Smelly Farts Are a Dank Bouquet of Sulfurous Bacterial Waste

The olfactory implications of breaking wind are much more complex than you might think.

The new film Swiss Army Man — better known as Daniel Radcliffe’s farting corpse movie — has thrust flatulence back into the cultural spotlight. In the film, Radcliffe’s dead butthole literally gets corked when its malodorous offerings become too heady to bear, inviting us to reconsider the olfactory minutiae of breaking wind. To suggest that an extra-odorific flatus is simply a direct consequence of the foods we eat or can’t digest would be overly facile, like saying a gorgeously ripe-smelling Stilton is merely moldy cream. The complex bouquet offered up by a seething fart is the hot consequence of a series of physiological machinations designed to balance ingestion and digestion.

But first, take a second to appreciate that farts are, for the most part, made up of benign, odor-free air. In fact, a team of researchers publishing in the Journal of Chromatography B went so far as to scientifically determine that 99 percent of the average fart is a cocktail of innocent-smelling gases, like oxygen, carbon dioxide, nitrogen methane, and hydrogen.

(The latter two, incidentally, are flammable, providing fuel for cigarette lighters in the general vicinity of your butt.) In the world of flatulence, just as in real life, the 1 percent is an incredibly influential minority with the power to overwhelm the masses.

When your gut bacteria finish off the digesting that your stomach can't do, the consequences can be rank.

The stench-wealthy elite of the fartverse are a closely related bunch of potent chemicals known as the Volatile Sulfur Compounds. This squad, bound together by the central atom they all share, are responsible for some of the worst smells known to humankind: Poop, bad breath, certain shitty-smelling lilies, and, yes, farts.

Among the members of the VSC are compounds like the classic rotten egg-scented hydrogen sulfide and its cousin methanethiol, which gives asparagus pee its rank odor. Then you have a trio of dimethyl sulfides — namely, dimethyl sulfide, dimethyl disulfide, and dimethyl sulfide, which are respectively responsible for the intoxicating fetor associated with the sea, garlic, and Limburger cheese.

The VSCs don’t just show up uninvited. Naturally, sulfur-containing foods make for more sulfurous gas. Pretty much any food that’s been condemned as a fart inducer — beans, cabbage, eggs, broccoli and its relatives, onions, and red meat — are not only harder to digest but also contain a higher-than-average amount of sulfur in its protein chains, which are released during digestion.

But when it comes to smelly farts, perhaps even more important than what you eat is what eats what you eat. Pause for a moment to take this in: What is eating, anyway? Getting nutrition from food is the process of breaking down, say, a bite of bean-dense chili, into the individual strings of proteins and carbohydrates that make up its ingredients. Chewing and blasting our half-digested food with stomach acid is just the first part of the eating process. Anything left over — some foods are harder to digest than others — winds up in the lower depths of your bowels, where hungry commensal bacteria, which specialize in breaking down specific types of proteins and carbohydrates, pick up where our guts left off.

The bacteria that specialize in breaking down lactose, a sugar found in dairy, are also thought to burp up particularly sulfurous gases.

Bacteria, like us, belch when they eat. And some of the species that colonize our guts cough up more noxious VSC-rich gases than others. These gases, joining forces with the air naturally ingested when you eat and talk, pool at your sphincter’s doorway, where they’re eventually squeezed out — sometimes stealthily, sometimes like triumphant brass fanfare through your back end. Whether silent farts are actually smellier is still up for debate; the sound of a fart has little to do with its volume or contents and everything to do with the positioning of your ass cheeks when your butt decides to blow. Still, it’s possible that smaller — and thereby more discreet — farts contain a higher proportion of VSCs in its gaseous mix.

All this is to say that: If your farts smell particularly rank, you probably know exactly why. Our bodies may each contain different levels of VSC-belching bacteria, but, if we’re honest, we each also know what sets them off. To know why your fart truly smells is to know yourself.

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