We've all been there: you try your best to keep it in, but you just can't hold it anymore. You have to let it slip - how bad could it be? Then the unpleasant smell wafts your way, and all you can do is hope that no one comes near you for the next couple of minutes.
Author
- Maximilienne Toetie Allaart
Postdoctoral Researcher in Gut Microbiome Research, University of Tübingen
However uncomfortable or embarrassing they are, farts are natural and a sign that your digestive system is alive. Quite literally, actually. It's not just your own body that's responsible for producing gases. Trillions of microbes live in your gut, helping you digest your food - and producing farts in the process.
Our gut microbes play an indispensable role in our health. This is why it's so important to take good care of them. And, bizarre as it might sound, the smell of your farts can actually tell you something about what's keeping your gut microbes busy.
Gassy gut microbes
Your gut microbiome is as personal as your fingerprint. There can be significant variation between people in the specific microbes present in their guts.
In general, your gut microbes work together to turn large molecules (the sugars, fats, proteins and fibres that are extracted from the foods we eat) into small molecules - mainly volatile fatty acids and gases. These fatty acids feed the cells lining the colon, while the gases naturally escape our body - sometimes quietly, sometimes explosively.
The large molecules that we consume in our food mainly consist of carbon, oxygen, hydrogen and, in smaller amounts, nitrogen and sulphur atoms . These give our gut microbes the capacity to produce different gases - such as carbon dioxide, hydrogen gas, methane and hydrogen sulphide.
While carbon dioxide, hydrogen and even methane gas are odourless, hydrogen sulphide is extremely smelly. This gas produces that rotten egg smell you might be familiar with in your own farts' fragrance profile.
The gases that are produced by our gut microbes also serve as a vehicle for other smelly molecules - such as the volatile fatty acids mentioned earlier.
Our fatty acid molecules mainly exist in the gut as dissolved compounds. But when there's gas present, a part of these molecules becomes " volatile ". This means they can be carried around by the gas, making them a bit like hitchhikers on their way out of the gut.
The three most important volatile fatty acids are acetate, propionate and butyrate. While these are all associated with good gut health , they're also rather smelly. They reek of vinegar, smelly gym socks and vomit, respectively, and I can tell you from experience with them in the lab that they're quite pungent.
There are also molecules that simply smell like, well, farts - such as indole and skatole. These molecules are produced when the amino acid tryptophan - one of the building blocks of protein - ferments in the large intestine. The molecular structure of indole and skatole not only makes them very adept at lingering, but also gives them the capacity to partition into gas. This means they can also be carried out of our guts and into the world by our non-smelly, friendly gut gases.
Food versus farts
There's a correlation between what you eat, how much gas your belly creates and how the gas smells. This is because each food affects your body and your gut microbes differently.
For instance, hydrogen sulphide - the smelliest of the gases our gut microbes make - can only be produced if your food contains sulphur. Sulphur is typically found in the amino acids cysteine and methionine, which are part of proteins. There are generally higher levels of these amino acids in animal proteins (such as eggs and red meat) than in plant proteins.
In general, proteins are more likely to produce malodorous gas because they contribute to a process called putrefaction - the fermentation of excess protein in the large intestine. This same process makes those extra-stinky indole and skatole molecules. So, too much protein fermentation can cause foul smells - and is also linked to negative health effects , such as ulcerative colitis and bowel cancer.
But don't worry, there's no need to cut out proteins altogether. Your body actually needs them. If you eat the right amount for your body , most protein will be digested in the small intestine to fuel our cells . It's only when you eat way too much protein that the excess can't be used and ends up in the colon, where smelly molecules of all kinds will be produced from it.
You might also have noticed that fibre-rich foods, such as beans, make you gassy. Fibres cause more gas production because our body lacks the capacity to break down fibre by itself. This means that all the fibre we ingest will reach our large intestine, where the microbes do the heavy lifting of breaking them down into health-promoting volatile fatty acids. Fortunately, fibre-rich foods are mainly associated with the production of hydrogen and carbon dioxide , our non-smelly intestinal gases.
Your gut is a complex jungle of interactions between the body, its microbes and your food. And just as each person's microbiome is unique, so is the scent of the gas it produces.
Although farts aren't exactly ideal, it's important to remember they're a sign that your microbes are working. Having a diverse microbiome is related to good gut health . Eating diverse foods will help you maintain a diverse set of microbes. Exercise is also a good way to ensure your digestive system can move everything - including gases - around as it should.
Maximilienne Toetie Allaart receives funding from the Alexander von Humboldt foundation.
