A fart has to be one of the most mortifying bodily functions.
While they are completely natural and a sign your digestive system is working, every so often there is an unexpected stinker.
An expert has lifted the lid on why some farts smell worse than others – and how the stench could provide clues about what’s going on in your gut.
It all starts with the large molecules present in the food we eat, which mostly consist of carbon, oxygen, hydrogen and, in smaller amounts, nitrogen and sulphur atoms.
Once they reach our gut, the microbiome – the ecosystem of bacteria that live in our intestines – starts to break them into smaller molecules.
The gases produced during this process naturally escape our body – sometimes in the form of a loud fart.
While most of this gas is odourless, hydrogen sulphide produces the rotten smell we’re all familiar with.
And if your farts are particularly pungent, your love of eggs or red meat could be to blame, according to Dr Maximilienne Toetie Allaart, from the University of Tübingen.
There’s nothing more mortifying than accidentally letting out a smelly fart. Now, experts have revealed what food tends to make them smell worse (stock image)
Beans are often the scapegoat for bad gas but there are also other, more surprising offenders such as high-protein foods. Artificial sweeteners have also been linked to smelly farts
Writing for The Conversation, Dr Allaart said: ‘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.’
She explained that there’s no need to cut protein out of the diet altogether, as the body needs it.
However, moderation is key as it’s only when you eat too much that it ends up in the colon – where smelly molecules will be produced from it.
The microbiome’s activity also produces volatile fatty acids, she explained. These feed the cells lining the colon but can also hitch a ride out of the body with the released gas.
In general, proteins are more likely to produce smelly farts because they contribute to a process called putrefaction – the fermentation of excess protein in the large intestine
The three most significant volatile fatty acids are acetate, propionate and butyrate – all of which are linked to good gut health but also reek of vinegar, smelly gym socks and vomit.
These are mostly produced when the gut bacteria breaks down fibrous food such as whole grains, fruits, vegetables and legumes.
‘Your gut is a complex jungle of interactions between the body, its microbes and your food,’ Dr Allaart added.
‘And just as each person’s microbiome is unique, so is the scent of the gas it produces.’
She added that farts are a sign that your gut microbes ‘are working’, and that exercise is a good way to make sure the digestive system can move everything around as it should.
Flatulence is a natural part of life. The average person passes gas 13 to 21 times a day whether consciously or unconsciously.
That gas can travel up to ten feet per second – nearly seven miles per hour.
What other food can cause smelly farts?
Beef and pork
Frequent steaks and pork chops could be behind offensive gas.
Beef contains the amino acid methionine, which contains sulfur. When the body breaks this amino acid down, gut bacteria turn it into hydrogen sulfide, giving rise to foul flatulence.
Fatty pieces of beef and pork can slow down digestion due to the complex molecules that take longer for the body to break down.
Garlic and onions
Garlic is full of compounds called fructans, fermentable carbohydrates that are also found in onions, leeks, scallions and wheat.
The human digestive system cannot metabolise fructans.
As a result, they travel to the large intestine from the small intestine largely undigested.
In addition to containing sulfur compounds, garlic and onions contain fructans, which travel past the large intestine to the small intestine mostly undigested. Bacteria ferments those foods, producing smelly gas
Once there, bacteria that makes up the gut microbiome ferments the food, resulting in foul-smelling gas.
Garlic and onions, which both belong to the allium family of plants, also contain sulfur compounds which help give them their signature pungent aroma.
Poultry
Like other foods on this list, chicken contains a lot of sulfur, mainly due to the amino acids methionine and cysteine.
When we eat chicken, the digestive system breaks down proteins into amino acids, which are absorbed into the bloodstream.
As these amino acids are metabolised in the body, sulfur compounds are the byproducts.
Fried chicken is high in fat due to the amount of oil used to prepare it. High fat foods digest slowly, leading to fermentation in the gut.
Certain cuts of poultry, such as chicken thighs, contain more fat than other cuts, which could also contribute to gassiness.
Chicken, especially fried chicken, contains sulfur which translates to a rotten-smelling gas
Artificial sweeteners
The body can struggle to digest artificial sweeteners like stevia and aspartame, the sweetener in sodas.
Dr Patricia Raymond, a Virginia-based gastroenterologist, said: ‘High-fructose corn syrup can also cause excessive gas and diarrhoea.’
Many sweeteners are not fully digested by the body and can pass through the small intestine to the large intestine, where the fermentation process begins.
The result is gasses like hydrogen, methane, and hydrogen sulfide.
The carbonation and sweeteners in popular sodas such as Diet Coke can also lead to bloating and changes in the gut microbiome.
People with irritable bowel syndrome should be especially cautious about the amount of sodas and other sweets they consume.
