A new study suggests your coffee habit may be doing more to shape your mood than the caffeine alone would explain — by changing which bacteria live in your gut.
Researchers at APC Microbiome Ireland, a research centre based at University College Cork, published the findings in Nature Communications after tracking 62 adults — 31 regular coffee drinkers and 31 people who rarely drink it. Regular drinkers kept consuming three to five cups a day for two weeks, then stopped entirely; both groups were later given either caffeinated or decaffeinated coffee under blinded conditions while researchers collected stool and urine samples and tracked mood and cognitive measures.
Both caffeinated and decaffeinated coffee were linked to lower scores for perceived stress, depression and impulsivity, and to measurable shifts in gut bacteria — including higher levels of species such as Eggertella and Cryptobacterium curtum in habitual coffee drinkers. The two versions diverged in other ways: decaf was associated with better learning and memory performance, while caffeinated coffee was linked to reduced anxiety, sharper vigilance and attention, and markers suggesting lower inflammation.
Coffee is more than just caffeine — it's a complex dietary factor that interacts with our gut and emotional wellbeing.
— Professor John Cryan, APC Microbiome Ireland
The research adds to a growing body of work on the so-called gut-brain axis, the communication network between digestive bacteria and the nervous system that scientists increasingly tie to mood and cognition. Earlier coffee research had mostly focused on caffeine's direct effects on alertness; this study's blinded, decaf-versus-caffeinated design was aimed specifically at separating caffeine's role from whatever else is in the cup — plant compounds, fiber-like components and other chemicals that survive into the gut largely intact.
The sample size is modest, and the authors note their results describe associations over a short, controlled window rather than long-term habits in the wild; a 62-person trial is not the final word on how millions of people's daily coffee routines interact with decades of gut microbiome development. Still, the design — randomizing caffeinated versus decaf rather than just comparing drinkers to non-drinkers — gives the findings more weight than typical observational coffee studies. The APC Microbiome team says larger follow-up trials are planned to test whether a person's existing gut bacteria predict how they respond to coffee, which could eventually inform more individualized dietary advice.
