Life Coffee to go: Why a strong latte is such a moving experience
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Coffee to go: Why a strong latte is such a moving experience

Scientists are flush with success in explaining why coffee makes people go to the toilet. Photo: Getty
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US scientists are flush with success, because they’re one step closer to solving one of modern life’s most pressing mysteries: why drinking coffee causes one in three people to run to the toilet.

The common assumption has been that caffeine played a role. After all, it gives life in general a little pep, so why not on the inside?

In 1998, researchers placed six-sensor solid-state probes in the colons of 12 volunteers, who were given various combinations of coffee (caffeinated and decaffeinated) and a meal, or water in a meal.

The scientists found caffeine stimulated colonic activity on par with a meal. But decaffeinated coffee was found to be almost as equally powerful at calling for a sit-down meeting with the porcelain.

Caffeine doesn’t explain it

So something else in coffee, apart from caffeine, is behind the sometimes inconvenient rush.

An earlier study, from 1990, published in the journal Gut, found coffee induces a “gastro-colonic response” in some people within four or five minutes of drinking their favourite brew.

The scientists theorised that coffee somehow affects the epithelial tissue lining the stomach and the small intestine – a theory that has held firm for nearly 30 years.

The Gut study also found coffee promotes the release of gastrin, a hormone produced within the stomach and known to increase motor activity in the colon.

Stomach acid and hormones are boosted

Subsequent studies found a compound found in coffee called chlorogenic acid triggers higher stomach acid levels and also higher production of gastric acid.

One of these studies found “a dark brown roast coffee blend is less effective at stimulating gastric acid secretion in healthy volunteers compared to a medium roast market blend”.

The bottom line of the gastric acid theory is that it might stimulate the stomach to evacuate more quickly than usual.

And now, the latest: University of Texas researchers, in experiments with rats, found coffee suppressed gut 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 Dr Xuan-Zheng Shi, lead author of the study and 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.”

Researchers examined changes to bacteria when faecal matter was exposed to coffee in a petri dish, and by studying the composition of faeces after rats ingested differing concentrations of coffee over three days.

The study also documented changes to smooth muscles in the intestine and colon, and the response of those muscles when exposed directly to coffee.

Strange link with gut bacteria shows promise

The study found growth of bacteria and other microbes in faecal 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.

After the rats were fed coffee for three days, the overall bacteria counts in their faeces were decreased.

The link with gut bacteria is new, but until the link can be explained, it sort of muddies the waters rather than solves the mystery.

Dr Shi said results supported 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.

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