at government guidelines doesn’t negate these misperceptions. Regulations often discuss fiber as one broad entity, with the primary goal being to simply get enough.

The reality is that dietary fiber is made up of a vast array of unique bioactive structures. We’re not just talking about soluble or insoluble fibers, either. The diversity of benefits derived from fiber is as abundant as the 100s of unique fiber structures found in whole foods. 

But before we can understand and discuss the benefits of fiber, we first need to define what fiber is, and we need to know where and how to source a diversity of unique fiber structures in our diets. 

We feel it’s time to take the conversation about fiber beyond blanket recommendations for intake levels, with a more targeted focus on the different fibers found in foods. Throughout this article are links to supplemental information detailing which foods contain which types of fiber. 

The information supplied in this article is meant to be eye-opening, not overwhelming. In the end, we hope that the complexity of fiber detailed here points to one straightforward conclusion: The need for an abundance of plant and fiber diversity to promote optimal health.

What is Dietary Fiber? 

The world of nutrition always has a good controversy on its hands. While nutrition debates about carbs, sugars, fats, and proteins may have stolen the spotlight over the years, many of you were likely unaware of the riveting, decades-long fiber definition debate.

Despite this behind-the-scenes nutrition saga, for the most part, there appears to be a general global agreement about what constitutes dietary fiber. S We’ll be touching on points of agreement and disagreement when it comes to defining fiber, but first, to understand what fiber is, we need to understand three other important concepts:

1. Available versus unavailable carbohydrates
2. The structures and composition of different carbohydrate types
3. The presence of other non-carbohydrate fibers in plant cell walls 

Available Carbohydrates -- Not Fiber

Available carbohydrates are those which the body can digest and absorb in the small intestine. From there, our bodies use these carbohydrates to produce energy. 

Available carbohydrates include sugars and starches, or more technically, monosaccharides, disaccharides, and digestible polysaccharides, respectively. These carbohydrate names may sound confusing; however, the terms simply describe the unique structures of different carbohydrate types.

Available Carbohydrate Structures

The word “saccharide” is a term derived from the Latin word for sugar. Monosaccharides are carbohydrates containing a single (mono) sugar (saccharide). Monosaccharides are the simplest form of carbohydrates, and all digestible carbohydrates must be broken down into this form to be absorbed in the small intestine. S Likewise, disaccharides are made up of 2 chemically bound sugar units. Monosaccharides and disaccharides are classified as “sugars” and are not considered a source of dietary fiber. S

Polysaccharides are longer chains of many sugar units. The majority of polysaccharides (starches) are digested in the small intestine and upper GI tract. S These starches are not considered dietary fiber because digestive enzymes in the mouth and small intestine can break the bonds between connected sugars in starches. This works to liberate single sugar units (monosaccharides). From there, the body can absorb these monosaccharides for energy in the small intestine.  S

Key Takeaways: Available carbohydrates are those which the body can digest, absorb, and use for energy.  Available carbohydrates include starches and sugars.  Sugars include monosaccharides and disaccharides. Monosaccharides are made up of a single sugar unit. Disaccharides are made up of two chemically bound sugar units.  Starches are polysaccharides, meaning they’re made up of many (poly) sugars (saccharides).  Digestible starch polysaccharides are not fiber because digestive enzymes can break apart these long chains of sugars into monosaccharides. From there, the monosaccharides can be absorbed in the small intestine to provide energy.

Unavailable Carbohydrates -- Fiber 

The remaining carbohydrates in the diet are unavailable. This means that our body cannot break down these carbohydrates into monosaccharides in the small intestine to be absorbed and used for energy.

Unavailable Carbohydrate Structures

Carbohydrates may be unavailable due to the structures of the bonds holding together sugar units in a carbohydrate. This is because human digestive enzymes cannot break down certain types of bonds between linked sugars. S When we can’t break these carbohydrates down into monosaccharides, this makes them “unavailable” to us to be absorbed and used for energy. The result is that unavailable carbohydrates pass through the small intestine undigested, making their way to the large intestine.

Other structural components can also prevent digestive enzymes from breaking carbohydrates into monosaccharide units. Examples of when this might happen would be if a carbohydrate is in a specific crystalline structure or entrapped by other indigestible plant components. These factors make it challenging or impossible for digestive enzymes to break the carbohydrate down into absorbable monosaccharides. 

Unavailable carbohydrates that resist digestion in the small intestine are considered to be dietary fiber. These unavailable carbohydrates are naturally present in plant foods such as grains, legumes, fruits, and vegetables. Unavailable carbohydrates can also be synthetic (i.e., man-made or “functional fiber”) as well as extracted, purified, or isolated fibers added to packaged foods or provided through supplements. S Broad categories of unavailable carbohydrates include non-starch polysaccharides, resistant starch, oligosaccharides, and synthetic fibers. S

Key Takeaways: Unavailable carbohydrates are those which the body cannot digest in the small intestine due to the carbohydrate structure.  Because the body does not break down these carbohydrates into monosaccharides, we cannot absorb them and use them for energy. Instead, these carbohydrates pass through the small intestine undigested, making their way to the large intestine.  Unavailable carbohydrates count as dietary fiber. Broad categories of unavailable carbohydrates (fibers) include non-starch polysaccharides, resistant starch, oligosaccharides, and synthetic fibers.

Other Fibers 

Plant Cell-Wall Components

Aside from unavailable carbohydrates, plants contain other non-carbohydrate compounds that resist digestion in the small intestine. These compounds count as fiber, too, and often include structural components of plant cell walls such as lignin, cutin, suberin, and waxes. S, S

Animal and Fungi-Based Fiber

While the bulk of dietary fiber comes from plants, an exception to this rule is the fiber called chitin. Chitin is derived from the exoskeleton of crustaceans, insects, and some fungi. Chitin resists digestion in the small intestine and is made up of a unique amino-polysaccharide structure. This fiber is compositionally similar to the non-starch polysaccharide cellulose found in plants. S

Defining Fiber: What Meets the Criteria? 

Dietary fiber comprises linked carbohydrates (polymers) of 3 or more sugar units (monomeric units) which are neither digested or absorbed in the human intestine. S

• These include: 
• Non-starch polysaccharides from fruits, vegetables, cereals, tubers, and fibers isolated and extracted from these sources. S
• Fibers that are synthetic (man-made) when extracted and chemically, physically, or enzymatically modified. S
• Scientific evidence has to support a health benefit for synthetic and/or extracted fibers to be considered a source of dietary fiber. S
• Non-digestible oligosaccharides S
• Oligo (few) saccharides (sugars) composed of 3-9 linked sugar units 
• Resistant starch S
• Non-digestible form of starch with >10 linked sugar units 

Dietary fiber includes non-carbohydrate ‘associated substances.’ These other fibers are generally found in plant cell walls and include lignin, cutin, suberin, and waxes. S S

Defining Fiber: Points of Discussion and Controversy

While much overlap is seen in the definition of fiber from one regulatory agency or country to another, the following two defining parameters are points that may differ from one regulatory entity to the next.

‘Associated Substances’

• All regulatory agencies do not consistently include ‘associated substances’ (primarily lignin) across the globe as dietary fiber. S
• We have chosen to include associated plant substances (lignin, cutin, suberin, and waxes) as dietary fiber for this resource. 

Short-chain oligosaccharides

• Historically, there has been controversy over the minimum number of sugar units in a non-digestible carbohydrate that should be counted as dietary fiber. The consensus now appears to lean toward including short-chain non-digestible oligosaccharides containing 3-9 (monomeric) sugar units. 
• We have included short-chain non-digestible oligosaccharides containing 3-9 sugar units as part of our definition of dietary fiber for this resource. This is in alignment with guidance from the following regulatory agencies and countries: 

Organizations Which Classify Short-Chain Oligosaccharides as Dietary Fiber: List of Institutions List of Countries American Association of Cereal Chemists (AACC) Association Official Analytical Chemists (AOAC) Codex Alimentarius Commission (CAC) European Food Safety Authority (EFSA) Food and Drug Administration (FDA) Food Standards Australia and New Zealand (FSANZ) Institute of Medicine (IOM) International Life Science Institute (ILSI) Brazil Canada Chile China Indonesia Japan Korea Malaysia Mexico Singapore Thailand Taiwan Source: S

Not Fiber  Available Carbohydrates	Fiber                                                                Unavailable Carbohydrates                Other Fibers
-Sugars Monosaccharides Disaccharides -Starch  Digestible Polysaccharides	Naturally Occurring -Non-starch polysaccharides -Resistant Starch -Oligosaccharides	Synthetic  -Type 4 Resistant Starch -Dextrins -Polydextrose -Cellulose Derived Fibers -PolyGlycoplex (PGX)	Plant Cell-Wall Components: Lignin Cutin Suberin Waxes Animal/Fungi-Based Chitin

Table Sources:  S, S

Properties of Fiber

Soluble vs. Insoluble: An Outdated Paradigm? 

You may have heard of fiber being broken down into two categories: soluble or insoluble. If a fiber is soluble, it can dissolve in water, whereas insoluble fibers cannot. 

Solubility has been used as a general descriptor to classify how fiber might behave in the body. In the past, soluble fibers were associated with aiding blood sugar control and lowering cholesterol. In contrast, insoluble fibers were thought to help with bowel movement frequency and stool bulking. 

While there are some truths in those associations, fiber categories are a bit more nuanced and complex beyond just solubility. Three better characteristics to consider when evaluating isolated fiber types and their impact on the body include: fermentability, viscosity, and bulking. 2, 6 

Fermentability

Fermentability is a characteristic describing gut microbes' ability to either totally or partially ferment fiber in the large intestine. Roughly 70% of dietary fiber is fermented. 2

Fermentable fibers can promote the growth of beneficial bacteria in the intestine. 6 

Gut microbes can also ferment certain fibers to produce beneficial byproducts called short-chain fatty acids (SCFAs). 2 SCFAs help maintain cells' health and integrity in the colon. They also trigger a cascade of beneficial health impacts throughout the body. 6

Fermentation of fiber produces intestinal gas, which is usually a worthwhile trade-off given fermentable fibers' benefits. 

However, in some cases, the gases produced after eating fermentable fiber can lead to discomfort in sensitive individuals. Under certain circumstances, such as in IBS, choosing well-tolerated and low gas-producing fibers may be necessary to lessen irritation and pain. 8

Are Fermentable Fibers Prebiotics? Defining Prebiotics: The following criteria are used to classify a compound as a prebiotic: It should be resistant to the acidic pH of the stomach, and it cannot be broken down by digestive enzymes. Prebiotics should also not be absorbed in the gastrointestinal tract.   It can be fermented by intestinal microbiota. This compound can selectively stimulate the intestinal bacteria’s growth and/or activity to improve health.  S Fermentability might be thought of as a ‘prebiotic’ effect; however, not all fibers are considered prebiotics, even if they’re fermentable. That is because the definition of a  prebiotic requires selective use by the microbiome for a more targeted effect. S For instance, fructooligosaccharides (FOS) are known to selectively increase bifidobacteria and lactobacillus. On the other hand, pectin is a fermentable fiber that is broadly fermented by several microbe types in the microbiome. 7 Pectin may still offer benefits, but due to the lack of specificity, it would not technically fit the definition of being a ‘prebiotic.’7 Examples of Prebiotics:  Fructans Inulin, Fructooligosaccharides (FOS), Oligofructose Galactooligosaccharides (GOS) Other Oligosaccharides  Starch or glucose-derived oligosaccharides, polydextrose, pectic oligosaccharides.

Viscosity

Viscosity is the fiber’s ability to thicken and form a gel. 9 Viscous fibers reduce the absorption of cholesterol, sugar, and other nutrients such as vitamins and minerals. 6  

Viscous fibers may also block the absorption of bile salts. 6 Bile salts are produced from cholesterol in the liver, so blocking their absorption is another way in which viscous fibers may help to lower cholesterol levels directly. 6  

Some fermentable non-viscous fibers may also indirectly improve blood sugar control and cholesterol through effects on the microbiome.

Fibers that are both viscous and fermentable, such as beta-glucans found in oats, may improve health markers both from the fiber’s viscous consistency and the effects on the microbiome. 10

Bulking 

Some fibers create a bulking effect, increasing stool volume, improving regularity, and alleviating constipation.6 Often, this effect is associated with insoluble fibers such as cellulose and lignin found in plant cell walls. However, some soluble fibers such as psyllium or oat bran can also improve stool bulking. 9 

Fermenting soluble fibers may also increase stool bulk inadvertently by promoting the growth of our gut microflora. This leads to the creation of byproducts such as gas or SCFAs, which help to soften hard stools or firm up loose stools to improve diarrhea. 6

Tying it all together

Fermentability, viscosity, and bulking, along with the structure and dose of the fiber, 11 shapes the actions, benefits, and fiber tolerability in individuals.

It’s critical to know that no single dietary fiber can provide the full scope of health benefits on its own. Therefore, it’s recommended to choose various fiber-rich foods and sources in your daily diet to reap as many health benefits as possible. 6 

Fiber Type & Examples	Health Effects
Insoluble Fibers
Insoluble Poorly Fermented Examples: Cellulose, lignin, wheat bran, plant cell walls12	Laxative effect from large coarse particles. 12 Small smooth particles such as wheat bran ground into a flour or bread will not produce a laxative effect 12  May stimulate gut movement/motility 13 Creates bulkier stools 13 Dilutes and helps to eliminate toxic compounds from the intestines 13 Poorly fermented
Insoluble Fermented Example: Resistant Starch 6 Found in:  Whole grains, legumes, seeds, pasta, raw or cooked and cooled potatoes, green bananas, bread, and cornflakes	Fermented --  may support the growth of microbes that produce beneficial SCFAs 14
Soluble Fibers
Soluble Non-Viscous Fermented Examples: Inulin, oligosaccharides, wheat dextrin, partially hydrolyzed guar gum (PHGG)	Many prebiotics with specific microbiome effects are found in this category. Fermented, so supports the growth of microbes in the gut.   Not associated with a laxative effect. May indirectly improve stool volume and laxation by promoting the growth of the microflora.6 May indirectly help lower cholesterol and support blood sugar control through effects on the microbiome. 15
Soluble Viscous Fermented Examples: Beta-glucans found in oats, barley, wheat, and rye, pectins found in fruits such as apples or oranges, guar gum	Helps with blood sugar control directly12  Direct cholesterol-lowering effect 12   Fermented, so supports the growth of microbes in the gut that may be beneficial or that may create SCFAs Generally not associated with a laxative effect; may indirectly improve laxation through the growth of the microflora  12, 13, 6
Soluble Viscous Poorly Fermented Example: Psyllium	Improves blood sugar control directly 12  Direct cholesterol-lowering effect 12  Poorly fermented -- low gas-producing fiber.  While not considered a well-fermenting fiber, psyllium increases fecal water content, which may result in a better overall fermentation profile in the gut. 16 Helps to normalize stools -- may improve both constipation and diarrhea 12  Stool bulking effect  12

 

Key Takeaways Dietary fibers are made up of specific carbohydrate structures or components of plant cell walls that we can’t digest and absorb. Fiber is commonly broken down into two categories: soluble or insoluble. A fiber that can dissolve in water is a soluble fiber. Fibers that do not dissolve in water are insoluble. While solubility is often used to describe different fiber types, this broad classification does not give enough information about how different fibers behave in the body. Characteristics of fiber that better describe the behavior of a given fiber in the body are fermentability, viscosity, and bulking.  Fermentable fibers promote the growth of bacteria in the intestines. While fermentation may be associated with a prebiotic effect, not all fermentable fibers are prebiotics. In order to be a prebiotic, a fiber must have a selective beneficial impact on the microbiome.  Non-prebiotic fermenting fibers may still have benefits through broad, rather than selective, impacts on the microbiome, for example, by feeding several types of microbes rather than selectively stimulating a select few. Viscous fibers hold water to thicken and form a gel. This property helps to lower blood cholesterol and post-meal blood sugar.  Bulking fibers help to increase stool volume. No single fiber type can provide all beneficial effects.  It's essential to eat a diet rich in a diverse array of plant foods and fiber types to reap all of fiber's benefits.

 

Final Takeaways: 

• Depending on their unique structure and composition, dietary fibers have varying characteristics, including solubility, viscosity, fermentability, and bulking. These characteristics help to determine the beneficial health impact from ingesting a given fiber type. However, knowledge gaps still exist, and the subject of fiber is underexplored. S

• Future development of food composition databases exploring specific fiber types and content of foods might allow for more targeted fiber recommendations and observations. S For now, we recommend eating a diversity of plants and fiber types to reap as many benefits as possible.  

 

	Fermentability		Viscosity		Bulking
	High	Low	Yes	No	Yes	No
Cellulose		✔		✔	✔
Lignin		✔		✔	✔S
Resistant Starch	✔			✔		✔
Arabinogalactans
Arabinoxylans S, S	✔		✔		✔
Glucuronoxylan
Xyloglucan S, S	✔		✔
Galactomannans
Beta-glucans
Pectin	✔		✔
Galactomannans
Guar Gum	✔		✔
Acacia Gum
Psyllium		✔	✔		✔
Inulin	✔			✔		✔
Oligosaccharides  (FOS, GOS, AXOS, XOS)*	✔			✔		✔

	Solubility	Fermentability		Viscosity		Bulking
Cellulose S	-	+		-		+++
Lignin S	-	-		-		✔S
Waxes S	-	-		++
Chitin S	-	-		-
Resistant Starch (types 1, 2, &3)	-	++		-
Resistant Starch Type 4	-	+		-
Arabinogalactans
Arabinoxylans S, S		✔		✔		✔
Glucuronoxylan S	-	+		-
Xyloglucan S, S		✔		✔
Galactomannans
Beta-glucans
Pectin		✔		✔
Galactomannans
Guar Gum		✔		✔
Acacia Gum
Psyllium			✔	✔		✔
Inulin		✔			✔		✔
Oligosaccharides  (FOS, GOS, AXOS, XOS)*		✔			✔		✔

 

Table Key:  No Degree of Effect: - Mild/Poor Degree of Effect: + Moderate Degree of Effect: ++ High Degree of Effect: +++ Positive effect but degree of effect not clear or specified: ✔