Men fart more when eating a plant-based diet due to good gut bacteria

Plant-based diets cause men to fart more and have larger stools, researchers have found – but that seems to be a good thing, because it means these foods are promoting healthy gut bacteria.

Anecdotally, it is well-known that eating more plants – including fruit, vegetables, grains and legumes – creates bulkier stools and increases flatulence. However, few studies have measured these changes or related them to changes in gut bacteria.

Claudia Barber at the Liver and Digestive Diseases Networking Biomedical Research Centre in Barcelona, Spain, and her colleagues compared the effects of a Mediterranean-style diet mostly comprised of plants with a Western-style diet containing fewer fruit and vegetables on the guts of 18 healthy men aged between 18 and 38. Each participant was randomly assigned to follow one of the diets for two weeks, then after a break, they switched to the other diet for two weeks.

The men did a similar number of poos per day on the two diets, but each one was about double the size while they were on the plant diet. The men collected and weighed their own stools using digital scales and found they produced about 200 grams per day on the plant diet, compared with 100 grams on the Western diet.

This is because eating plants promotes certain types of bacteria in our guts that make food for themselves by fermenting plant fibre, says Rosemary Stanton at the University of New South Wales in Sydney, Australia. The added stool weight is made up of the spent bodies of these extra bacteria plus water and a small amount of undigested plant fibre, she says.

Some of the specific fibre-fermenting bacteria that became more abundant in the men’s guts while they were on the plant diet included Agathobaculumand anaerostipes and Agathobaculum butyriciproducens, an analysis of their waste showed.

The participants logged how many times they farted per day using a handheld counter and found they farted seven times more per day on average while on the plant diet than when they were on the Western diet. Each fart contained about 50 per cent more gas, as revealed when the researchers gave the men a test meal of stewed beans and measured subsequent gas production using balloons fitted to the men’s rectums.

Eating plants promotes farting because most fart gas is odourless hydrogen, methane and carbon dioxide that is produced by gut bacteria when they ferment plant fibre, says Stanton. The smell of farts comes from traces of hydrogen sulphide gas, which is a by-product of protein digestion.

Fibre-fermenting bacteria are known as “good” bacteria because they release short-chain fatty acids. These chemicals keep the large intestine healthy and protect against bowel cancer. Short-chain fatty acids can also be absorbed into the bloodstream where they protect against heart disease and diabetes by lowering cholesterol and regulating blood sugar.

The findings suggest that flatulence associated with eating more plants should be welcomed, says Stanton. “Our Western idea that farting is a sign of something being wrong is totally false,” she says. In most cases, “farting is a sign of a healthy diet and a healthy colon”, she says.

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