Why can drinking coffee trigger diarrhoea?

Why coffee makes you need the toilet

Coffee is known to stimulate the digestive system, but researchers suggest there could be various different reasons for this reaction. One factor is believed to be the high caffeine content in coffee, compared to other hot drinks such as tea.

"Coffee is one of many food substances that is high in caffeine and the caffeine in the coffee can lead to the gut symptoms experienced," says Dr Bridgette Wilson, a registered dietitian from City Dietitians in London. "Other common foods that are also high in caffeine include tea, energy drinks, cola beverages and dark chocolate."

Various studies support the theory that caffeine has a laxative effect. In one study, researchers gave participants either water or caffeine powder dissolved in water and measured their anorectal function. The results suggested that ingesting caffeine led to stronger anal sphincter contractions and an increased desire to poo.

"Caffeine is a gastrointestinal stimulant which means that it speeds up peristalsis (muscle contractions that move food through the digestive tract)," says registered dietitian Caroline Bletcher. "Therefore it speeds up transit through the bowel, resulting in symptoms such as diarrhoea and stomach cramping."

However, research has suggested there may be other factors at play too. Both caffeinated and decaf coffee stimulate the production of a hormone called gastrin, which signals the stomach to release gastric acid. As gastrin promotes digestion, it may contribute to our urge to go to the toilet after drinking coffee.

Another reason why coffee impacts our bowels is the gastrocolic reflex - a physiological response in which the act of eating or drinking stimulates movement in the gastrointestinal tract. Therefore, the activity of drinking a cup of coffee with breakfast may be enough to trigger the need to 'go'. Additionally, this reflex is more reactive in the mornings, which is when many people reach for their cafetiere.

The acidic nature of coffee can also stimulate the bowels, which may explain why some people experience stomach trouble with decaffeinated coffee as well as regular coffee. Both decaf and caffeinated versions contain chlorogenic acid, which triggers higher stomach acid levels and higher production of gastric acid. In turn, this acid helps to move food through the gut.

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Can coffee affect irritable bowel syndrome?

"People with irritable bowel syndrome (IBS) may have a more sensitive gut and therefore the effect of caffeine could be more pronounced," says Wilson. "Caffeine is what is known as a stimulant, meaning that it can lead to over-excitation of the nervous system, including the nervous system attached to our gut.

"When the gut nervous system is overstimulated, it can lead to increased contractions within the large bowel, leading to cramping sensations and looser and more urgent stools. Some people with IBS already experience symptoms associated with over-stimulation of the gut nervous system. So, coffee and other sources of caffeine could make this worse."

However, Wilson adds, it is worth remembering that there are many causes of IBS and people react differently to substances like coffee. "Not everyone with IBS will have an over-excited gut nervous system, so for those people caffeine is no more likely to lead to gut issues than in people without IBS," she says.

What can you do to negate the negative effects of coffee?

Of course, getting things moving along isn't always a bad thing. It is generally advised that people limit their caffeine intake to 400 mg a day, which is between three to five cups. Pregnant women should consume no more than 200 mg of caffeine a day. However, some people may be more sensitive than others to the effects of a flat white.

Cut down your coffee intake

Those who struggle with stomach problems after drinking coffee may find it useful to cut down their caffeine intake. Decaf or half-caffeinated coffee may help and there is some research to suggest that the type of coffee consumed could have an impact too. In 2014, a study suggested that dark roast coffee produced less stomach acids than a medium roast, due to its different balance of chemicals.

Don't drink coffee on an empty stomach

Although many people reach for the coffee machine before breakfast, drinking a fresh brew on an empty stomach may exacerbate negative effects like diarrhoea and stomach cramps. Having a coffee with or after food may help.

Be wary of extras

Sometimes, how coffee is taken can have an effect too. Some may find that adding cow's milk, cream, syrups, sugar or sweeteners can trigger stomach discomfort.

However, coffee sensitivity is personal and it may be a case of trial and error to find out what works best.