Why Does Asparagus Make Our Urine Smell? - BBC Future

After eating asparagus, some people can detect a strange smell, while others claim not to notice a thing. What’s going on?

We’ve been aware of the pungent properties of asparagus for a long time. Back in 1731 the Scottish mathematician and doctor, John Arbuthnot wrote that asparagus gives urine “a foetid smell”. Not everyone dislikes the smell, though: Marcel Proust said it had the effect of transforming his “humble chamber pot into a bower of aromatic perfume”.

A quick straw poll over dinner (assuming you feel it’s the right time to raise this kind of subject), typically reveals your guests fall in to certain camps. Some people will have never detected anything strange about their urine after they’ve eaten asparagus. Others assume that because their urine smells so strong afterwards, then so must everyone’s. Then someone else may complicate matters by revealing they’ve smelt it after their partner has been in the bathroom, but can’t detect it in their own urine.

On the basis of these observations, it seems that some people are producers of pungent asparagus-tainted urine, while some are detectors – but the two don’t necessarily go together. But data is not the plural of anecdote, so more evidence is needed.

It’s no good just asking people whether or not they can smell asparagus in their own urine. If they say yes, they’re both a producer and a detector, but if they say no, we don’t know whether they simply couldn’t detect the smell. So what we need are laboratory experiments. Sniffing other people’s urine might not be everyone’s idea of fun, but several experimenters have managed to recruit enough volunteers to do it.

In 1956, a team of British researchers demonstrated that fewer than half of people produce the odour in their urine, which was assumed to be down to the influence of a single gene. Another British study from 1987, with a larger sample of 800, found a similar proportion. Confusingly, other studies have found a much higher percentage of producers. An American study from 1985 put the number at 79%, and one in 2010 at almost 92%. This raises the possibility there may be ethnic differences in the trait.

Perhaps further clues might come from the chemicals that produce the signature smell. One prime suspect from many studies is a sulphur compound called methanethiol. In the 1956 study, though, the researchers found methanethiol present in the urine of some asparagus-eaters and not others. But the researchers were only looking for compounds in the urine itself, which doesn’t necessarily mean the smell could be detected. For that you need to examine the vapour given off by the urine. Analysis of the vapour using gas chromatography revealed four compounds. The strongest smelling are methanethiol and dimethyl sulphide which smell like old cabbage. There are also two compounds which might give the odour that tinge of sweetness.