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Let it rip. Eating slow-release carbs and cutting down on protein may prevent rotten-egg farts according to a study of the gases emitted by human faeces samples.

Farts are mostly composed of odourless gases. There is oxygen and nitrogen from swallowed air, while hydrogen, methane and carbon dioxide are produced when bacteria in the large intestine ferment the carbohydrates we eat.

The distinctive rotten-egg whiff is caused by traces of hydrogen sulphide, which gut bacteria are known to produce from protein. In addition to causing red-face moments, this gas can exacerbate inflammatory bowel disease and increase the risk of bowel cancer. This prompted Chu Yao at Monash University in Melbourne, Australia, and her colleagues to investigate how different foods affect the amount of hydrogen sulphide is produced in the gut.


Protein vs carbs

Examining the faeces from seven healthy people, the team found that mixing it with cysteine – a major sulphur-containing component of meat, eggs, dairy and other types of protein – caused hydrogen sulphide emissions from gut bacteria to increase more than seven-fold.

“This explains why bodybuilders who consume lots of protein powder are known to have smelly farts,” says Yao.

But hydrogen sulphide production declined substantially when the team mixed the faeces with four slowly absorbed carbohydrates. These pass through the small intestine without being fully digested, and are then fermented by bacteria in the large intestine.

Two of these carbs – resistant starch, which is found in potatoes, bananas, legumes and cereals, and fructans, which are found in wheat, artichokes and asparagus – both reduced hydrogen sulphide production by about 75 per cent. These foods are highly fermentable, meaning they are preferentially broken down ahead of protein, Yao says. “The focus is taken away from the protein, so hydrogen sulphide is not produced.”

Bad farter? Try eating more of these Potatoes

Bananas

Legumes

Cereals

Wheat

Artichokes

Asparagus

The other two carbs that decreased hydrogen sulphide emissions were psyllium and sterculia , but only by 25 per cent. These two forms of dietary fibre are poorly fermentable, but can mop up rotten-egg gas as they soak up water and swell in size.

Backfiring advice

The team’s findings, presented at the annual scientific meeting of the Gastroenterological Society of Australia in Adelaide on Tuesday, overturn conventional wisdom that people with stinky gas should eat less fibre.

People with excessive flatulence are often advised to avoid fibre to reduce how much hydrogen, methane and carbon dioxide are produced by bacterial fermentation. However, this may increase hydrogen sulphide emissions, which are more likely to clear the room than the three odourless gases, Yao says.

The team’s findings are consistent with research showing that high fibre intake increases people’s fart production but not the subjective smell. Rosemary Stanton at the University of New South Wales, Australia, who conducted this study, says there is no good reason for self-conscious farters to avoid fibre.

“When we dug down, it wasn’t the act of farting that worried people, it was the smell,” she says. “But the concerning thing is that there are all these people walking around constipated because they are too scared to eat fibre in case they do a bad fart.”

Yao’s team now plans to test whether getting people to eat only small amounts of protein and lots of slow-release carbs can prevent rotten-egg farts in both healthy volunteers and people with inflammatory bowel disease.

Read more: Farting 101: The questions you’re too embarrassed to ask