Coffee Boosts Your Gut & Mood? New Research Says Not So Fast
Your daily coffee almost certainly changes the bacteria in your gut. Whether that does you any good is still unknown. Two pieces of research in the news this month land on opposite sides of that question.
On September 28, ScienceDaily ran a University College Cork study under the headline "Scientists just discovered what coffee really does to your gut and brain." Twelve days earlier, researchers at Xi'an Jiaotong University in China published a review in the journal Metabolites that examined the wider body of evidence. Their conclusion was blunter. No human study has yet shown that coffee's effect on gut bacteria is what drives its health benefits.
The "discovery" is five months old
The Cork study first appeared in Nature Communications in April. ScienceDaily ran a nearly identical headline in May. The research was sponsored by the Institute for Scientific Information on Coffee (ISIC), a nonprofit whose members are five major European coffee companies: illycaffè, JDE Peet's, Lavazza, Paulig and Tchibo. Industry funding doesn't make a study wrong, but readers deserve to know about it.
What the Cork study found
Researchers followed 62 adults. Half drank three to five cups of coffee a day, and half didn't drink coffee at all. The coffee drinkers quit for two weeks, then got coffee back without knowing whether it was regular or decaf.
After coffee returned, both groups reported lower stress, depression and impulsivity. Only the decaf group showed better learning and memory. The caffeinated group reported less anxiety and sharper attention. Coffee drinkers also carried more of two bacteria, Eggerthella and Cryptobacterium curtum.
The researchers acknowledge that a study this small can't prove coffee caused any of it. The design also leaves an open question. People who quit a daily coffee habit usually feel worse for a while, so some of the improvement after coffee came back may simply be recovery from withdrawal. The decaf group improving too complicates that explanation. It points either to compounds beyond caffeine or to the comfort of the ritual itself.
What the review found
The Metabolites review traces what happens to coffee after you drink it. Caffeine is absorbed early in the digestive tract. Other compounds, including chlorogenic acids, fiber-like sugars and dark compounds formed during roasting, travel to the colon, where gut bacteria break them down.
Coffee does seem to shift specific bacteria. Small human trials show increases in Bifidobacterium. The most consistent signal across large studies is a bacterium called Lawsonibacter asaccharolyticus, which is more common in coffee drinkers. Nobody yet knows what it does. It also isn't on the Cork study's list, which is a sign that science hasn't settled which microbes matter.
The health picture is mixed as well. Large population studies link coffee to lower type 2 diabetes risk and healthier livers, but clinical trials are inconsistent. In one trial, patients with diabetes and fatty liver disease took caffeine and chlorogenic acid supplements, and the supplements performed no better than a placebo. Much of the evidence for how coffee might help comes from cell and animal studies that used doses far higher than anyone gets from a cup.
The review has its own funding footnote. It was partly funded by a coffee research project from an industrial park administration in Yunnan, China's main coffee-growing province. The authors declare no conflicts of interest, and their conclusions are notably cautious.
Why both can be true
The studies don't contradict each other as much as the headlines suggest. Coffee drinkers have different gut bacteria than non-drinkers, and they tend to report feeling better with coffee than without it. What remains unproven is the chain connecting those facts: that coffee changes your bacteria, and that those bacteria then improve your mood, blood sugar, or liver.
What actually holds up
The strongest practical finding in either piece has nothing to do with your microbiome. It comes down to your filter.
Unfiltered coffee from a French press or a boiled brew contains oils called cafestol and kahweol, and a paper filter traps most of them. In a six-month trial, people drinking unfiltered French press coffee saw their LDL cholesterol rise compared with people drinking filtered coffee.
What goes into the cup matters too. Sugar and creamers change the drink's effects, and research tracking more than 100,000 adults found that people who increased their unsweetened coffee intake gained less weight over time. Adding sugar weakened that link.
Individual biology also plays a part. Your genes affect how fast you clear caffeine, and your existing gut bacteria shape how your body processes everything else in the cup. That helps explain why the same coffee can suit one person and not another.
The bottom line
Coffee is a complex drink, and scientists are still working out what it does inside you. If you enjoy it and it agrees with you, nothing in either study suggests you should stop. Just don't drink it for your gut. The evidence isn't there yet.
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