Science1 publisher2 min readPublished
Memory gains in a 31-person coffee trial appeared only in the decaf group
John Cryan's lab at University College Cork found only the decaf half of 31 coffee drinkers improved on two memory tests. Cryan suspects gut microbes, but the blinded trial ties them to the result without showing they caused it.
The Scientist · Science desk
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What happened
- Participants first gave up their usual 3 to 5 cups of coffee a day for two weeks, and the break alone substantially shifted the gut microbes in their stool samples.
- Half then drank decaf and half caffeinated instant coffee, four servings a day for three weeks, with no one knowing who had which.
- Coffee's return triggered acute changes in the gut microbiome in both groups, whether or not the coffee contained caffeine.
- Only the caffeinated group showed reduced anxiety and psychological distress alongside better attention and vigilance.
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Why it matters
- contradiction Cryan's gut explanation has to account for the caffeinated drinkers, whose microbes also changed but whose memory scores showed no significant gain.
- constraint With about 15 people per arm, the study cannot say decaf outperformed caffeinated coffee on memory, only that one group's change crossed a significance threshold.
- decision Coffee drinkers have no basis here for switching to decaf to help memory; the evidence is three weeks of instant coffee after a forced two-week break.
The decaf gains were specific: better scores on several parts of a word-learning test and fewer errors on a location test [9]. If gut bacteria produced them, the caffeinated arm needs explaining, since its microbes moved too [5]. Either caffeine offset a benefit, or the two groups' microbes changed in different ways. According to ScienceAlert, scientists can only speculate about why without further research. One suggestion is that coffee's polyphenols feed gut microbes and set off changes that reach cognition [10].
Chance is the other candidate, because each arm was small. Thirty-one people split in half gives groups of about 15 and 16 [1]. The decaf group's memory gains reached statistical significance and the caffeinated group's did not [8]. That pattern does not by itself show the groups differed. A modest gain among about 15 caffeinated drinkers could fall short of significance. ScienceAlert's account of the paper, published in Nature Communications in April [1], does not report a direct comparison of the two groups' memory scores or a coffee-free arm during reintroduction [4].
The design does rule some things out. Participants did not know which coffee they were drinking [4], so expectation should not have favored decaf. Both arms also started from the same coffee-free baseline [2].
The effects that turned up in both arms are the stronger case for something beyond caffeine. Decaf and caffeinated drinkers alike reported lower perceived stress, depression and impulsivity after resuming [6]. The caffeine-only effects match what other studies have reported [15]. Coffee contains more than a thousand compounds, among them polyphenols known to feed gut microbes [11]. With only two coffee arms, the return of a daily habit is also a candidate for the shared effects [4].
Cryan described what his lab saw as a relationship. "There was a clear relationship between some of their cognitive effects, immune effects, their stress effects, and their microbiome," he said [12]. The trial manipulated the coffee and measured the microbes. I think the result supports the narrower claim Cryan also made: "So it wasn't just the caffeine." [13] His suspicion that gut microbes account for more of coffee's benefits than caffeine does [16] is still a hypothesis. "It's a small study," Cryan said, "it asks a lot of questions." [14]
What to watch
- Whether the full Nature Communications paper reports a direct decaf-versus-caffeinated test on the word and location scores, and how large the gap is.
- A trial that alters gut microbes while holding coffee intake fixed, the test the microbe explanation needs.
- A larger replication with a coffee-free control arm during reintroduction, to separate coffee's compounds from the return of the habit.