Question: why do brussel sprouts make you fart?

Simon Langley-Evans answered on 19 Nov 2013:

At last! A fart question!

We fart because we accumulate gas in our intestines. The gas gets in there because we swallow air when we eat and drink, because gases formed in the biochemical reactions of the body pass from the blood into the gut, but most importantly gas is produced by bacteria in our intestines.

When we eat food, some of the food is not digested. We call that material ‘fibre’. A lot of the fibre comes out of our bodies as poo, but some of it is used as a food source by the millions of bacteria that live in the large intestine. When they use that fibre they produce gas as a by-product. That gas is what we fart.

Any food that is rich in fibre or hard to digest starch will make us fart. Much of the ‘fartogenic’ (my word, it isn’t official!) food comes from plant sources. Potatoes that have been cooked and then allowed to cool are especially potent. This is because their fibre is changed to a form that is particularly friendly to bacteria. Sprouts are a fartogenic food because they are rich in a type of starch called raffinose. Our guts cannot break raffinose down and so the bacteria ferment it and produce those sprouty emissions that you asked us about.

The other important thing about a sprout fart is the smell. Some farts smell because the gases that the bacteria produce have components that have an odour. The smell is not methane (as many people think) but comes from sulphur containing gases like hydrogen sulphide and some nitrogen rich molecules like indoles and skatoles. Some foods will deliver more of these chemicals to the gut and make smellier farts. Baked beans cause lots of farts but they don’t usually smell. Sprouts cause smelly farts because they are loaded with indole glucosinolates.

Is that enough on farts? I enjoyed your fruity question.