Why does coffee make you poop?

• Your coffee-drinking habit is likely to send you to the loo, and it's not just the caffeine that causes it.
• Studies have looked at the potential reasons for coffee promoting bowel movements, although latest research is lacking. 
• Not everyone is affected by coffee's laxative effects - in fact some people aren't affected by it at all.

Coffee is the fuel that keeps the world going. Apart from it fuelling your morning, you have probably realised the taste of a fresh brew also helps to get your bowel movement going.

The phenomenon is so well-known that it's become plastered on mugs.

A past study found that nearly 30% of participants needed to use the bathroom within 20 minutes of having a cup of coffee, but that drinking plain hot water didn't have the same effect. 

READ MORE | 10 ways to make yourself poop first thing in the morning

Another study found that caffeinated coffee stimulated the colon more than decaffeinated coffee and 60% more than plain water.

According to a 2020 analysis, coffee consumption in patients who had undergone colorectal surgery reduced their time to their first bowel movement by an average of 15 to 18 hours.

There are millions of people who love - and need - to get their caffeine fix every day. And as one of the top consumed beverages, after water, we have to wonder: what is in the more than 1 000 chemical compounds in coffee that gets us moving?

Despite the drink being widely popular, research on why it gets our bowels moving is fairly limited. But researchers have a few theories.

Not just caffeine

It's tempting to think that your bowel-stimulating effects are caused by the caffeine in your cuppa joe. But this isn't the case, as even Coke and energy drinks, which also contain caffeine, don't cause a laxative effect. Research also shows that decaffeinated coffee can send you to the loo, although it's not as potent as caffeinated coffee.

While caffeine certainly does play a role, a 1999 review concluded that it "cannot solely account for these gastrointestinal effects". So there are clearly other factors and compounds in coffee that's provoking our bowel movement.

Stimulating colon contractions

Our colon experiences contractions that work to mix, knead and push out poop, and limited research suggests that drinking coffee might stimulate our colonic motor activity within minutes of consuming it, Dr Kyle Staller, director of the Gastrointestinal Motility Laboratory at Massachusetts General Hospital, told CNN. 

This was proven in an old study, where a group of participants experienced this activity after drinking coffee. Researchers could observe this effect as the participants agreed to have a sensor probe inserted into their colons. 

READ MORE | 10 things you didn’t know about poop

Staller explained that coffee doesn't directly contact your colon's lining. Rather, it "triggers either a nervous system or hormonal response that causes the colon to start contracting".

"Colon contractions move stool in the colon toward the rectum and voila -- you have the urge to have a bowel movement," he said.

Interaction with our stomach acid?

Studies have also found that drinking coffee can stimulate the production of stomach acid, which helps break down food and move it swiftly through our gut.

Specifically, it stimulates the production of a hormone, called gastrin, which makes the colon more active.

As McGill University explains: "… we can say that gastrin promotes digestion. Therefore, if coffee stimulates gastrin and gastrin stimulates digestion, this pathway may be a mechanism by which coffee makes us defecate."

A past study found that drinking coffee raised gastrin levels by 2.3 compared to drinking water. 

Other compounds in coffee

There's also melanoidins, which are compounds formed during coffee roasting, that are known to have dietary fibre. Since this aids digestion, it could be another explanation for coffee's effect on our bowels, Jerlyn Jones, a registered dietitian nutritionist, told CNN.

As the researchers of one study write, "in the gastrointestinal tract, melanoidins [also found in bread crust and cocoa] behave not only as antioxidants, but also as dietary fibre".

Lactose intolerance and pre-existing health conditions

Someone who is lactose intolerant (unable to digest the sugar in milk) may go on to experience symptoms, such as bloating or diarrhoea, after consuming dairy. Around 65% of the world's population cannot fully digest lactose after infancy, according to Medical News Today.

So, in people with this condition, drinking a cup of coffee with milk can easily trigger the need to use the loo.

Similarly, people with irritable bowel syndrome (IBS) have reported that consuming certain foods and drinks may worsen their symptoms, such as experiencing diarrhoea after drinking coffee.

Could it just be a coincidence?

Since your bowels are twice as active when you wake up, compared to while you're asleep, this means that coffee-induced bowel movements could just be a coincidence, Healthline suggests.

While the research on coffee's effect on our colon remains murky and it appears to involve a combination of factors, one thing is clear: you shouldn't rely on coffee to stay regular. 

READ MORE | Drinking coffee before you shop could make you spend more, researchers say

Sonya Angelone, a registered dietitian, told the New York Times that, if someone is constipated, "it is not because they have a deficiency of coffee".

For constipation, Angelone recommended eating foods that are high in fibre, such as fruits and vegetables, as well as keeping hydrated and getting moving. 

"What I find for a lot of people is they don't start off with a fibre punch in the morning," she said.