Why do we fart?

Seriously, where do they come from? Eating baked beans makes them particularly potent, but why?


Reason 1: you ingested gas

While you eat or drink, you ingest some gas. Think like when you have a soft drink with all those bubbles, in the more extreme scenario. If you don’t immediately expel this gas by burping, then it sticks around and can travel the whole digestive system and come out the other end.

This is the most simple explanation, but if you think about it, most people fart far greater volumes than the volume of gas they ingest. So that means we’ve got to look a bit further!


Reason 2: bacteria produce gas

This is the main reason we fart. Your intestines are full of trillions of bacteria and they are there to break down food that wasn’t already broken down in your stomach. In doing so they produce some useful compounds for you, at little cost, given you didn’t digest that food to start with.

But as these bacteria digest the food, they release gasby breaking down the compounds. The majority of this gas release is in the forms of carbon dioxide and hydrogen- making up around 3/4 of fart gas composition. Around a third of people also have bacteria that produce methane as well.

But this combination of gases is hardly a gag-worthy odour. The bad part comes from a few different gases, making up only 50 ppm (parts per million- 0.005%) of the total fart, but gifting the awful smell.

It’s hydrogen sulphide … the charming “rotten egg” smell. It’s produced by the bacteria again and there’s not really much you can do to stop it either. There’s also methanethiol, giving a charming “rotten cabbage” smell and dimethyl sulphide (said to smell garlic-like) and a few others. The combination varies from person to person just as much as their own personal gut microbiota vary. Some people will have higher concentrations of some gases and lower of others, giving each person a unique scent.


How to reduce it

Although flatulence is a natural process, eating certain foods will increase the amount of gas that your wonderful bacteria produce for you.

  • Fatty foods: these take longer to digest and so they’re more likely to pass through to the intestines incompletely digested and ready for the bacteria to dig in. Fatty meats are also broken down to release sulphur which becomes part of the nasty-smelling sulphur gases. So eating fatty meats generally will make your gas smell worse.
  • Fibres: yes, fibre is not so well-digested, so again it’s great for bacteria. But it doesn’t make your farts smell much worse, and it is important for your overall health and promoting the growth of populations of good gut bacteria, so please don’t cut it out!
  • Sugars and sweeteners: these digest ok, but if you eat too many of them, then you won’t digest all of them. And chocolate cake becomes a bacteria feast!
  • Additionally if you’re gluten or lactose intolerant, these foods will also make you gassier because (same story again), your body doesn’t digest them properly.



For all we don’t talk about it much, flatulence (or just farts) is a very interesting topic and is an integral part of our body’s functioning and microbiota. Hope you enjoyed it!

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