Prebiotic Spotlight: Fiber

Each edition of Global Prebiotic Association’s (GPA) Prebiotic Spotlight focuses on a specific prebiotic type to raise awareness around the prebiotic itself, its sources, any notable and/or recent research, and how it is used in the marketplace. This edition takes a new approach by addressing the common misconception that all fibers are prebiotics and vice versa, by reviewing their use in scientific literature.

Overview

The assumption that all dietary fibers qualify as prebiotics, and conversely that all prebiotics are dietary fibers, represents a common but inaccurate interpretation. This misconception has been reinforced by ongoing developments in nutraceutical research and the evolving definitions associated with prebiotics. Although the term “prebiotic” has been in use for approximately three decades, its definition has undergone several revisions, resulting in continued ambiguity surrounding both the term “prebiotic” and “prebiotic effect” (Deehan et al., 2024).

Prebiotics and dietary fibers share certain functional characteristics, including resistance to digestion in the upper gastrointestinal tract and susceptibility to fermentation by the intestinal microbiota. However, these shared attributes do not make the terms interchangeable. Not all dietary fibers meet the established criteria required to be classified as a prebiotic, and not all substances recognized as prebiotics are categorized as dietary fibers (Deehan et al., 2024).

Prebiotics and Dietary Fibers: Key Definitions

Dietary fibers are derived from a variety of plant-based sources, including fruits, vegetables, whole grains, potatoes, tubers, and legumes. They comprise a structurally and functionally diverse group of compounds that may be characterized according to physiochemical properties such as solubility (soluble versus insoluble fiber), molecular weight (low versus high), viscosity, and fermentability (Alahmari, 2024; Partula et al., 2020). Despite these commonly applied descriptors, the classification of dietary fiber remains inconsistently defined across scientific and regulatory frameworks (Feng et al., 2025).

Given this variability, it is not scientifically appropriate to assume that the broad category of dietary fiber inherently encompasses prebiotics. Rather, classification should be based on clearly defined criteria. In 1995, Gibson and Roberfroid first introduced the term “prebiotic”, as “nondigestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of bacterial species already resident in the colon, and thus an attempt to improve host health” (Gibson et al., 1995). As research into the gut microbiome progressed, this concept evolved. In 2017, the International Scientific Association for Probiotics and Prebiotics (ISAPP) updated the definition to describe a prebiotic as “a substrate that is selectively utilized by host microorganisms conferring a health benefit” (Gibson et al., 2017). More recently, in 2024, GPA proposed defining a prebiotic as “a compound or ingredient that is utilized by the microbiota producing a health or performance benefit” (Deehan et al., 2024).

In light of these evolving definitions and the absence of a universally standardized dietary fiber classification system, it is essential to clearly distinguish between prebiotics and dietary fibers in scientific and regulatory contexts to minimize ambiguity and ensure accurate classification.

Evidence-Based Prebiotics: Dietary Fibers with Demonstrated Prebiotic Activity

Certain dietary fibers demonstrate prebiotic effects by selectively stimulating the growth and/or activity of gut microorganisms and thereby conferring measurable health benefits, consistent with established definitions of a prebiotic. Examples include non-starch polysaccharides such as pectin, inulin, and arabinoxylans; resistant starch types 1-4; and oligosaccharides, including fructooligosaccharides (FOS) and galactooligosaccharides (GOS) (Deehan et al., 2024). In particular, FOS and GOS are characterized by relatively low molecular weight and high solubility, properties that facilitate their rapid fermentation by colonic microbiota (Guan et al., 2021). These attributes contribute to their established role as well-recognized prebiotic substances.

Similarly, inulin has been shown to increase the abundance of Bacteroidales, Bacteroidia, and Lactobacillus, while decreasing Firmicutes, Clostridia, and Ruminocococus. These microbiota changes were reported to be accompanied by several health benefits, including improved glucose levels, fasting insulin, and reduced homocysteine in individuals with overweight or obesity (Li et al., 2025). In healthy participants, daily supplementation with either 1.3 or 2.0 g of GOS for three weeks showed an increased relative abundance of Bifidobacteriumim in fecal samples, as well as shifts in overall microbiota composition with the 2.0 g dose, although no significant health benefits were observed (Looijesteijn et al., 2024). In participants with prediabetes, supplementation with 20 g/day short-chain FOS (scFOS) led to increases in Bifidobacterium, Anaerostipes and reductions in Blautia and Ruminococcus2. These changes were accompanied by increased fecal short-chain fatty acids (SCFAs) including acetate, propionate, and butyrate, in addition to improved body composition (Le Bourgot et al., 2025). Similarly, in individuals with overweight or obesity, 40 g/day of resistant starch type 2 for eight weeks significantly increased in Bifidobacterium adolescentis, Bifidobacterium longum, and Ruminococcus bromii, while reducing body weight, fat mass, waist circumference, and improving glucose tolerance (Li et al., 2024). Overall, prebiotics such as inulin, GOS, FOS, scFOS, and resistant starch can modulate gut microbiota composition and confer health benefits.

Evidence-Based Prebiotics: Non-Dietary Fibers with Demonstrated Prebiotic Activity – Polyphenols

Polyphenols are constituents of plant-derived foods and encompass a wide range of polyphenolic compounds, including flavonoids (e.g., flavonols and isoflavones), phenolic acids (e.g., flavanols and anthocyanins), and stilbenes (e.g., flavones and flavanones) (Lippolis et al., 2023; Plamada & Vodnar, 2022). Although polyphenols are not classified as dietary fibers due to their structure, they meet recent definitions of prebiotics due to their ability to modulate the gut microbiota and confer health benefits. Complex polyphenols remain undigested until they reach the large intestine, where they are converted into low-molecular-weight metabolites by gut microbiota (Lippolis et al., 2023). These microbial metabolites can act as prebiotic-like molecules by modulating the growth of specific bacterial strains and promoting health effects such as anti-inflammatory, immunomodulatory, antidiabetic, cardioprotective, and gastroprotective properties (Lippolis et al., 2023).

The diversity of polyphenols and their various sources highlight their extensive diversity and the broad scope for research on microbiome effects. A systematic review of human in vivo studies showed that supplementation with polyphenols from 6.4 to 2364 mg/day promoted the production of beneficial gut bacteria such as Bifiobacterium and Lactobacillus (Ma & Chen, 2020). In addition, daily consumption of a polyphenol-rich juice (100 mL of Aronia melanocarpa juice with a polyphenol concentration of 8,330 mg/L) for six weeks led to enhanced microbiome alpha diversity, increased abundance of Anaerostipes and Bacteroides, and improved lipoprotein metabolism (Lackner et al., 2024). More specifically, stilbenes found in red grapes and red wine enhance the growth of Lactobacilli and Bifidobacteria, while exerting antioxidant, anti-inflammatory, and other beneficial health effects (Plamada & Vodnar, 2022). While polyphenols are not classified as dietary fibers, many exhibit prebiotic effects by modulating the gut microbiome and supporting a range of health benefits. Further research is needed to determine the impact of different polyphenols on the gut microbiota and associated outcomes in humans.

Non-Prebiotic Dietary Fibers: Evidence of Reduced Fermentability

Not all dietary fibers meet the definition of a prebiotic. Although the different forms of fiber do confer health benefits, their mechanisms differ from fermentable fibers in the gut, resulting in minimal production of fermentation byproducts such as SCFAs. Dietary fibers are associated with a range of health benefits, including reduced risk of obesity, type 2 diabetes, and cardiovascular diseases (Ehret et al., 2023). For example, psyllium husk has shown beneficial effects on body weight, body mass index (BMI), waist circumference, glycemic indices, and stool consistency, without being digested or fermented by humans, limiting its prebiotic potential

(Gibb et al., 2023). Similarly, cellulose, hemicellulose, and lignin have low fermentability, but evidence on their health benefits in humans is limited (Feng et al., 2025).

To expand on fermentability outcomes, co-administration of psyllium fiber and inulin reduces the inulin-induced gas production, as indicated by reduced colonic gas production and breath hydrogen, compared to inulin alone, but does not inhibit fermentation (Gunn et al., 2022). This could be due to the gel-structures formed by viscous fibers which reduce the accessibility of fermentable fibers to the microbiota, slowing fermentation (So et al., 2023). Furthermore, fibers such as sugarcane bagasse contain cellulose, hemicellulose, and lignin which are highly resistant to fermentation. So et al. (2023) showed that SCFA concentrations were lower after supplementation with sugarcane bagasse compared to a combination of sugarcane bagasse and resistant starch from high-amylose starch, suggesting the reduced fermentability of these fibers. An in vitro fermentation study comparing different dietary fibers such as inulin, microcrystalline cellulose, corn straw soluble dietary fiber, and corn straw insoluble dietary fiber found that inulin produced the highest amount of SCFAs and gas, whereas microcrystalline cellulose and insoluble dietary fiber reduced gas production (Fan et al., 2025). Overall, dietary fiber consumption is associated with numerous health promoting effects. However, fibers with limited fermentability, as reflected by reduced gas and SCFA production, have a negligible prebiotic effects.

Conclusions

Dietary fibers are characterized by their structure and physiochemical properties, including solubility, molecular weight, and fermentability, highlighting the heterogeneity within this broad category. Current evidence indicates that both prebiotic and non-prebiotic dietary fibers can contribute to various aspects of human health. However, the extent to which specific functional outcomes are influenced by fermentability, fiber type, and dosage remain incompletely characterized. While many fibers undergo microbial fermentation and generate metabolic by-products, these processes do not uniformly translate into clinically meaningful health effects. Accordingly, rigorous human clinical investigation and robust scientific substantiation of fiber- and prebiotic-based ingredients are necessary to confirm efficacy, support appropriate classification, and reduce potential consumer misunderstanding.