A 2026 University College Cork trial found that both caffeinated and decaffeinated coffee altered gut bacteria and were linked to lower stress, depression and impulsivity scores. Caffeine alone improved attention and reduced anxiety, while decaf alone improved memory and learning, according to the study published in Nature Communications.
Scientists at University College Cork (UCC) in Ireland have published evidence that drinking coffee — with or without caffeine — changes the composition of gut bacteria and is associated with measurable shifts in mood and cognitive performance. The study, led by neuroscientist John Cryan of UCC's APC Microbiome Ireland centre, appeared in the journal Nature Communications in 2026; outlets reporting on the paper give different dates for its publication, with Sci.News citing April 21, 2026 and ScienceDaily citing May 3, 2026.
What the study did
The research team recruited 62 healthy adults in Ireland, aged 30 to 50: 31 who habitually drank three to five cups of coffee a day, and 31 non-drinkers. Regular drinkers first stopped coffee for two weeks. All participants then went through a blinded period in which they drank four cups a day of instant coffee that was either caffeinated or decaffeinated, while researchers collected stool and urine samples and ran psychological assessments, according to coverage of the paper by ScienceDaily, PsyPost and News-Medical.
Changes in gut bacteria
Regular coffee drinkers showed higher relative levels of two bacterial species, Cryptobacterium curtum and Eggerthella sp., compared with non-drinkers, according to the reports. These organisms have been linked in prior microbiome research to acid secretion and bile acid metabolism. The study also noted higher levels of Firmicutes bacteria, which outlets reporting on the paper said have been associated with positive emotions in women. Researchers said these shifts occurred within specific bacterial populations without changing the overall diversity of the gut microbiome, according to PsyPost.
Mood, stress and impulsivity
Both the caffeinated and decaffeinated coffee groups reported lower perceived stress, fewer depressive symptoms and reduced impulsivity after coffee was reintroduced, according to ScienceDaily, News-Medical and Sci.News.
Where caffeine and decaf diverge
The two coffee types did not produce identical effects. Only the decaffeinated group showed improved performance on memory and learning tests and reported better sleep quality, the outlets said. Only the caffeinated group showed improved attention and alertness alongside reduced anxiety and lower markers of psychological distress and inflammation, according to News-Medical and PsyPost.
Reported effects by coffee type, per UCC study coverage
Outcome
Caffeinated coffee
Decaffeinated coffee
Perceived stress
Reduced
Reduced
Depression symptoms
Reduced
Reduced
Impulsivity
Reduced
Reduced
Anxiety
Reduced
No reported change
Attention / alertness
Improved
No reported change
Memory / learning
No reported change
Improved
Sleep quality
No reported change
Improved
Coffee is more than just caffeine — it's a complex dietary factor that interacts with our gut microbes, our metabolism, and even our emotional well-being.
— John Cryan, University College Cork, quoted by ScienceDaily and News-Medical
Sci.News's summary of the paper suggested that the overlap between the two groups points to non-caffeine compounds in coffee, such as polyphenols, driving part of the shared mood and cognitive effects, while caffeine itself was linked only to the attention and anxiety changes seen in that group.
What is still unknown
The UCC study involved 62 adults over a period of weeks, a scale that lets researchers detect short-term biological and psychological changes but that the coverage does not describe as settling the question of long-term effects. The cited reports did not include specific percentage changes for the mood or cognition scores, and the study does not establish that changes in gut bacteria directly cause the reported mood or memory effects, only that the two were observed together under controlled conditions.
Questions
Does decaf coffee have the same effect on gut bacteria as regular coffee?
According to the UCC study, decaffeinated coffee produced many of the same effects as caffeinated coffee on gut bacteria and on stress, depression and impulsivity scores, suggesting non-caffeine compounds play a major role.
What is different between caffeinated and decaffeinated coffee's effects on the brain?
In the study, only caffeinated coffee was linked to improved attention, alertness and reduced anxiety, while only decaffeinated coffee was linked to improved memory, learning and sleep quality.
Who led the coffee and gut microbiome study?
The study was led by neuroscientist John Cryan at APC Microbiome Ireland, University College Cork, and was published in Nature Communications in 2026. News outlets covering the paper give different exact dates, from April 21 to May 3, 2026.
How many people took part in the research?
The trial included 62 healthy adults aged 30 to 50 in Ireland: 31 regular coffee drinkers and 31 non-drinkers.
Does this mean coffee cures depression or anxiety?
No. The sources describe an association between coffee consumption, gut bacteria changes and lower scores on stress, depression and anxiety measures in a small trial; they do not claim coffee treats or cures any mental health condition.
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