Why do Brussels Sprouts make you fart?

Every Christmas, one vegetable divides opinion - Brussels sprouts. Some of us love them, some of us hate them, but eating them can have some, er, embarrassing consequences...

Sprouts aren't the only things that cause gas to build up in our intestines - baked beans are notorious in this department too. But what actually is flatulence?

Well, some of it is caused by swallowed air - we all swallow some air when we eat and drink, but the worst offenders are gum chewers.

Some of this swallowed air comes back up again as a burp, but any that doesn't can pass through the digestive tract and emerge again at the other end in the usual tuneful fashion.

But most of the gas that ends up as flatulence is actually formed "fresh" inside our intestines by the colonies of bacteria that live there; as a normal part of their microbial metabolism, they can pump out variable volumes of nitrogen, methane, carbon dioxide and hydrogen.

These are thankfully all odourless and largely harmless gases, although hydrogen and methane are quite combustible, as some party pranksters armed with a lighter and a convenient bout of wind will attest to...

Unfortunately, some of the other gaseous products of bacterial digestion are much less easy on the olfactory system. Hydrogen sulphide reeks of rotten eggs, and methyl mercaptan, which stinks of mouldy cabbages, is the same stuff that is deployed by skunks as part of their repellent arsenal...

But why are some foods far more "fartogenic" than others, and why does the bouquet of some airborne toxic events place them on the cusp of being classified as chemical weapons?

As a rule, foods that trigger flatulence are those that can't be completely broken down in the stomach or small intestine. This means that partially-digested foodstuffs then make their way into the colon where they can fuel a large-bowel bacterial banquet, with predictable odiferous effects...

And this is where sprouts come in. Sprouts, along with onions, beans and dairy products, are hard to digest in the stomach and small intestine because our bodies can't produce the enzymes needed to break down some of the chemical components they contain.

The main culprit in sprouts is a complex sugar called raffinose, which is also found in cabbage, broccoli, cauliflower, kale, and, in fact, all members of the brassica family of vegetables.

Raffinose is broken down by an enzyme called alpha-galactosidase, but as we don't make this enzyme in our guts, the raffinose, together with other complex sugars like the inulin which is present in beans, arrive in the large intestine. Some of the bowel bacteria are armed with the necessary chemical knives and forks to break these sugars down, but in the process, they churn out hydrogen, methane and carbon dioxide.

OK, so that's why sprouts make you produce gas, but why the particularly pungent smell that you often get as an unwelcome addition to the Christmas table?

Well, one thing that all brassicas also have in common is that they are full of sulphur-containing defensive chemicals. They're there to deter animals from feeding on their leaves, and are also what cause the strong and sometimes bitter flavours of these vegetables that puts some people off eating them at all.

And it's these sulphur-containing chemicals that the bacteria turn into hydrogen sulphide and methyl mercaptan. Added in small amounts to the bulky sugar-fuelled fart gas already being produced, these gases are offenders that can clear a room in seconds.

But is there any way of solving the problem, apart from avoiding sprouts in the first place of course?

Unfortunately, some people are just more prone to producing their own airborne toxic events owing to the unique community of bacteria with which they are colonised. Some guts are just more fart-friendly, you could say.

And if this is the case for you, then perhaps Buck Weimer, of Pueblo Colorado can help? He won an IgNobel prize in 2001 for his invention of underwear that contains a removable filter to help soak up nasty niffs.

For those who don't like the sound of charcoal-stuffed pants, there are some enzyme-containing dietary supplements that can help break down the complex sugars, reducing the personal fart-risk, but fart experts agree, there's no sure fire way to prevent those Brussels sounding a bum note on boxing day...

Merry Christmas!