2026: The Year of Fibre – Fibre & the Gut Microbiome

Part Two of the Fibre Series

Welcome back to The Fibre Series. Last week, we explored the basics of fibre, what it is, its molecular structure, and why diversity matters. This week, we’re taking a deeper dive into the gut microbiome, the complex ecosystem of trillions of microorganisms in your digestive tract and how fibre shapes it.

The gut microbiome is a dynamic, metabolically active community composed of bacteria, archaea, viruses, and fungi. These microbes play a crucial role in human health, influencing digestion, metabolism, immune function, and even brain signalling. Fibre is their primary source of energy and a key regulator of microbial composition and activity [1,2].

How Fibre Feeds the Microbiome

Not all fibres are equal when it comes to microbial fermentation. Microbes in the colon specialise in breaking down different types of carbohydrates, and each fibre feeds a specific microbial community:

• Soluble fibres (e.g., oats, barley, legumes) dissolve in water and form viscous gels. They are highly fermentable and tend to increase populations of beneficial bacteria such as Bifidobacteria and Lactobacilli [3].

• Insoluble fibres (e.g., wheat bran, vegetable skins) resist fermentation but improve gut transit and provide structural support to the microbial environment [3].

• Resistant starches (e.g., cooled potatoes, green bananas, legumes) bypass digestion in the small intestine and are fermented in the colon, producing short-chain fatty acids (SCFAs) like butyrate, which nourish colonocytes and help maintain gut barrier integrity [4,12].

• Prebiotic fibres (e.g., inulin, fructooligosaccharides) selectively stimulate beneficial microbes, enhancing microbial diversity and resilience [2,5].

Each fibre type acts as a signal and substrate, shaping which microbes thrive and what metabolic products they produce. The interactions are highly specific: some fibres feed a narrow set of microbes, while others support a broad spectrum, reinforcing the importance of diversity in dietary fibre intake [1,4].

Fibre and Microbial Diversity

Microbial diversity is a hallmark of a healthy gut. Diets high in diverse plant-based fibres lead to a richer microbial ecosystem, whereas low-fibre diets are associated with reduced diversity, loss of beneficial microbes, and increased susceptibility to inflammation and metabolic disorders [5,6].

Research shows that microbial diversity contributes to:

• Efficient fermentation of fibre and production of SCFAs, which are key microbial metabolites linking diet to host metabolism [1,2,13]

• Balanced immune function, through modulation of immune signalling pathways and support of barrier integrity [3,4,11]

• Protection against pathogens and microbial imbalance (dysbiosis), by maintaining a resilient, competitive microbial community [5,6]

• Metabolic regulation, including effects on glucose and lipid homeostasis [1,2,10]

Importantly, different microbes specialise in fermenting different fibres, so a varied diet is critical. For example:

• Ruminococcus bromii is a key starch-degrading bacterium that initiates the breakdown of resistant starch, enabling other microbes to ferment the products further [7,8].

• Bifidobacterium adolescentis and related species possess enzymes to utilise resistant starch and prebiotic fibres, contributing to SCFA production and microbial interactions [8,9].

Including multiple fibre sources ensures these microbes are nourished and active, supporting a diverse and functional gut ecosystem.

The Role of Short-Chain Fatty Acids (SCFAs)

Microbial fermentation of fibre produces short-chain fatty acids (SCFAs), primarily acetate, propionate, and butyrate, which act as messengers between the gut and the rest of the body [4,12].

• Butyrate: Primary fuel for colonocytes; reduces inflammation; supports gut barrier integrity and mucosal health [3,15,16]

• Propionate: Travels to the liver; helps regulate glucose production, lipid metabolism, and colonic health [1,13,15]

• Acetate: Enters systemic circulation; influences appetite regulation, energy balance, and hormonal signalling, including peptide YY and GLP-1 [1,2,11]

Together, SCFAs link dietary fibre to systemic health, influencing metabolism, immunity, and overall wellbeing. They also act via G-protein-coupled receptors (GPCRs) and epigenetic mechanisms, illustrating how the microbiome translates fibre intake into broad physiological effects [15,2].

Fibre-Microbe Interactions Beyond SCFAs

In addition to SCFAs, microbial fermentation of fibre produces a range of other biologically active compounds:

• Gases (hydrogen, methane, carbon dioxide), fermentative activity generates gases that can influence gut motility and fermentation dynamics [7,16]

• Vitamins, certain gut microbes can synthesise B‑vitamins and vitamin K; fibre-rich diets enhance the pool of microbiota-produced vitamins, supporting metabolism and immune function [4]

• Neuroactive compounds and precursors, microbes metabolise amino acids such as tryptophan into signalling molecules (e.g., indole derivatives) that influence host metabolism, immunity, and gut–brain communication [5,18]

These additional products highlight the complex dialogue between dietary fibre, the gut microbiota, and human physiology, showing that fibre’s influence extends far beyond digestion.

Next week, we’ll follow the trail of fibre a little deeper, beyond the microbes themselves and into the molecules they produce. Short-chain fatty acids (SCFAs) act as powerful messengers, linking your gut to metabolism, hormones, and overall health.

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11. SCFAs and regulation of appetite/blood glucose via hormone signalling. Lipids Health Dis.

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13. Short-chain fatty acids: linking diet, the microbiome and immunity. Nat Rev Immunol. 2024; (review).

14. Dalile B, Van Oudenhove L, Vervliet B, Verbeke K. Nat Rev Gastroenterol Hepatol. 2019;16:461–478.

15. A widespread hydrogenase supports fermentative growth of gut bacteria in healthy people. Nat Microbiol. 2025.

16. Dietary fibre directs microbial tryptophan metabolism via metabolic interactions in the gut microbiota. Nat Microbiol. 2024.