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What causes diarrhoea to travel so quickly through your body?

Dr Adam Simon photo

Created: 23 June, 2016

Updated: 18 June, 2019

Why does digestion normally take so long, but when you have diarrhoea, the process only takes a matter of minutes? We get to the bottom of this fascinating topic.


It can take more than two days between eating something and it being expelled from your body, but when you have diarrhoea, this process takes no time at all.

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In this guide, we’ll explore the fascinating mechanism that makes this possible and look into one of the most ingenious ways the body has developed to defend itself from external nasties.

Digestion 101

Eating: Your digestive system gears up before you even take a bite – with the sight and even smell of food being enough to kick start the production of saliva.

Once you start to chow down, things start to really gear up, with more saliva being produced by glands in your mouth and throat. Saliva contains a mixture of substances that help to break down food and lubricate it.

In combination with the act of chewing, this turns the food in your mouth into blobs (known as boluses) that are easy to swallow.

From there, the food travels through your oesophagus into the stomach. Here, it’s bathed in digestive enzymes and stomach acids and churned into smaller and smaller bits.

The small intestine: Once the food’s been processed into a paste-like substance known as ‘chyme’ – it’s cycled out of the stomach and into the small intestine.

Here the pancreas, liver and gallbladder leap into action – continuing the digestive process by adding a variety of substances to the mix.

The small intestine is a powerhouse, breaking down the food into tiny molecules of nutrients that can then be absorbed a little further down the line.

Once it’s had its fill of nutrients – all that remains is a mixture of water and electrolytes, plus the bits that can’t be digested, like fibre and dead cells.

The large intestine: The remains of your meal then make their way through the large intestine, which absorbs the moisture content – leaving you with the stool substance you’ll be familiar with.

Onward journey: When it’s finished its travels through your digestive system, what remains of your food heads to the rectum to await the final part of its journey.

The rectum is filled with nerves that react to pressure and once the amount of stool in the colon reaches critical mass, it’ll alert your brain that it’s time to defecate (poo).

When you’re ready to go, you take conscious control of the sphincters that control the stool’s passage, relaxing them and allowing the stool to pass through.


What Changes When You've Got Diarrhoea?

If we imagine your digestive system as a conveyor belt, when the overseer (your body) notices a quality control issue, they activate the emergency fast track – overriding the usual controls and shunting everything currently making its way through the system right to the end of the production line.

Once the emergency button is hit, your body begins to flush everything out of your digestive system – pushing the food you’ve recently eaten, as well as anything else currently making its way through, out as quickly as possible.

The usual mechanism for re-absorbing water is bypassed, which is what causes the watery stools typical of diarrhoea.

How Does Your Body Know When to Engage the Diarrhoea Mechanism?

Your body has a sophisticated suite of sensors (nerves), whose role is to detect when something isn’t as it should be in your digestive system.

So if you’ve eaten something that causes inflammation (e.g. spicy food), have a bacterial infection, or contract a parasite – these trigger abnormal reactions, which alert the nerves to the fact something’s wrong.

In short, that’s why when you’ve eaten something that disagrees with you it takes no time at all for your body to get rid of it.


The Dangers of Diarrhoea

While most people get the occasional attack of diarrhoea after eating something that upsets their stomach, in more serious cases – it can be caused by viruses, parasites and various types of bacteria.

Chronic conditions like irritable bowel syndrome (IBS) can also lead to regular bouts of diarrhoea and even psychological problems like anxiety can prompt an attack.

If you’re worried about diarrhoea, or think you may have a chronic condition that’s causing regular attacks, see a doctor as soon as you can.

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Usually, diarrhoea will clear up without treatment after a few days – but one of the biggest dangers is posed by loss of fluids, which can lead to dehydration, especially in young children.

For those particularly vulnerable to dehydration – an oral rehydration solution, which will help replace lost fluids and electrolytes, is often recommended. There’s also a range of medications that can reduce the symptoms – but these are only needed in more extreme cases.