The gut and brain maintain continuous communication through a complex network of nerves, hormones, and microbial signals, collectively known as the microbiota–gut–brain axis. Intestinal bacteria influence a wide range of functions, including mood, memory, stress responses, and impulse control.
While diet’s influence on the microbiome is well established, coffee’s specific role within this system remained unclear until it was recently studied.
Researchers at University College Cork recruited 62 healthy adults aged 30 to 50, including 31 habitual coffee drinkers (3–5 cups daily) and 31 non-drinkers. Over several weeks, participants followed a structured protocol comprising a baseline comparison, a 14-day coffee-abstinence period, and a 21-day reintroduction phase with either caffeinated or decaffeinated coffee. Biological samples (stool, urine, blood, saliva) and cognitive assessments were collected throughout the study.
The results revealed notable differences, offering new insights into coffee’s effects on the gut–brain axis and setting the stage for more detailed findings.
Coffee Drinkers Have a Distinctly Different Gut Microbiome
A comparative analysis of fecal microbiomes between coffee drinkers and non-drinkers revealed clear, consistent differences at the species level. Coffee drinkers exhibited significantly higher levels of Cryptobacterium curtum and Eggerthella species, which are bacteria currently under investigation for their potential roles in bile acid metabolism and gut homeostasis. In contrast, non-coffee drinkers had elevated levels of Veillonella parvula and Haemophilus parainfluenzae.
Overall gut microbial diversity did not differ substantially between the two groups. Instead, specific bacterial strains were altered, indicating that coffee consumption selectively targets certain microbial populations rather than inducing broad changes in the gut ecosystem.
During the two-week coffee-abstinence period, levels of Cryptobacterium curtum in coffee drinkers declined steadily at multiple time points. Upon reintroduction of coffee, whether caffeinated or decaffeinated, all seven species that were differentially abundant shifted again. These results strongly suggest that non-caffeine compounds in coffee, such as polyphenols, play a significant role in modulating the gut microbiome.
The Metabolite Story: What Coffee Does Inside Your Gut
Coffee drinkers had higher fecal levels of theophylline, a coffee alkaloid, 1,7-dimethylxanthine, a caffeine metabolite, and hippuric acid, a polyphenol byproduct, evidence of active coffee metabolism.
Coffee drinkers also showed reduced levels of key gut-derived metabolites:
- Indole-3-propionic acid (IPA) may support cognitive health.
- Indole-3-carboxyaldehyde (ICA) is tied to gut and inflammation regulation.
- GABA, the primary calming neurotransmitter, is produced by gut bacteria
These neuroactive compounds, found at lower levels among coffee drinkers, may be associated with behavioral differences between groups.
Following two weeks of coffee abstinence, ICA levels began to recover, indicating that these metabolic shifts are at least partially reversible.
Coffee, Impulsivity, and Memory: The Behavioral Findings
This section explores how coffee consumption influences impulsivity, emotional reactivity, energy levels, attention, and fatigue, and tracks changes during abstinence and reintroduction.
The study found that habitual coffee drinkers showed greater impulsivity, emotional reactivity, and sensation-seeking than non-drinkers, suggesting that regular coffee intake may be associated with these traits.
Non-coffee drinkers performed better on memory assessments during the study period.
After two weeks without coffee, coffee drinkers reported reduced impulsivity and emotional reactivity, with improvements in attention and cognitive vigilance. Contrary to expectations about caffeine withdrawal, they also reported feeling more energetic and less fatigued.
Then came the reintroduction phase, and the results were nuanced:
- Caffeinated coffee reduced anxiety, psychological distress, and improved attention and stress coping. It also lowered blood pressure.
- Decaffeinated coffee improved sleep quality and physical activity scores and led to meaningful gains in memory performance.
- Both types reduced perceived stress, depression symptoms, and impulsivity and boosted mood.
These findings indicate that many of coffee’s psychological and physiological benefits are attributable to its non-caffeine components. Polyphenols, melanoidins, and other bioactive compounds exert significant effects independent of caffeine’s stimulant properties.
In addition to cognitive and emotional outcomes, the study addressed markers of stress and inflammation.
The study also examined cortisol, the primary stress hormone. Coffee drinkers and non-drinkers exhibited similar cortisol levels at baseline and under stress conditions (using a cold pressor test to induce acute stress). These results suggest that habitual coffee consumption does not meaningfully alter the stress hormone axis in the short term.
On the immune side, however, coffee drinkers had lower baseline CRP levels (a key marker of inflammation) and higher IL-10 levels (an anti-inflammatory cytokine). When coffee was withdrawn, CRP and TNFα both rose, indicating increased inflammation. When coffee was reintroduced, these levels normalized.
Both caffeinated and decaffeinated coffee reduced lipopolysaccharide (LPS)-induced interleukin-6 (IL-6) secretion by about half. This finding suggests that coffee’s anti-inflammatory properties are primarily independent of caffeine and likely mediated by polyphenols.
Broader Implications: Coffee as a Modulator of the Gut-Brain Axis
Integrating microbiome, metabolomics, and behavioral data, the researchers built a statistical model that identified nine key metabolites, including theophylline, caffeine, ICA, fumaric acid, and selected phenolic acids, that are tightly linked to specific gut microbial species and cognitive/behavioral outcomes.
For example, Veillonella species were strongly associated with theophylline, which correlated with stress, sleep quality, and memory performance. This sequence of coffee compounds, microbial shift, metabolite, and cognitive outcome represents the type of pathway that researchers have sought to elucidate for years.
A key practical implication of the study is that the gut microbiome may predict individual coffee consumption habits. The distinctive microbial signatures observed in coffee drinkers suggest that microbiome profiles could potentially serve as biomarkers for dietary behavior.
What This Means for You
This study reveals that coffee is not just a caffeine delivery system; it’s a complex mixture of bioactive compounds that actively communicates with your gut microbiome, which in turn shapes your neurochemistry. Key evidence-based conclusions include the following:
- Individuals who consume coffee and experience heightened emotional reactivity or impulsive tendencies may be influenced by mechanisms of the gut-brain axis.
- Switching to decaffeinated coffee does not eliminate all potential benefits. Polyphenols, which remain after decaffeination, contribute substantially to coffee’s microbiome and anti-inflammatory effects.
- Brief periods of coffee abstinence appear to partially restore metabolite levels, particularly the neuroactive compounds ICA and IPA.
- The gut microbiome plays a central role in coffee metabolism, suggesting that individuals who consume the same amount of coffee may experience substantially different physiological effects.
The Bottom Line
Historically, coffee research has focused primarily on caffeine. This study shifts the perspective, emphasizing that the relationship between coffee and health is mediated by the gut microbiome, which responds to coffee intake by adjusting metabolic output and transmitting chemical signals to the brain.
Morning coffee does more than just provide alertness; it interacts with trillions of gut microbes, shaping the brain’s neurochemical environment. Understanding these interactions may inform future dietary recommendations.
References
Boscaini, S., Bastiaanssen, T.F.S., Moloney, G.M., et al. (2026). Habitual coffee intake shapes the gut microbiome and modifies host physiology and cognition. Nature Communications, 17, 3439. https://doi.org/10.1038/s41467-026-71264-8