Coffee’s grip on the morning routine may run deeper than a caffeine hit. Researchers at University College Cork say regular coffee drinking, including decaf, changed gut bacteria and was linked to better mood and lower stress.
The study, published in Nature Communications, was carried out by researchers at APC Microbiome Ireland at University College Cork and supported by the Institute for Scientific Information on Coffee.
Scientists compared 31 regular coffee drinkers with 31 people who do not drink coffee. Participants completed psychological assessments, tracked their diet and caffeine intake, and provided stool and urine samples to measure changes in gut bacteria and emotional state.
In the study, coffee drinkers were defined as people who usually consume 3 to 5 cups of coffee a day, a level described as safe and moderate by the European Food Safety Authority.
At the start of the experiment, regular coffee drinkers stopped consuming coffee for two weeks. During that period, researchers kept collecting biological samples and monitoring mental health. The break produced clear shifts in metabolites made by gut microbes in regular coffee drinkers, separating them from non-coffee drinkers.
After that abstinence phase, researchers gradually reintroduced coffee without telling participants if it was caffeinated or decaffeinated. Half received decaf and half received regular coffee.
Both groups reported improved mood, including lower stress, depression and impulsivity. The researchers said that suggests coffee can improve mood without caffeine.
The study also found some bacteria were more common in coffee drinkers. Levels of Eggertella sp and Cryptobacterium curtum were higher among regular coffee consumers. Researchers said these microbes are believed to be involved in acid production in the digestive system and bile acid synthesis, which may help protect against harmful bacteria and infections.
They also observed an increase in Firmicutes, a group of bacteria previously linked to positive emotions in females.
The effects were not identical between decaf and regular coffee. Improvements in learning and memory were seen only in participants who drank decaffeinated coffee, suggesting compounds other than caffeine, such as polyphenols, may drive some cognitive effects.
Caffeinated coffee showed different effects. Only participants consuming caffeine had reduced anxiety, along with better attention and alertness. Caffeine was also associated with a lower risk of inflammation.
Professor John Cryan, principal investigator at APC Microbiome Ireland, University College Cork, said: “Public interest in gut health has risen hugely. The relationship between digestive and mental health is also increasingly being better understood, but the mechanisms behind coffee’s effects on this gut-brain axis have remained unclear.
“Our findings reveal the microbiome and neurological responses to coffee, as well as their potential long-term benefits for a healthier microbiome. Coffee may modify what microbes do collectively, and what metabolites they use. As the public continues to think about dietary changes for the right digestive balance, coffee has the potential to also be harnessed as a further intervention as part of a healthy balanced diet.”
He added: “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,” said Professor Cryan. “Our findings suggest that coffee, whether caffeinated or decaffeinated, can influence health in distinct but complementary ways.”
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