Scientists in Ireland have reported new evidence that coffee can influence the gut-brain axis, the two-way signaling system linking digestion and brain function. The research suggests both caffeinated and decaffeinated coffee may shape gut microbes and track with changes in mood-related measures.
The study, led by APC Microbiome Ireland at University College Cork and published in Nature Communications, compared 31 regular coffee drinkers with 31 non-coffee drinkers. Researchers combined diet records, psychological questionnaires, and stool and urine samples to map links between coffee intake, microbiome activity, and wellbeing.
A two-week coffee break tested
Regular coffee drinkers were first asked to stop drinking coffee for two weeks, allowing scientists to observe what changed when coffee was removed. During this abstinence period, the team reported shifts in microbe-related metabolites that helped distinguish habitual coffee drinkers from non-drinkers.
Coffee was then reintroduced in a way that did not tell participants whether they were receiving caffeinated or decaffeinated coffee. According to the findings, both groups showed improvements across several mood measures, including lower reported stress and depressive symptoms.
Decaf and caffeine showed differences
One of the more closely watched results was that improvements in learning and memory were seen in the decaffeinated group. The authors say this points to coffee compounds beyond caffeine, such as polyphenols and other bioactives, as possible drivers of certain brain-related effects.
Meanwhile, the caffeinated group showed changes consistent with caffeine’s known effects, including better alertness and attention, alongside reduced anxiety in the study’s assessments. The paper also reported differences in biological markers tied to inflammation, though it did not position coffee as a treatment.
What changed in the microbiome?
The researchers identified specific bacterial signals associated with coffee drinking, including higher levels of Eggertella species and Cryptobacterium curtum in coffee drinkers. They also noted shifts in broader bacterial groupings that have been linked in other research to metabolic and emotional outcomes.
Experts caution that microbiome studies often show associations rather than direct cause-and-effect, and larger trials are still needed. Still, the work adds to a growing body of evidence that everyday dietary habits, including coffee consumption, may influence both gut activity and mental health.
In Europe, moderate caffeine intake is generally considered safe for most healthy adults, with guidance often referenced around 400 mg per day from all sources. People who are pregnant, sensitive to caffeine, or managing anxiety, reflux, or sleep issues are typically advised to discuss intake with a clinician.
