Why coffee reduces gut bacteria and increases bowel movements: surprisingly, it is not due to caffeine

An image of coffee to illustrate its effects on gut bacteria, with or without caffeine.
© iStock/ArisSu

 Why does coffee keep bowel movements regular and suppress gut bacteria? Researchers in Texas have found that this does not seem to be due to the caffeine content.

Coffee drinkers may recognise the effect of the drink on bowel movements. A study presented at Digestive Disease Week® (DDW) 2019 showed that coffee suppressed gut bacteria and increased muscle motility in rats, but this was regardless of the caffeine content. The researchers fed it to rats and also tested a mix of coffee and gut bacteria in petri dishes.

The effects on gut bacteria and bowels

The study found that:

  • The growth of gut bacteria and other microbes in faecal matter in a petri dish was suppressed with a solution of 1.5 percent coffee;
  • The growth of microbes was even lower with a 3 percent solution;
  • There were changes to the smooth muscles in the intestine and colon; and
  • This was not limited to caffeine. A decaffeinated version had a similar effect on the gut microbiome.

Decaf can have the same effects as caffeine

Xuan-Zheng Shi, PhD, the lead author of the study and associate professor in internal medicine at the University of Texas Medical Branch, Galveston, explained: “When rats were treated with coffee for three days, the ability of the muscles in the small intestine to contract appeared to increase. Interestingly, these effects are caffeine-independent, because caffeine-free coffee had similar effects as regular coffee.”

Further research

After the rats were fed coffee for three days, the bacteria counts in their faeces decreased. However, the researchers note that further research is required to determine whether the changes favour firmicutes (‘good bacteria’) or enterobacteria (‘bad bacteria’).

The researchers added that their results support the need for clinical research on whether drinking it could be an effective treatment for the following:

  • Post-operative constipation; or
  • IIleus (when the intestines stop working after abdominal surgery.
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