Why does asparagus make our urine smell?

It's a food question that has puzzled scientists for decades.

Beloved for its flavour, symbolic of the start of spring (and for fruitful fertility, thanks to its aphrodisiac qualities), and simply sublime as boiled-egg soldiers, French novelist Marcel Proust called asparagus “celestial” and “not of this world”. But for many others the potent effect it has on our pee is not heaven scent.

If asparagus were a dinner guest, it would have bad breath and body odour. This is thanks to what scientists believe is a pungent combination of ‘mercaptan’, a compound also found in rotten eggs, and the eponymously named ‘asparagusic acid’, which is unique to asparagus.

While these compounds themselves do not emit odours on cooking or eating, it is thought that when they get into our guts the enzymes in our bodies break them down, causing a noxious niff. This turns our bathrooms into something akin to a skunk den thanks to its sulphurous qualities.

A selective scent

So why can’t everyone smell it? Asparagus wee, it seems, is the, ahem, piddle in the middle of two conflicting theories about whose wee smells and whose doesn't. And while you might pity the poor boffins who have spent decades studying other people’s asparagus-infused waters in the name of science, they’ve come up with some interesting discoveries.

Some research has suggested that minute mutations in genetic make-up means not everyone produces the smell. “Some people produce it and others don’t, with studies suggesting ethnic and country differences of 50-90%”, says Dr Duane Mellor, Assistant Professor in Dietetics at Nottingham University’s School of Biosciences.

But, conversely, other studies have proposed that all urine takes on the smell – it’s just some people can’t smell it. So, in the same way no two people wee alike, no two people smell alike (and that’s thought to be true of sweat too, while we’re on the subject of bad smells).

In fact, with the average nose containing 400 smell receptors, each one encoded with a different gene, it’s no wonder a few blind spots have evolved. After all, being able to smell asparagus wee is not central to our species’ survival.

Is this a good thing?

So is there any nutritional benefit to this smelly substance? “Other sulphur containing compounds are thought to be beneficial such as sulforalphanes found in broccoli,” says Dr Mellor. “But asparagusic acid benefits, apart from probably causing smelly urine, are yet to be defined.”

If you are one of the unfortunates, there may be ways to reduce the pong. These include drinking more water with your meal to dilute the smell and choosing younger spears that are thought to be less pungent.

One study has even suggested cutting the tips off the asparagus, as this is thought to where the compounds tend to congregate. But this culinary blasphemy is surely missing the point.

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