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Fermentation of Fibre and Short-Chain Fatty Acids

The Microbiota and Fibre Fermentation

The human colon harbours a diverse community of microorganisms—bacteria, archaea, and other microbes—collectively referred to as the gut microbiota or microbiome. These organisms play a central role in metabolising dietary fibre that reaches the colon unchanged, a process known as fermentation.

Soluble Fibre as a Substrate

The Fermentation Process

When soluble fibre reaches the colon, it becomes an energy source for resident microbiota. These microorganisms lack the enzymes required to digest insoluble fibre or highly resistant starches, so they have evolved to metabolise soluble carbohydrates through fermentation—an anaerobic metabolic pathway occurring in the oxygen-limited colon environment.

Bacterial Enzyme Systems

Different bacterial species possess distinct enzymatic capabilities, allowing them to ferment different types of soluble fibre. For example, some bacteria specialise in fermenting inulin (found in garlic, onions, and chicory), while others ferment beta-glucans (found in oats and barley). This microbial diversity enables the utilisation of varied dietary fibre sources.

Short-Chain Fatty Acids (SCFAs)

Definition and Production

Short-chain fatty acids are organic acids containing two to six carbon atoms. The three most abundant SCFAs produced from fibre fermentation are:

• Butyrate: The primary energy source for colonocytes (colon epithelial cells)
• Propionate: Primarily metabolised by the liver
• Acetate: Readily absorbed and distributed systemically for metabolism in various tissues

The relative proportions of these three SCFAs depend on the type of fibre fermented and the composition of the microbiota performing fermentation.

SCFA Production Rates

SCFA production from dietary fibre fermentation occurs over hours to days, depending on transit time through the colon and microbial fermentation kinetics. The concentration of SCFAs in colonic fluid rises progressively as fermentation proceeds.

Physiological Effects of SCFAs

Colonic and Intestinal Effects

Butyrate serves as the primary fuel source for colonocytes, supporting the integrity and function of the colonic epithelium. Propionate and acetate are absorbed across the colonic epithelium into the bloodstream, where they circulate and are metabolised by various tissues.

G-Protein Coupled Receptors

SCFAs act as ligands (binding molecules) for specific G-protein coupled receptors, particularly GPR41 and GPR43, found on intestinal epithelial cells and immune cells. Activation of these receptors by SCFAs triggers intracellular signalling cascades that may influence multiple physiological processes.

Enteroendocrine Cell Signalling

SCFAs and the fermentation of fibre influence the secretion of enteroendocrine hormones, including GLP-1 and PYY, from L-cells distributed throughout the intestinal epithelium. This represents a key mechanism linking dietary fibre fermentation to appetite-related hormone secretion.

Individual Variation in Fermentation

Microbiota Composition

The composition of an individual's gut microbiota varies considerably between people, influenced by genetics, dietary history, antibiotic use, and other factors. Different microbial communities ferment fibre at different rates and produce different SCFA ratios.

Fibre Adaptation

When fibre intake increases, the microbiota composition can shift over days to weeks. Bacteria capable of fermenting the newly abundant fibre source expand in population, whilst those lacking the requisite enzymes may decline. This adaptation process explains why some individuals experience adjustment symptoms when initially increasing fibre intake, symptoms that typically resolve as the microbiota adapts.

Dietary History

Individuals with long-standing high-fibre diets typically harbour microbial communities more efficient at fermenting diverse fibre types. In contrast, those accustomed to low-fibre diets may have microbiota less equipped for rapid fermentation, leading to transit-related symptoms when fibre intake suddenly increases.

Fermentation and Gas Production

A byproduct of microbial fermentation is the production of gases, primarily hydrogen, carbon dioxide, and sometimes methane. In individuals with efficient fermentation and rapid substrate availability, gas production can exceed absorption rates, leading to bloating or flatulence—common experiences when significantly increasing fibre intake.

Key Takeaway

Soluble dietary fibre undergoes fermentation by colonic microbiota, producing short-chain fatty acids that interact with intestinal tissues through multiple mechanisms. This fermentation-based pathway represents a key physiological connection between dietary fibre intake and downstream metabolic and signalling effects. Individual responses depend on microbiota composition and fermentation capacity, both of which vary significantly between people and adapt to dietary changes over time.