Ending some confusion around soluble and insoluble fiber

Dissolves, attracts, absorbs water? Adds bulk and causes constipation? Adds bulk and causes diarrhea? Keeps stool loose but helps solidify??! Let’s try to end the confusion surrounding the differences between soluble and insoluble dietary fiber!

Dietary fiber

Depending on who you ask, there are different definitions of what constitutes fiber. Common to most, if not all, is fiber’s inability to be digested in the human digestive system.

The proposed definition of fiber, according to the US Institute of Medicine, distinguishes a difference between dietary fiber and added fiber. Dietary fibers are regarded as carbohydrate polymers and lignin, present in plants, that are not broken down by the body’s digestive enzymes. Added fiber is proposed to include both natural isolated/extracted and synthetic undigestible carbohydrate polymers that have physiological benefits in humans. Total fiber is the sum of the two.¹

Both insoluble and soluble fiber can effect nutrient digestion and rate of uptake, digestive tract residence times and passage rates, and microbiota populations and fermentation products.² Fiber intake is correlated with reduced risks of all-cause mortality, cardiovascular diseases, diabetes and cancers of the digestive tract.^2,3 Both insoluble and soluble dietary fiber can be found in fruits, vegetables, seeds, legumes and all other plant parts.

However, as will be mentioned later, the beneficial qualities of fiber cannot be described by their solubility alone and, for this reason, insoluble and soluble fiber are terms that many groups suggest phasing out.^1,2,3

This oversimplification of fiber categorization has likely lead to some of the contradictions and confusion surrounding the functional properties of fibers.

Both soluble and insoluble carbohydrate types of fiber are considered hydrophillic - water-loving - due to their hydroxyl groups which can interact with/attract water. Hydrophillicity, however, does not completely explain how a fiber will actually interact with water. For example, two fibers that are both considered hydrophillic, may not dissolve in water equally due to their different size and molecular structure, giving us an insoluble and soluble fiber.

Soluble fiber

Soluble fiber dissolves in water. This means that the water, H₂O, molecules bind to the hydrophillic groups of the fiber in a way so that the fiber and water become a homogenous solution.

It is generally stated that soluble fiber creates a viscous gel, slows down digestion, is fermentable and can help relieve darrhea.

While this can be true, it is an oversimplification.

Soluble fiber can increase the viscosity of digesta, in turn, slowing gastric emptying, slowing nutrient absorbtion and relieving diarrhea. Not all soluble fiber is viscosifying, however. Some soluble fibers are small in molecular weight and some ferment, both making them unable to meaningfully add to viscosity.^2,4 Conversley, some hydrated but insoluble fibers show “viscosifying” behaviour.²

Soluble fiber can relieve diarrhea but can also relieve constipation. Low stool weights are correlated with constipation so both insoluble and soluble fiber can help with constipation by increasing fecal bulk. Some types of soluble fiber form a viscous gel that holds onto water well and resists dehydration. This bulking of the stool increases regularity of bowel movements. It must also resist fermentation to do this.⁴

Fermentability reduces the ability of a fiber to increase the stool’s water content, add bulk and improve regularity.^4,5 It is generally said that soluble fibers are fementable and insoluble are not, but again this is an oversimplification.

Examples of soluble fiber

An example of a soluble fiber is psyllium. It forms a gel, is viscous and does not ferment in the digestive tract. Because of this, it is able to hold onto water and improve bowel movements (relieves/prevents constipation and diarrhea).^6,7

Another example of a soluble fiber is inulin. It does not form a gel and is not considered viscous. Inulin readily ferments.

Pectin and guar gum are soluble, gel-forming, fermentable fibers. Becuase they are fermented, they lose their gel structure and water-holding capacity, resulting in no significant laxative effect.^8,6

Insoluble fiber

Insoluble fiber does not dissolve in water. This means that the H₂O molecules are not able to bind to the fiber and bring it into a homogeneous solution. Some portions of the fiber may hydrogen bond to water, but it won’t dissolve completely. This is similar to how cotton wool absorbs water, but does not dissolve in water. It remains a heterogenous mixture. This is because the intramolecular hydrogen bonds are strong enough to keep the fiber intact.⁹ Hydrophobic fibers will also not dissolve in water.

Insoluble fiber is generally stated to bulk up the stool, resist fermentation and help relieve constipation.

It is sometimes said that it is insoluble fiber’s water-absorbing/holding capacity (like a sponge) that contributes to it’s bulking and laxative effect. Insoluble fiber, however, has an insignificant ability to hold onto water in the digestive tract. The ability for insoluble fiber to increase stool water weight and produce a laxative effect instead comes from it mechanically irritating the bowel. In defense, the large intestine mucosa secretes water and mucous.^4,7

Examples of insoluble fiber

An example of an insoluble fiber is cellulose. Cellulose is an important component of a plant’s cell wall, made from linear D-glucose units linked by β(1→4) bonds. It is poorly fermented and doesn’t hold onto water well like a soluble, viscous, gel-forming fiber.^4,10 Cellulose forms intramolecular hydrogen bonds that hold the polymer chains together and prevent water from binding.^11,8

Lignin, a non-carbohydrate fiber, is hydrophobic and is classed as an insoluble fiber. It is a polyphenol polymer that is an important component of cell walls, particularly in wood and bark. Lignin binds covalently with carbohydrate fibers and alters the physiological effects of fiber.^1,3

Soluble/insoluble categorization has limited power to predict beneficial fiber properties

Many of the important beneficial physicochemical properties of fiber are not dependent on solubility. Sometimes, definitions confusingly state that fibers are different due to properties that, actually, they both can have. So, knowing only solubility will not automatically predict how a fiber will benefit you and your gut. For this reason the phasing out of these terms is suggested.

Properties that help describe the functional benefits fiber will have in your gut include viscosity and fermentability.¹

One paper suggests categorising fibers by properties such as bulk structuring, molecular binding and transport barriers.²

References