Science1 publisher2 min readPublished
Decaf and caffeinated coffee both lowered stress scores after a two-week break, Cork study reports
University College Cork researchers report that 31 habitual coffee drinkers scored lower on stress and depression once coffee returned, decaf or not. Gut microbe and metabolite differences make a gut-brain route plausible, though arms of about 15 people keep it an association.
The Scientist · Science desk
What happened
- The 31 drinkers, who normally had 3 to 5 cups a day, quit coffee for two weeks and then got it back without knowing whether it was decaf.
- During the two-week break, the drinkers' gut metabolite profiles shifted substantially compared with 31 people who do not normally drink coffee.
- Eggerthella and Cryptobacterium curtum, bacteria tied to acid secretion and bile acids, were more abundant in coffee drinkers than in non-drinkers.
- The Nature Communications study, run at APC Microbiome Ireland, was sponsored by the Institute for Scientific Information on Coffee.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- constraint Relief from caffeine withdrawal cannot explain the mood result on its own, so any account of coffee and mood now has to include what else is in the cup.
- constraint Claims that decaf helps memory or that caffeine sharpens attention rest on arms of about 15 people and are not yet a basis for choosing one cup over the other.
- contradiction The release headline promises what coffee 'really does', while the evidence beneath it supports only a correlation between gut chemistry and self-reported mood.
Pulling coffee from habitual drinkers for two weeks and then returning it blind, half of it decaffeinated, is a neat way to separate caffeine from the rest of the cup [3][4]. If the mood gains were only relief from missing caffeine, the decaf arm, still largely without it, should not have improved. Its scores fell anyway [5]. The release concludes that some of coffee's effects on mood "may not depend entirely on caffeine" [6].
Only the 31 drinkers went through that blind reintroduction [2][4], so each arm held about 15 people [1]. The findings that differ by arm rest on those small groups: better learning and memory with decaf [9], lower anxiety and better attention with caffeinated coffee [10]. I would want both replicated before treating either as a property of the drink.
Participants did not know which coffee they had, but they knew coffee was back, and mood came from psychological assessments they completed themselves [4][14]. The release does not describe a non-coffee control drink or report effect sizes. Without such a control, the return of a familiar daily habit remains a competing explanation for lower stress in both arms.
Two kinds of gut evidence sit in the study. The metabolite result belongs to the withdrawal period and is measured against non-drinkers [7]. The bacterial differences compare people with different habits [8][11]. Everyone kept diet records [14], so some of what else separates the groups can be checked. The release picks its verbs carefully: one of the species "is thought to contribute" to acid secretion, and the other "may participate" in bile acid production [12]. The Firmicutes link to positive emotion comes from earlier work in females [11].
Gut chemistry and mood scores moved in the same people over the same weeks. They are correlated, and a gut-brain route is one hypothesis that fits. Testing it would take an experiment that changes the microbes and watches whether mood follows. For what might act on the gut, the release looks past caffeine [18] to the thousands of other compounds in coffee, including polyphenols [16].
What to watch
- Whether the full Nature Communications paper gives effect sizes, and whether stress scores fell below pre-break levels or only below the withdrawal lows.
- A replication with a non-coffee control drink, to separate what is in the cup from the expectation of getting coffee back.
- Whether the decaf-only gain in learning and memory holds in arms larger than about 15 people.