Like opening the blinds and stepping into the shower, a cup of coffee gets people moving in the morning – in more ways than one. This satisfying brew revs energy levels with a dose of caffeine and, for many people, quickly and reliably jump-starts gut activity and an urgent need to poop.
But given coffee’s popularity, it’s surprising that we know so little about how it affects the gastrointestinal tract, says Dr Robert Martindale, a professor of surgery and medical director for hospital nutrition services at Oregon Health and Science University in the States.
Some studies on the topic – which tend to be small, old and limited – have suggested that it’s probably not the caffeine that triggers the urge to go. A paper published in 1998, for instance, found that decaffeinated coffee had a similar stimulatory effect on the colon as caffeinated coffee, whereas a cup of hot water did not.
Coffee is a complex beverage containing more than 1,000 chemical compounds, many of which have antioxidant and anti-inflammatory properties. And determining how they affect the intestines is challenging.
One thing we do know is that coffee doesn’t affect everyone the same way. In one study published in 1990 in the journal Gut, 92 young adults filled out a questionnaire about how coffee affected their bowel habits; just 29 per cent of the respondents said it “induced a desire to defecate,” and most of them – 63 per cent – were female. (But Martindale says the percentage of people who have a bowel response after drinking coffee is likely to be much higher in the general population – he estimates that about 60 per cent of his patients do – and he hasn’t noticed any differences between men and women.)
We also know that a gut response to coffee can happen fast. In the same study, some volunteers agreed to have a pressure-sensing probe inserted into their colon to measure intestinal muscle contractions before and after drinking a cup of Joe. Among those who said coffee usually stimulated a bowel movement, the probe showed a significant increase in pressure within four minutes of drinking coffee, while the so-called non-responders had no change in colon activity.
That drinking a cup of coffee can stimulate the opposite end of the gastrointestinal tract within minutes means “it’s probably going through the gut-brain axis,” Martindale says. That is, the arrival of coffee in the stomach sends a message to the brain, which then “stimulates the colon to say, ‘Well, we’d better empty out, because things are coming downstream’,” he explains. The coffee itself would move through the intestines much more slowly, probably taking at least an hour to traverse the long path from the stomach through the small intestine and on to the colon.
This communication between the stomach, brain and colon, called the gastrocolic reflex, is a normal response to eating. But coffee seems to have an outsized effect; a study published in 1998 found that 240ml of coffee stimulated colonic contractions similar 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 effect 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.
“A couple of sips of coffee can do it. It doesn’t take much,” says Martindale, who routinely offers his patients a cup of coffee the morning after surgery.
Martindale also suggests coffee, along with other dietary changes, when he counsels patients with chronic constipation. And he said it’s not uncommon for patients who have given up coffee for one reason or another to tell him, “Doc, I can’t go to the bathroom without a coffee.”
Sonya Angelone, a registered dietitian and spokesperson for the Academy of Nutrition and Dietetics, cautioned against relying too much on coffee to stay regular. If someone is constipated, “it is not because they have a deficiency of coffee,” she says. Angelone recommends eating more fruit and vegetables, which are high in fibre, as well as increasing fluid intake and physical activity to address constipation. “What I find for a lot of people is they don’t start off with a fibre punch in the morning,” she says. Brewed coffee contains a small amount of fibre, about one gram per 240ml cup.
Some people find that coffee causes an upset stomach and loose stools, as well as side effects linked to excess caffeine, such as insomnia, anxiety, heart palpitations and headache, Angelone says.
The US Food and Drug Administration says that it’s safe for most people to drink 400mg of caffeine – the amount in about four or five cups of coffee – per day. But keep in mind that people metabolise caffeine differently, so this threshold can vary from person to person. “Coffee is one of those things, unlike other foods, that if it bothers you, you know it,” Angelone says.
But for the rest of us, coffee can be part of a comforting morning routine – waking us up in a multitude of ways.
This article originally appeared in The New York Times.