What does drinking coffee every day do to your gut bacteria?

Daily coffee drinking demonstrably changes the composition of your gut bacteria. Moderate consumption, fewer than 4 cups per day, is associated with higher levels of bacteria considered beneficial, such as Bifidobacterium and broader groups like Firmicutes and Actinobacteria. At the same time, lower levels of Bacteroidetes and Enterobacteria have been found in coffee drinkers, which is generally regarded as a positive sign.

The diversity of bacterial species in the gut also appears to be greater in people who drink coffee. Greater diversity is generally seen as a hallmark of a healthier gut environment. Not all studies agree on this point entirely: there are also studies that find no effect or a different effect, so the finding is consistent but not undisputed.

Coffee also speeds up the motility of the large intestine, which may contribute to more regular bowel movements. Newer research suggests that this effect is partly linked to coffee's influence on the gut bacteria themselves, although that relationship has not yet been fully worked out.

Laboratory and animal studies have also found indications that coffee protects the gut lining against oxidative damage and inflammation. That sounds promising, but this type of research does not always translate one-to-one into an effect in humans.

People who drink more than 5 cups per day may have a higher risk of acid reflux, gum inflammation, and, in people with Crohn's disease, a worsening of symptoms. These are associations from observational research, but they are a reason not to drink too much. All of the research described is observational: it shows associations, but does not prove that coffee directly and causally changes gut bacteria.