A Coffee Habit Tracks With a More Acidic Small Intestine

Plenty of people will swear blind that a morning coffee gets things moving in the gut. A Danish study of 61 adults found that regular coffee drinkers’ guts did differ in a few ways, but it cannot say coffee caused them. The differences were modest, the groups lopsided, and most of the headline findings come with a hefty asterisk.

Researchers at the University of Copenhagen, writing in the peer-reviewed journal Gut Microbiology, mined data from an earlier nine-day observational project that was not originally designed to study coffee. Volunteers kept weighed food diaries for eight days. Of those, 50 also swallowed a SmartPill, a wireless capsule that radios back acidity, temperature and pressure as it travels through the digestive tract. Pressure spikes in the capsule’s readings serve as a stand-in for gut muscles contracting. The team also tested urine for caffeine byproducts and sequenced stool samples to profile gut bacteria.

Coffee habits were sorted by how often people drank it, not how much. Thirteen people had coffee on fewer than three of the eight days and counted as rare drinkers. Another 48 had it on more than three days and counted as frequent drinkers. The frequent group averaged roughly 340 milliliters a day, a big mug or so, against about 18 milliliters for the rare group. Caffeine byproducts in urine rose with reported coffee intake, which gives the diaries some backing.

A More Acidic Small Intestine

The firmest result sits in the small intestine, where food gets most of its digesting. Frequent drinkers had a statistically significant lower pH there than rare drinkers, meaning the contents were more acidic. Across all 50 capsule swallowers, more coffee also went with lower small-intestine pH. The authors cannot yet say whether that is good, bad or neutral. Acidity matters because it affects how well digestive enzymes work and how food breaks down.

Contractions were muddier. Caffeine byproducts in urine went with more contractions in the colon, a pattern the authors read as a possible nudge to gut movement. Yet colon contractions did not differ between rare and frequent drinkers, and neither coffee nor its byproducts showed a link with how long the capsule took to cross the colon. More squeezing without faster passage would fit muscles mixing the contents rather than shoving them along, but that remains a hypothesis. Older work does fit the idea that coffee can stir the colon: an older experiment in 14 healthy volunteers recorded more colon activity within minutes of coffee, decaf included, in those who said coffee sends them to the toilet.

A statistical caveat applies. The authors did not correct their capsule results for the many comparisons they ran, because the measurements overlap heavily. Run enough comparisons and some will look meaningful by chance, so these links are leads, not verdicts.

Richer Bacteria, Fuzzier Evidence

Frequent drinkers’ stool samples held a greater number of distinct bacterial types, a measure called richness, than rare drinkers’ samples. A second diversity measure tilted the same way without reaching significance, and the overall bacterial mix did not differ significantly with coffee intake. Three bacterial groups (Lawsonibacter, Slackia and a family called Eggerthellaceae) were more abundant in frequent drinkers, but none of those differences survived correction for multiple testing.

Other labs have seen echoes. A much larger analysis of more than 22,000 people tied a microbe from the Lawsonibacter genus to coffee drinking, though its own release said it is not known whether that microbe helps or harms health. A recent trial from University College Cork also found more Eggerthella, a member of the Eggerthellaceae family, in coffee drinkers.

Where the Release Ran Ahead of the Paper

The university’s press release is warmer than the paper beneath it. The release says the results confirmed that coffee affects the makeup of gut bacteria; the paper’s test of overall bacterial makeup found no significant link, and only richness and a few bacterial groups moved, none of which held up after correction. The release says higher coffee consumption went with more colon contractions; in the paper that link runs through urinary caffeine byproducts, and the rare and frequent groups did not differ. The release also floats better constipation treatments, yet this analysis did not test constipation.

Senior author Henrik Munch Roager argues in the release that coffee probably plays a part: “So there are good reasons to believe that what we are seeing is not merely coincidence.” His proposed next step is an intervention study, with researchers controlling how much coffee people drink and comparing periods with and without it.

Several limits sit under all of this. Everyday habits were observed, not assigned, so anything else that differs between coffee drinkers could drive the results; the frequent group was a few years older and had slightly higher, though still normal, fasting blood sugar. The group of 13 rare drinkers is small. The paper did not record what kind of coffee people drank, and tea or energy drinks can also supply caffeine. All of the volunteers were healthy Danish adults, mostly women (43 of 61), so the results may not carry over to other groups.

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