From espresso to filter coffee, brewing may shape what reaches your gut

From the way coffee is roasted and brewed to the microbes that process its compounds in the colon, several factors may shape its effects on the body. Researchers are piecing together what happens along the way.

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In a recent focused narrative review published in the journal Metabolites, researchers summarized evidence on the gastrointestinal (GI) fate, systemic availability, and microbial transformation of coffee constituents.

Coffee is among the most consumed beverages worldwide and a primary dietary source of polyphenols and caffeine. Coffee and its derivative preparations may affect the gut microbiota, metabolic phenotypes, microbial metabolites, and hepatic inflammation.

Observational studies have linked habitual coffee consumption to lower risk of type 2 diabetes and more favorable liver outcomes, but they cannot establish whether gut microbes contribute to these associations. While caffeine is rapidly absorbed in the upper GI tract, other coffee constituents, such as chlorogenic acids (CGAs), polysaccharides, and ester-bound phenolics, may evade absorption in the small intestine.

Colonic microbes ferment some nondigestible coffee fractions to produce short-chain fatty acids (SCFAs) and transform CGAs and bound phenolics into low-molecular-weight (LMW) phenolic acids. The health effects of coffee may result from the interactions among coffee-derived inputs, host metabolic networks, and the gut microbiome.

GI fate of coffee constituents

Caffeine is rapidly and efficiently absorbed after oral ingestion. It is then metabolized in the liver into paraxanthine, theobromine, and theophylline by cytochrome P450 1A2. Its effects on the gut microbiota may be largely indirect since it is not a typical substrate for colonic fermentation. Human cohort studies have associated high caffeine consumption with alterations in mucosal microbiota richness and the abundance of specific bacterial groups.

CGAs are a predominant class of polyphenols in coffee. A substantial fraction of CGAs evades absorption in the upper GI tract and is transformed by colonic microbes. Their initial hydrolysis can involve esterase-producing bacteria, including Eubacterium ramulus and Bifidobacterium. Caffeic acid undergoes sequential microbial degradation, yielding various LMW microbial metabolites, some of which are absorbed and conjugated in the gut mucosa and liver before entering the systemic circulation.

In laboratory fermentation studies, purified CGAs and coffee extracts have been found to influence the growth of specific taxa and promote the synthesis of different phenolic acids. Roasting reactions generate high-molecular-weight (HMW) melanoidins and modify the structure of coffee polysaccharides. Some fractions of these carbohydrate-associated melanoidin complexes, soluble polysaccharides, and bound phenolics reach the colon after resisting digestion in the upper GI tract.

Galactomannans and arabinogalactans, the primary constituents of the cell-wall polysaccharide matrix in coffee beans, can undergo microbial degradation in the colon, producing SCFAs. The fermentation of coffee fractions enriched in arabinogalactans is accompanied by the generation of dihydroferulic and dihydrocaffeic acid. Fractions enriched in melanoidins may influence fermentation in the colon, but whether they affect microbial bile acid transformation remains unknown.

Brewing changes the mixture of compounds in the cup. Water temperature, contact time, pressure, grind size, and filtration affect extraction, so espresso, drip-filtered, French press, boiled, cold-brew, and instant coffee may deliver different proportions of compounds that resist digestion and reach the colon. These differences could change what is available to gut microbes, but no brewing method has been shown to produce a consistent microbiome or metabolic benefit in humans.

Filtration has a separate, better-established implication for health. Paper filters reduce levels of the coffee compounds cafestol and kahweol compared with unfiltered methods such as French press and boiled coffee. Sustained consumption of unfiltered coffee can raise LDL cholesterol.

Host metabolic regulation via microbiota–coffee interactions

Among microbial metabolites, SCFAs are plausible mediators of coffee's metabolic effects as the intestinal microbiota ferments nondigestible coffee fractions to produce butyrate, acetate, and propionate in vitro. Free and bound polyphenols may also influence SCFA profiles by altering community composition and microbial substrate preferences. CGAs, caffeic acid, and other phenolics have been implicated in inflammatory, metabolic, and redox signaling in animals and cell cultures.

Microbial bile acid transformation is another potential mechanism linking coffee to the regulation of the liver–gut axis. Preclinical interventions with CGAs or caffeine have been associated with changes in the serum metabolome, including aspects of bile acid metabolism. Microbes modify the bile acid pool through deconjugation, oxidation, dehydroxylation, and epimerization, transforming primary bile acids into secondary bile acids.

Bile acids and their microbial derivatives act as ligands for G protein-coupled bile acid receptor 1 (TGR5) and farnesoid X receptor (FXR), which help regulate metabolic and immune responses. In mice, intestinal FXR activation has been reported to promote FGF15 secretion, contributing to the feedback regulation of bile acid synthesis. Humans have a related FGF19 pathway. TGR5 activation can modulate glucagon-like peptide 1 (GLP-1) release and immune cell function. Yet clinical studies directly demonstrating coffee's effects on metabolic outcomes via bile acid receptor pathways are sparse.

Concluding remarks

Taken together, several limitations prevent firm clinical or mechanistic conclusions. For instance, many studies used purified caffeine, extracts, CGAs, or experimental doses that do not recapitulate the exposure and composition of routinely consumed coffee. Evidence on microbial responses is often based on taxonomic abundance, without parallel measures of microbial pathways or metabolite production.

Associations between coffee consumption, cardiometabolic outcomes, and microbial features do not imply that the observed effects are mediated by the gut microbiota. Future work should develop a standardized coffee exposure platform and compare decaffeinated and caffeinated coffee with a non-coffee control, while controlling for brewing, filtration, the degree of roasting, dosage, and additives.

Translating available evidence into precision coffee nutrition requires evidence from studies using realistic doses and clinically meaningful outcomes. This hinges on whether experimental doses reproduce achievable human exposures, biomarkers capture heterogeneity in exposure and response, and potential benefits remain favorable after accounting for safety, individual susceptibility, and beverage preparation.

Journal reference:

• Yang, L., Wang, C., Cui, L., Zhu, Y., & Wang, X. (2026). Coffee, the Gut Microbiome, and Host Metabolism: Gastrointestinal Fate, Microbial Transformation, Mechanistic Insights, and Prospects for Precision Nutrition. Metabolites, 16(9), 680. DOI: 10.3390/metabo16090680, https://www.mdpi.com/2218-1989/16/9/680