Decaf sharpened memory while caffeinated coffee improved attention, a gut study found

Thirty-one habitual coffee drinkers, each used to three to five cups a day, gave up coffee entirely for two weeks. Then half were handed caffeinated coffee and half decaf, without knowing which. When the dust settled, the decaf group had improved on memory tests and the caffeinated group had improved on an attention test.

The experiment, which is now more than five months old, was run by APC Microbiome Ireland at University College Cork, was designed around the gut, not the brain. Stool and urine samples were collected alongside the psychological assessments throughout, and the interest was in how habitual coffee drinking changes the microbes, which metabolites result, and what those microbes might do to mood, learning and thinking. The same battery was given to a comparison group of 31 people who did not drink coffee at all, which gave the drinkers a reference point for what a coffee-free baseline looks like.

Decaf Memory Gains on the ModRey Tests

The study, published April 21, 2026 in Nature Communications as “Habitual coffee intake shapes the gut microbiome and modifies host physiology and cognition”, enrolled 62 healthy adults: 31 habitual coffee drinkers and 31 non-drinkers. After two weeks of abstinence, the drinkers consumed either caffeinated or decaffeinated coffee for three weeks. In the paper’s results, the decaffeinated group showed significant improvements in several components of the ModRey memory test, while the caffeinated group did not show similar gains.

University College Cork’s own release frames the same result as improved learning and memory in the decaf group, and suggests polyphenols rather than caffeine may drive it. Professor John Cryan, the principal investigator, Dr. Serena Boscaini and Dr. Gerard Moloney led the work.

Caffeinated Coffee and the PASAT Attention Score

The attention result runs the other way, with caffeine rather than decaf associated with the better score. Participants who drank caffeinated coffee posted increased scores on the PASAT, a test of sustained attention and processing speed, according to the paper. The university release describes the caffeinated arm as showing reduced anxiety along with enhanced vigilance and attention, and a decreased inflammation risk.

Both arms shared one benefit. Per the university, participants in either group reported significantly lower perceived stress, depression and impulsivity, regardless of caffeine content. That overlap fits the line Cryan gave in the university release, that coffee is more than just caffeine and is a complex dietary factor interacting with gut microbes, metabolism and emotional well-being.

Decaf is not strictly caffeine-free, a point that matters for how cleanly the two arms can be separated. The FDA notes that a cup of decaffeinated coffee typically carries about 2 to 15 milligrams of caffeine, against its 400-milligram daily benchmark for amounts not generally associated with negative effects in adults, which it equates to roughly two to three 12-ounce cups of coffee. The small residue is far below what the caffeinated arm consumed, though it is not zero, and the study’s authors treat the decaf group’s gains as pointing toward non-caffeine compounds in coffee rather than toward any complete absence of the stimulant.

Eggerthella, Cryptobacterium and the Gut Link

The gut side of the story centers on two bacteria. Coffee drinkers carried higher levels of the bacteria Cryptobacterium curtum and Eggerthella, two names that recur in the microbiome results, and urine samples showed changes in compounds tied to the breakdown of caffeine and polyphenols. PsyPost’s account adds that people varied widely in how they processed polyphenols, and that this variation tracked specific differences in gut bacteria.

The cohort was small and specific, and its makeup limits how far the results travel. Medical News Today notes only 62 participants, predominantly Caucasian, who drank instant coffee exclusively, which may differ chemically from freshly brewed coffee. Participants were healthy adults aged 30 to 50 living in Ireland, so the results say nothing directly about older people, people with existing memory problems, or populations whose diets and gut bacteria differ from those of an Irish cohort. The memory test used for the decaf finding and the attention test used for the caffeine finding are different instruments, which means the two results are not a head-to-head comparison on one scale.

Cryan himself has been careful about the size of the effect. Quoted by PsyPost, he described the changes as “subtle but measurable” and said the study was observing associations and physiological responses rather than diagnosing clinical effects. That is a long way from showing that switching to decaf will improve anyone’s memory, or that a morning coffee will sharpen attention at a desk, and the Cork team asserts neither. Cryan also pointed, in the coverage, to a mix of explanations behind the benefits: biological effects of coffee compounds including caffeine and polyphenols, microbiome-mediated mechanisms, and psychological effects, so the microbes are one pathway among several rather than the whole account.

A larger, more varied group drinking brewed coffee has not been tested in this trial, so the memory and attention differences remain unreplicated in that setting. The numbers that stand are 62 adults, three weeks on caffeinated or decaffeinated coffee, and one dated record in Nature Communications.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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