I’m a gastro expert – here’s how your bum can tell the difference between a poo and a fart
EVER heard of the expression ‘never trust a fart’?
Well, it turns out you can.
In most cases, your body can easily tell the difference between a fart and a poo.
It’s down to a process called the rectoanal inhibitory reflex and the huge amount of nerve endings in your bum.
Everybody farts and poops and board-certified gastroenterologist Dr Will Bulsiewicz says we shouldn’t be so embarrassed about it.
“People don’t think or talk about this enough. And they should!” the ZOE expert said.
Basic anatomy
To understand how this process works, it’s helpful to have a basic grasp of how food moves through your body.
When you eat and swallow food, it passes from your oesophagus to your stomach, small intestine and large intestine over several hours.
Your large intestine has two sections.
The first is the rectum, which is where your poo is stored. The muscles in your rectum’s walls also help you poo when you need to.
Your anal canal sits right below it and connects your rectum to your anus, which is where poo exits your body.
Your anal canal is hugely important to telling the difference between a poo and a fart.
You have two sphincters in this part of your body – these are rings of muscle that can open and close.
You activate the external one when you squeeze your bum to stop a poo from coming out. You don’t consciously activate your internal sphincter – it’s always hard at work so nothing accidentally leaks out.
How can your body tell the difference between gas and solid?
Once your rectum fills up beyond a certain capacity, its walls are stretched, leading your internal sphincter to relax, Dr Bulsiewicz said.
This is the rectoanal inhibitory reflex.
It allows a small amount of what is in your rectum to descend into your anal canal.
This is when a sampling process happens, according to Dr Bulsiewicz.
Mucosas – the moist inner lining in your anal canal – “samples” whether what is in your rectum is gas, liquid or solid, and decides whether it is safe to release it.
Scientists still don’t know exactly how the mucosa obtain this information.
But your anal canal does have an unusually large amount of sensory nerve endings, making it incredibly sensitive to vibrations, changes in tension and pressure, Dr Bulsiewicz said.
Your anal canal can also detect temperature changes.
Some scientists think that the mucosa can tell the difference between farts and poo due to temperature.
So, once your anal canal has sampled the contents of your rectum and decided whether its gas or poo, it sends this information to your brain.
“Then, you can make an informed decision whether to relax your external anal sphincter and release the beast,” Dr Bulsiewicz explained.
According to Dr Sarah Jarvis, the number of times you poo each day varies hugely between people, but it’s considered normal to do so between three times a day and three times a week.
Meanwhile, Dr Karan Rajan said you should never put off going to the loo as holding in your poo can make it dryer and harder.
How much you pass gas also varies between people and is influenced by your diet and cycle, GP Dr Sarah Garsed told The Sun.
NHS guidance states that: “Flatulence is a normal biological process and is something everyone experiences regularly.
“Some people pass wind only a few times a day, others a lot more, but the average is said to be about five to 15 times a day.”
If you’re worried about any of your symptoms and you’ve noticed a change in your poo habits you should see a GP.