For years, coffee was mostly discussed in terms of caffeine: how much is too much, and when to stop drinking it. A team in Ireland looked somewhere else, at the bacteria living in our gut. Their results suggest that even a cup of decaf does more than people think.
In short
- Researchers at University College Cork studied 31 regular coffee drinkers and 31 non-drinkers.
- Stopping coffee for two weeks was linked to changes in gut microbiome metabolites.
- After coffee was reintroduced, both caffeinated and decaf groups reported lower stress and depression scores.
- Some effects were specific: memory improved with decaf, attention with caffeinated coffee.
The study
The research was led by Professor John Cryan, principal investigator at APC Microbiome Ireland, University College Cork, and published in Nature Communications, the university announced in April 2026. Its findings later spread widely, including in National Geographic Spain.
The team recruited 31 people who regularly drink 3 to 5 cups of coffee a day, and 31 people who don’t drink coffee. The coffee drinkers first stopped for two weeks, giving psychological assessments and stool and urine samples along the way. Then coffee was reintroduced without them knowing which kind they got: half received decaf, half caffeinated.
What happened in the gut
Going without coffee was linked to significant changes in the gut microbiome’s metabolite profiles compared with non-drinkers. Coffee drinkers also had higher levels of certain bacteria, and Firmicutes bacteria increased; the university notes these have been associated with positive emotions in women.
“Coffee is more than just caffeine,” Cryan said, describing it as a complex dietary factor that interacts with our gut microbes. “Coffee may modify what microbes do collectively, and what metabolites they use,” he explained.
Caffeinated or decaf: different effects
After coffee came back, both groups reported lower perceived stress, depression and impulsivity, whether or not their coffee contained caffeine. But other effects split. Learning and memory improved only in the decaf group, which suggests that other compounds, such as polyphenols, may play a role. Only caffeinated coffee was associated with reduced anxiety and better vigilance and attention, and caffeine was also linked to a lower risk of inflammation.
That’s the meaning of Cryan’s conclusion: “coffee, whether caffeinated or decaffeinated, can influence health in distinct but complementary ways.” The everyday foods we take for granted can surprise researchers, as a CDC ranking did with the vegetable that beat kale and spinach.
Why it matters
For many people who have switched to decaf for sleep or blood pressure reasons, this is reassuring: the cup may still bring something beyond taste. Warm drinks have plenty of fans at home, from coffee to lemon peels boiled with ginger.
The limit
The study involved 62 people, which is small, and it measured associations and short-term changes. It was sponsored by the Institute for Scientific Information on Coffee, an organization linked to the coffee industry. It doesn’t mean more coffee is always better: people sensitive to caffeine, pregnant women and those with heart conditions should ask their doctor.
That’s the trick.
