22/10/2026
Most people assume coffee's effects come down to caffeine, but in a new study of 62 adults, the gut bacteria shifts tied to coffee showed up with decaf too.
Boscaini, Cryan and colleagues at University College Cork reported the work in Nature Communications. They compared 31 healthy adults aged 30 to 50 who drank 3 to 5 cups a day with 31 who never drank coffee. The drinkers then stopped for 2 weeks and were randomly assigned, without knowing which, to 3 weeks of caffeinated or decaf instant coffee.
Coffee drinkers carried a different gut mix, with 7 bacterial species that differed from non-drinkers. After 2 weeks off, some drifted toward the non-drinker pattern. When coffee came back, all 7 shifted again, with or without caffeine.
The immune results followed the same pattern: when blood cells were challenged in the lab with a bacterial toxin, coffee drinkers' cells released less than half the inflammatory signal IL-6, 7,397 vs 17,634. Two weeks off coffee pushed it to 18,328. Three weeks back on brought it to 7,085 with caffeinated and 6,888 with decaf.
The authors think polyphenols are the link, because gut bacteria break down these coffee compounds and their breakdown products rose in urine and stool with both versions. The authors suggest these compounds, not caffeine, drive much of the effect.
Some results went the other way, since coffee drinkers scored higher on impulsivity and had lower stool levels of GABA and two indoles tied to gut and brain signaling.
The study was small, and the first comparison was a snapshot, not a trial. It was designed around the gut outcome and many measures were tested. It was funded by the Institute for Scientific Information on Coffee, an industry-backed group, and the trial coffee was Nescafé.
Larger studies support the gut finding. Manghi and colleagues, in Nature Microbiology, found the coffee and gut bacteria link held up across 22,867 people in the US and UK and 211 other cohorts. A BMJ umbrella review by Poole and colleagues found the biggest drop in risk of death from any cause at 3 to 4 cups a day, in observational data, with pregnancy the main exception.
If you drink coffee, the FDA cites 400 mg of caffeine a day, about 2 to 3 twelve-ounce cups, as not generally linked to harm. If caffeine costs you sleep, a sleep lab study found even a dose 6 hours before bed disrupted sleep, so make the afternoon cup decaf. If you are pregnant, keep caffeine low and talk with your doctor. If you do not drink coffee, this is not a reason to start.
Boscaini et al., Nat Commun 2026;17(1):3439
Manghi et al., Nat Microbiol 2024;9(12):3120-3134
Poole et al., BMJ 2017;359:j5024
Drake et al., J Clin Sleep Med 2013;9(11):1195-1200
