Drinking coffee keeps the bowels moving because it changes gut bacteria and improves ability of intestines to contract, find researchers.

Researchers, feeding rats coffee and also mixing it with gut bacteria in petri dishes, found that coffee suppressed bacteria and increased muscle motility, regardless of caffeine content.

"When rats were treated with coffee for three days, the ability of the muscles in the small intestine to contract appeared to increase," said Xuan-Zheng Shi, associate professor in internal medicine at the University of Texas Medical Branch, Galveston.

Interestingly, these effects are caffeine-independent, because caffeine-free coffee had similar effects as regular coffee, Shi informed during the Digestive Disease Week (DDW) 2019 here on Sunday.

Coffee has long been known to increase bowel movement, but researchers have not pinpointed the specific reason or mechanism.

The study found that growth of bacteria and other microbes in fecal matter in a petri dish was suppressed with a solution of 1.5 per cent coffee, and growth of microbes was even lower with a 3 per cent solution of coffee.

Decaffeinated coffee had a similar effect on the microbiome.

Muscles in the lower intestines and colons of the rats showed increased ability to contract after a period of coffee ingestion, and coffee stimulated contractions of the small intestine and colon when muscle tissues were exposed to coffee directly in the lab.

The results support the need for additional clinical research to determine whether coffee drinking might be an effective treatment for post-operative constipation, or ileus, in which the intestines quit working after abdominal surgery, the authors noted.