APC Microbiome Ireland trial of 62 adults records lower stress, depression and impulsivity scores after blinded reintroduction of either caffeinated or decaffeinated coffee

Researchers at APC Microbiome Ireland tracked stool, urine, diet logs and psychological inventories before and after coffee reintroduction. Eggertella sp. and Cryptobacterium curtum increased in coffee consumers. Decaffeinated intake correlated with higher learning and memory scores; caffeinated intake correlated with lower anxiety, higher vigilance and lower inflammatory markers.

The gut-brain shifts align with prior Cryan lab work on microbial metabolites modulating HPA-axis reactivity. Daily coffee therefore functions as a repeatable dietary lever that alters both microbial composition and self-reported mood metrics within weeks, directly affecting routine stress regulation for regular consumers.

Operational impact appears in adherence: participants maintained intake without external prompts once reintroduced, indicating the mood effect may reinforce habit formation. Larger longitudinal cohorts are required to test whether these microbiome changes persist beyond eight weeks and translate to measurable reductions in clinical depression scales.

Future trials should stratify by baseline microbiome diversity and track objective cortisol and actigraphy data to separate placebo from metabolite-driven effects.