Coffee contains roughly 1-2% caffeine by weight. The other 98-99% is chlorogenic acids at 7-10%, melanoidins formed during roasting, trigonelline, and diterpenes. The non-caffeine fraction is what does most of the work on the gut microbiome.
A new study quantified the shifts. 62 adults: 31 daily drinkers at 3-5 cups per day, 31 non-drinkers. 5-week controlled protocol. APC Microbiome Ireland.
Three findings stood out.
Cryptobacterium species increased in drinkers. These bacteria produce indoles from tryptophan. Indoles activate the aryl hydrocarbon receptor on intestinal cells, which supports gut barrier integrity and modulates immune tone.
Eggerthella species also increased. These bacteria metabolize coffee polyphenols, breaking down chlorogenic acids into smaller bioactive metabolites the gut can absorb. The increase is functionally relevant to coffee compound activation, though some Eggerthella species, particularly E. lenta, are linked to inflammation in inflammatory bowel disease and bloodstream infections in immunocompromised patients. The genus has a mixed clinical profile.
Indole-3-propionic acid decreased in drinkers. IPA is a tryptophan-derived metabolite produced by bacteria like Clostridium sporogenes. It's anti-inflammatory and supports tight junction integrity in the gut barrier. Lower IPA correlates with type 2 diabetes risk, gut barrier dysfunction, and inflammation across the literature. A reduction is not a benefit.
The same directional shifts appeared in decaffeinated coffee drinkers. Chlorogenic acids and melanoidins are present in decaf at similar concentrations. Caffeine alone does not explain the microbiome changes.
N=62 is small. The findings are exploratory and need replication.
The behavioral component of the study reported a mix of effects. Some measures of cognition shifted in expected directions. Others, including impulsivity and emotional reactivity, also moved. The picture on mood and cognition is more complicated than a single direction.
Whether the IPA reduction reflects a meaningful change in gut barrier function, or is a marker of broader bacterial community changes, isn't resolvable from this data.
Practical framing: coffee changes the gut whether or not it contains caffeine. If caffeine causes problems with sleep, anxiety, or blood pressure, decaf delivers most of the same microbiome effects. The IPA reduction applies to both forms.
Boscaini et al., Nat Commun, 2026 DOI: 10.1038/s41467-026-71264-8
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