As with opening the blinds and entering the shower, a cup of coffee in the morning gets people moving – in more ways than one. This energising brew boosts energy levels with caffeine and, for many people, jump-starts gut activity and an urgent need to poo.
However, given coffee’s widespread use, it’s surprising that we know so little about how it affects the gastrointestinal tract, according to Dr Robert Martindale, a surgery professor and medical director for hospital nutrition services at Oregon Health and Science University.
Several studies on the subject – most of which are small, old, and limited in scope – have suggested that it is unlikely that caffeine triggers the urge to urinate. For example, a 1998 study discovered that decaffeinated coffee stimulated the colon similarly to caffeinated coffee, whereas a cup of hot water did not.
The gastrocolic reflex, or communication between the stomach, brain, and colon, is a normal response to eating. However, coffee appears to have a disproportionate effect.
Coffee is a complex beverage that contains over 1,000 chemical compounds, many of which are antioxidant and anti-inflammatory. And determining their effect on the intestines is difficult.
We do know, however, that coffee does not affect everyone in the same way. In one 1990 study published in the journal Gut, 92 young adults completed a questionnaire about how coffee affected their bowel habits; only 29% of respondents indicated that it “induced a desire to defecate,” and the majority of respondents – 63% – were female. (However, Martindale asserts that the proportion of people who experience a bowel response after drinking coffee is likely much higher in the general population – he estimates that about 60% of his patients do – and he has observed no difference between men and women.)
Additionally, we know that a gut reaction to coffee can occur quickly. In the same study, some volunteers agreed to have a pressure-sensing probe inserted into their colon to measure intestinal muscle contractions prior to and following a cup of coffee. Within four minutes of drinking coffee, those who said it usually stimulated bowel movements had a significant increase in pressure, whereas the so-called nonresponders had no change in colon activity.
The fact that a cup of coffee can stimulate the opposite end of the gastrointestinal tract within minutes indicates that “it is most likely going through the gut-brain axis,” Martindale says. That is, when coffee enters the stomach, it sends a signal to the brain, which then “stimulates the colon to say, ‘We’d better empty out, because something is coming downstream,'” he explains. The coffee itself would move much more slowly through the intestines, likely taking at least an hour to complete the lengthy journey from the stomach to the small intestine and colon.
The gastrocolic reflex, a communication between the stomach, brain, and colon, is a normal response to eating. However, coffee appears to have a disproportionate effect; a 1998 study discovered that 235ml of coffee stimulated colonic contractions comparable to those induced by a 1,000-calorie meal. Researchers have hypothesised that coffee’s gut-brain messaging is probably caused by one or more of coffee’s many chemicals, and perhaps mediated by some of our own hormones that play important roles in the digestive process, such as gastrin or cholecystokinin – both of which can spike after coffee drinking.
Although the mechanism remains murky, coffee’s effects on the gut may be helpful for some people, including those recovering from certain types of surgery. Impaired bowel function is common after abdominal surgeries, for instance, which can lead to bloating, pain and an inability to pass gas or tolerate food. A 2020 analysis combined the results of seven clinical trials and found that drinking coffee allowed patients who had undergone colorectal or gynaecological surgery to tolerate solid foods an average of 10 and 31 hours sooner, respectively. Coffee also reduced the time to their first bowel movement, by an average of 15 to 18 hours.