Regular Coffee Drinkers May Host More Kinds Of Gut Bacteria

Swallowable Capsule Finds Regular Coffee Drinkers’ Small Intestines More Acidic

People who drink coffee regularly may carry a wider range of gut bacteria than people who rarely touch the stuff, and their small intestines measured more acidic, according to a small Danish study of 61 healthy adults. A swallowable capsule that travels through the digestive tract supplied the second finding.

Scientists at the University of Copenhagen and partner institutions compared food diaries, urine tests, stool samples, and capsule readings from the same volunteers, so coffee habits could be matched against conditions measured inside the gut. Results appeared in the journal Gut Microbiology.

Coffee ranks among the most consumed drinks worldwide, and earlier research has tied it to lower risks of conditions such as type 2 diabetes. Its knack for sending some people straight to the bathroom is well known, too. Yet how a daily cup interacts with the digestive system is poorly understood, and this project takes a close-up look.

Frequent Coffee Drinkers Averaged About 12 Ounces Daily in Food Diaries

Participants, 43 women and 18 men averaging about 39 years old, took part in a nine-day observational study in 2021. Observational means nobody was assigned to drink or skip coffee. For eight days, each person logged everything eaten and drunk in an online diary. Those who had coffee on fewer than three of the days were labeled rare drinkers (13 people), and those who had it on four or more days were labeled frequent drinkers (48 people). Rare drinkers averaged under an ounce daily, while frequent drinkers averaged about 12 ounces, close to a standard mug.

To check whether the diaries were accurate, the team tested urine for caffeine and the chemicals the body makes while breaking it down. Higher reported coffee intake matched higher levels, which supported the diaries.

Fifty participants also swallowed a SmartPill, a wireless capsule about an inch long that records acidity, temperature, and pressure from gut muscle contractions on its way through, then exits naturally. Everyone gave daily stool and urine samples, and genetic sequencing of the stool identified which bacteria were present and in what amounts.

Coffee and Gut Bacteria: More Types Detected, but Only Some Tests Back It Up

Frequent drinkers had more types of bacteria detected in their stool than rare drinkers, a measure called richness that works like a simple head count of the kinds present. A second diversity measure, which also weighs how evenly the bacteria are spread, showed no clear gap, and the full mix of bacteria was not significantly tied to coffee intake.

Three bacterial groups, Lawsonibacter, Slackia, and an unnamed member of the Eggerthellaceae family, appeared in larger amounts among frequent drinkers. Those differences did not hold up once the researchers adjusted for the many comparisons they ran, a safeguard against chance results. Other analyses fared better. Lawsonibacter levels tended to rise with coffee intake, matching earlier studies of other populations that connected it to coffee, and all three groups were more likely to be detectable in frequent drinkers.

Frequent Coffee Drinkers Showed More Acidic Small Intestines on Capsule Readings

Acidity was the capsule’s main result. Small intestine pH, the scale on which a lower number means more acid, tracked with coffee. People who drank more of it, and those with more caffeine and one of its breakdown chemicals in their urine, had lower readings. Frequent drinkers had measurably lower small intestine pH than rare drinkers. The capsule correlations were not corrected for chance results, and why coffee would shift acidity is unknown. Coffee has been tied to stomach acid release in other research, yet the capsule found no link between coffee and stomach pH.

Acidity and gut movement are thought to help decide which microbes do well in the digestive tract, so the authors float the idea that coffee affects bacteria indirectly by changing conditions in the gut. The study did not test it.

Muscle activity told a murkier story. Caffeine byproducts in urine were linked to more contractions in the colon, and caffeine itself to more contractions in the stomach, but frequent and rare drinkers did not differ significantly in either, and food moved through the colon at a similar pace in both groups.

Coffee and Gut Bacteria Link Is Exploratory, Not Proof of Cause

A handful of limits cap how much can be read into the results. The study was not designed to examine coffee, only 13 people made up the rare-drinker group, and participants were not asked to record whether coffee was caffeinated, decaf, espresso, or drip, though preparation changes what ends up in the cup. Rare drinkers were also younger on average, about 33 versus 41 years. Tea and other caffeinated drinks may have blurred the urine measurements, and the authors say the study “remains exploratory.”

More kinds of gut bacteria and a more acidic small intestine give researchers reason to keep studying regular coffee drinkers‘ insides, but this small snapshot is an early clue, not a reason to rethink anyone’s morning cup. Larger studies that record coffee type and test short-term effects could show whether coffee is the cause or just travels with other habits of people who drink a lot of it.

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