As a somewhat rabid vegetarian, I often enjoy a bit of asparagus (especially with hollandaise). And as a curious person, I also enjoy its interestingly-scented side effects. You know what I’m talking about. Or, as I recently discovered, perhaps you don’t…
I’m happy to admit that asparagus makes my wee smell. Plenty of people I know well enough to mention this to have said the same thing, but why doesn’t everyone remark on this? Science has the answer, and it turns out it could all be down to a genetic mutation.
I had a look on Google’s nerdy alter-ego PubMed to find out more, and happened across a paper by some researchers from the Monell Chemical Senses Center in Philadelphia, charmingly entitled ‘Excretion and Perception of a Characteristic Odor in Urine after Asparagus Ingestion: a Psychophysical and Genetic Study‘. You can read the whole thing here.
As I found out, it’s been known for a long time that some people notice the smell while others don’t. But although you might conclude that some people have less smelly wee after eating asparagus, it could be that those people simply can’t smell that ‘characteristic odour’. Of course science has a name for ‘the inability to smell certain odours’ – it’s called ‘specific anosmia’ (and it doesn’t mean you don’t have a nose, although that would have been my guess).
The paper explains that ‘because there is no known clinical problem associated with the inability to either excrete or detect the asparagus odor, the trait has received only scattered attention’. That’s a shame, in my opinion. But thankfully the human species has a yearning to find out more about almost everything, whether it’s clinically relevant or not.
So far researchers haven’t been able to distinguish between the two theories – the so-called ‘production’ (more or less smelly wee) and ‘perception’ (more or less rubbish nose) hypotheses. For example, one recent study used a database of people who had paid for genetic testing from the company 23andMe. The researchers set up an online questionnaire to ask about various traits, including the asparagus wee effect (I’m going to call it AWE). By comparing the participants’ answers and their genetic information, the researchers found that DNA changes near a gene for a particular olfactory receptor (OR2M7) were linked to AWE.
This suggests that the differences in AWE are down to sense of smell – perception, not production. But the question was worded like this: ‘have you ever noticed a peculiar odor when you pee after eating asparagus?’. Participants’ answers can’t distinguish between the two theories – if you don’t smell the odour you can’t tell if that’s because you don’t produce it or can’t smell it.
So the aim of the researchers at the Monell Chemical Senses Center was to try to solve this problem and find out which theory – production or perception – had real evidence to it up. Of course both could be true – you could produce less of the smelly chemical AND be less sensitive or unable to detect the smell.
They enlisted 37 volunteers who agreed both to donate urine samples and to smell the urine samples of others (selfless or what?). Each person donated samples before and after eating asparagus, and before and after eating bread as a control.
For each smell test, participants were first of all given two samples and were told which smelled of asparagus (so they knew what smell to look for, and to control for different strengths of the odour). Then they were given new sets of two samples and had to distinguish between samples taken after eating bread and after eating asparagus, or between samples taken before and after eating asparagus. In total they were given 3 sets of each, giving them a score out of 6 at the end of the tests.
The researchers also tested that all participants could smell phenyl ethyl alcohol (which has a floral scent) to assess their general sense of smell, and tested that they could cope with detecting odours in urine by adding basil odour to samples and checking they could tell the difference. Finally they also genotyped all the participants to find out which had the genetic changes near the olfactory receptor gene that were found in the earlier study.
After a lot of urine-smelling, the results were in. Overall 3 people didn’t produce smelly enough urine for the others to be able to tell the difference between the samples, and 2 people couldn’t smell the asparagus odour in samples from others. There wasn’t a clear link between the inability to smell the odour and the inability to produce it, but these are pretty small numbers so we’d need more willing urine-smellers to get a definitive answer.
The genetic profiles did show that the people who could smell the odour were more likely to have particular changes in their DNA near to the gene for the olfactory receptor OR2M7, backing up the results from the previous study. But there wasn’t a link between these genetic changes and the ability to produce the odour.
So the mystery isn’t quite solved, but it seems that whether you can or can’t smell asparagus odour in your wee is probably down to the genes controlling your olfactory receptors. Or to put it more neatly, the smell is in the nose of the beholder. As usual, more research is needed and hopefully a bigger study can tell us more about this sadly neglected topic! I’ll keep an eye out for asparagus updates…