Not all prebiotics are created equal — and not all are easy on the gut.In this post, we explore why certain prebiotic fibers, like inulin-type fructans, often cause bloating and gut discomfort, especially for individuals with IBD or functional bowel disorders. We contrast these with resistant starches, a gentler option that supports gut health without excessive gas or pain. You'll also learn how prebiotic fibers fuel beneficial gut bacteria, leading to the production of powerful metabolites like short-chain fatty acids. These compounds not only protect gut integrity but also regulate appetite, metabolism, blood sugar, inflammation, and even mood. Whether you're trying to optimize gut health, improve regularity, or simply choose a better-tolerated prebiotic, this guide covers the essential science behind smarter fiber choices.
We’ve all been there. Those days when our gut is feeling icky, so we head to the gut health aisle at the local pharmacy. Little did we know the vast number of gut health products that are available. Standing cross-eyed, staring at the products we begin to ask, “Will a ‘digestive health’ fiber do the trick?” “Why are some labeled ‘prebiotic fiber’, while others say ‘dietary fiber’?” Which one will turn gut gloom to gut bloom?
Not All Fibers Feed Gut Microbes
As a dietitian, prebiotic fibers are the #1 supplement that can improve gut health. Prebiotic fibers are found in plants and are not digested like normal carbohydrates. Therefore, their structure is unaltered when entering the large intestine, where most gut microbes live.
Prebiotic fibers provide fuel for the beneficial gut microbes, which allows them to survive and thrive in their large intestinal ecosystem. Providing health benefits to the host (aka, you and me) is also necessary for a fiber to be labeled prebiotic. Health benefits can include:
· Modulating immune function
· Strengthening gut barrier integrity
· Impacting appetite and metabolic health
· Improving mood and cognition
However, not all fibers act as a prebiotic. For example, fibers like psyllium (Metamucil) and methylcellulose (Citrucel) act as bulking agents and help relieve constipation and maintain regularity. However, they have no direct impact on the gut microbiome.
In contrast, many prebiotic fibers improve digestion and regularity while simultaneously nourishing beneficial microbes.
Types of Prebiotic Fibers
There are three main types of prebiotics: include inulin-type fructans, galacto-oligosaccharides, and resistant starches. Each prebiotic provides fuel for specific types of gut bacteria which also indicates the type of benefit to the host. Tolerability of prebiotic fibers also differ, especially among those with irritable bowel syndrome (IBS) and/or functional bowel disorders (FBD). Below is a list of prebiotic fibers and their structural components.
1. Inulin-Type Fructans
· Made of polymers of fructose linked in β-2,1 bonds with a terminal glucose.
· Found naturally in plants or created via enzyme reactions
· Bifidobacteria, bacteroides, and lactobacilli thrive on these fibers thanks to their fructanase. (1)
Types
o Inulin: Longer chain (2-60 fructose units) commonly derived from chicory root.
o Fructooligosaccharides (FOS): Shorter chains (2-8 fructose units).
2. Galacto-Oligosaccharides
· Composed of galactose and glucose linked in a way that cannot be digested
· Made of short polymers ( 2-8 glactose and glucose units).
· Promotes the growth of bifidobacteria due to the presence of the enzyme β-galactosidase. (1)
3. Resistant Starches
· A portion of starch that cannot be digested by human enzymes, thus entering the large intestine intact.
· Found naturally in green bananas, potatoes, cereal grains, beans and legumes.
· Amount varies across food type, and even within food species.
· Slowly broken down by the gut microbes due to its structure and complexity.
Types of Resistant Starch:
Type 1: Starch is entrapped in a hull or barrier in which enzymes cannot penetrate.
Type 2: Starch is too dense and cannot easily be digested, found in raw potatoes and bananas.
Type 3: Starch that has been cooled after being cooked, which is called retrogradation. The cooling causes some of the starch to be configured in a way that cannot be digested. Found in cooled potatoes and rice.
Type 4: A chemically-modified starch and is not found in natural food sources.
Type 5: Starch that is associated with a lipid to form a complex that cannot be digested. Only a few foods, like brown lentils and some cereals contain this type of resistant starch.
Specific microbes like Ruminococcus bromii and Bifidobacterium adolescentis degrade resistant starch, while Eubacterium rectale and Faecalibacterium prausnitzii also grow in the presence of resistant starch through other mechanisms (e.g., using a metabolite from the resistant starch degraders as food). (2)
In addition, Akkermansia, which does not directly feed on resistant starch, increases in abundance in the presence of potato resistant starch. (3) This gut microbe is associated with a healthy metabolism.
Prebiotic Tolerability: Choosing the Right Fiber for Your Gut
All prebiotic fibers increase the beneficial bacteria in the gut; however, not all are equally well tolerated. Individuals with IBD and/or FBD often cannot tolerate the shorter chain prebiotics, such as inulin-type fructans. These fibers are rapidly fermented by the bacteria, leading to gas production that causes bloating, flatulence, pain, and other gut symptoms. In fact, a meta-analysis found that inulin-type fructans worsened flatulence, even though they increased the abundance of Bifidobacterium. (4) Tolerability tends to worsen as the prebiotic dose increases.
In contrast, resistant starches are well tolerated even at doses up to 40 grams per day. Due to their structure, gut microbes ferment resistant starch much more slowly. Although fermentation still produces, the gradual process minimizes excessive flatulence or gut discomfort.
From Gut to Whole-Body Health: The Benefits of Prebiotics
The health benefits of prebiotics are largely mediated by their ability to increase the abundance of beneficial gut microbes. These microbes produce certain metabolites that promote the health of the host. When the microbes ferment prebiotics they release short-chain fatty acids (SCFA) such as acetate, butyrate, and propionate. Acetate and propionate serve as energy sources for the host, while butyrate specifically fuels the cells that line our gut. By nourishing these cells, butyrate helps maintain the integrity of the gut barrier, reducing the risk of toxins and pathogens entering the body.
SCFAs also interact with protein receptors on gut cells, stimulating the secretion of appetite and metabolism-regulating peptides, such as glucagon-like peptide-1 (GLP-1) and peptide YY (PYY). Additionally, SCFA lower the pH of the large intestine (they are acids after all), creating an environment less hospitable to harmful pathogens.
Beyond the local effects in the gut, prebiotics help reduce inflammation both locally and systemically (body-wide), regulate digestion and nutrient absorption, manage blood sugar and insulin levels, and even influence mood and cognition function. They also support regularity making bowel movements easier and reducing constipation.
In conclusion, prebiotic fibers are a key component to gut and overall health. Not all gut supporting supplements are equal as some only promote regularity. Prebiotics also improve regularity but also promote the growth and viability of beneficial gut microbes, which contribute to many host health benefits. Finding the right prebiotic depends on tolerability as not all are created equal!
(1) Wilson & Whelan, 2017 Gastro Hepatol
(2) Maiya et al., 2023 Nutr Res
(3) Bush et al., 2023 Nutrients
(4) Wilson et al., 2019 Am J Clin Nutr