Smelling asparagus wee is a mutant superpower

Some people report that their urine smells funny after eating asparagus, and some don’t. But is it because their wee is different or their sense of smell is different? Cue: science!

Actually, you can test this at home fairly easily. If you’re able to smell asparagus wee but you know someone who can’t, simply go into the bathroom after they’ve visited it following an asparagus meal. I’ve tried it – in the name of science – and I can say that I could definitely smell their urine, even though they couldn’t (a condition called specific anosmia).

However, that’s not a terribly rigorous experiment, and the plural of anecdote is not data. But answering it properly turns out to be a rather tricky puzzle, and one that has mildly interested scientists for centuries.

A pile of green asparagus spears (click to embiggen)
If you’re an asparagus-wee-smeller, you know exactly what the result of eating these will be. If you’re not, then you’re probably wondering what all the fuss is about (Photo by Jeremy Keith from Brighton & Hove, United Kingdom, via Wikimedia Commons)
The phenomenon of asparagus wee was first documented in the 18th century, by French botanist Louis Lémery in 1702 and English physician John Arbuthnot in 1735. Arbuthnot particularly observed that it’s more common after eating tastier, young asparagus.

Not a scientist, but renowned for his appreciation of smell and taste, was Marcel Proust, who said that asparagus “as in a Shakespeare fairy story transforms my chamber-pot into a flask of perfume.”

But the cause of it has taken a while to pin down, partly because it doesn’t appear to have any medical significance, so there isn’t really a pressing need to solve it. This is unlike, say, the tendency of some people to have red urine after eating beetroot, which has been linked to things like absorbing too much iron (see Mitchell SC 2001, “Food idiosyncrasies: beetroot and asparagus”, Drug Metabolism & Disposition, vol. 29, no. 4, pp. 539-543).

The other problem is that it’s subjective. Most studies have involved simply asking people “does your urine smell weird after eating asparagus?” But how do you define “weird”?

And we still don’t even know what causes the smell. Chemical analysis of urine can alter some of the volatile compounds that might be responsible. And even if you do find something unusual, how do you know that that’s actually causing the smell?

What you really need to look at is the vapour above the urine. Fortunately, a few dedicated souls have done just that, revealing a number of possible candidates, all of them sulphur compounds (see for example Waring RH, Mitchell SC & Fenwick GR 1987, “The chemical nature of the urinary odour produced by man after asparagus ingestion”, Xenobiotica, vol. 17, no. 11 , pp. 1363-1371, doi:10.3109/00498258709047166).

The main one that people have identified – and for a long time was believed to be the primary source of the smell – is methanethiol (CH3SH). It’s also found in faeces, bad breath, farts and other decaying organic matter, and it smells like rotten cabbage. It’s sometimes added to natural gas to give it a smell, for safety reasons.

So it’s famously smelly, which is one reason to doubt that it’s the culprit. After all, everyone can smell things like faeces and farts, but asparagus smell is a little more idiosyncratic. Which means it’s probably a combination of it and the other sulphur compounds; things like dimethyl sulphide, dimethyl disulphide, bis-(methylthio)methane, dimethyl sulphoxide and dimethyl sulphone.

Because all these chemicals contain sulphur, they have to originate from a sulphur compound that’s unique to asparagus. The only possible candidate is called, surprisingly, asparagusic acid (S2(CH2)2CHCO2H). It’s deadly to insects, and found more in young asparagus, presumably to protect them from pests. So Dr Arbuthnot back in 1735 was right.

However, it’s not the asparagusic acid itself that ends up in the urine, it’s what the body metabolises it into. Which is the methanethiol and all the dimethyl et ceteras.

So we have some possible culprits, but we still can’t isolate the recipe for the smell. Which means we can’t just hand people a flask of the odour, and instead we have to go back to smelling urine.

One of the most comprehensive urine-sniffing studies was published in 2010. It used a technique called ‘two factor forced choice’, where they presented people with asparagus and non-asparagus wee, both their own and from other subjects, and they had to say which was tainted. Pelchat ML, Bykowski C, Duke FF & Reed DR 2010, “Excretion and perception of a characteristic odor in urine after asparagus ingestion: a psychophysical and genetic study”, Chemical Senses, published online 27 September 2010, doi: 10.1093/chemse/bjq081

Unexpectedly, in this study they got a spread of results, rather than a simple yes-no, can-can’t smell it. Only 3 people out of 38 did not produce smelly urine, i.e. when other subjects were asked which of the samples came after eating asparagus, results were no better than chance. And only 2 were unable to smell it at all. But there was certainly a range of ability to smell, and a range of smelliness.

Alongside this they also did some genetic testing. This concentrated on a single nucleotide polymorphism – that’s a variation in just a single base-pair in the DNA molecule – near a gene call OR2M7 (the OR is for ‘olfactory receptor’). They showed that that variation was strongly associated with the ability to smell the asparagus in urine.

But there was no association with the ability to produce it. Although there was a range of smelliness, which would have to be related to how their bodies process asparagusic acid, the cause is as yet unknown.

So it seems that the main difference between people is due to this mutation in OR2M7. If you have it, you can smell asparagus wee, and if you don’t… Well, you can probably smell it at high concentrations, but not as well as the mutants.

Which all concurs with my not-so-scientific, DIY test. Which is reassuring, but I still wouldn’t expect my study to be published in a peer-reviewed journal.

(This story first aired on 24 January 2013 – you can listen to the podcast.)

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