What Am I? 17 was urine from someone who had recently eaten Asparagus. As its currently British Asparagus season (but the supermarket is still treacherously stocking Peruvian asparagus alongside its more local counterpart), I thought it might be appropriate to repost this blog post from April 27th 2009, originally on Endless Possibilities v2.0 over on Nature Network. I never did copy the archive over when I moved…never mind!).
Stinker or Smeller? 27th April 2009
One of the advantages of resolving to ‘buy British’ (or vaguely European) is rediscovering the latest trend in culinary adventure: seasonality! With the advent of the asparagus season comes that intriguing question – why does asparagus make some people’s pee smell?
It turns out that the answer isn’t as simple as I first anticipated. I thought it would be a simple exercise in looking up the chemical responsible, reading a little around metabolic pathways and degradation products and suggesting which volatile and vile molecule is responsible. Not everyone can smell asparagus pee however, and this initially lead to the assumption that not everyone breaks down asparagus in a manner that produces the smell. Adopting the naming convention of one paper1, those who produce asparagus pee are stinkers, and those who can smell it are smellers.
There have been a number of studies that investigate the origin of asparagus pee and range from feeding unsuspecting students various compounds that might be responsible for the smell, to making people eat asparagus every month to see if it keeps on happening. It is widely noted that the first reports of asparagus pee correlate well with the first use of sulfur containing fertilizers from the late 17th century1. This give a fairly decent clue (as if the smell didn’t) that sulfur compounds are to blame.
It isn’t that simple though, and the issue of stinkers versus smellers still exists. Some studies suggest that about half of the UK population are capable of producing the odour, but closer to three quarters of the American population are2. The sample size was small, and those results are from two different studies, also other studies contradict these findings so I’ll take those results with a pinch of salt. Another paper suggests that everyone is a stinker but not everyone is a smeller leading to the illusion that not everyone is a stinker, and also that some people may be hypersensitive, specifically sensitive to the pungent compounds3. Confused yet?
As far as I can tell (and as far as my journal access will let me go), the jury is still out on the precise nature of the variation between stinkers and smellers. There was one rather intriguing anecdote about women who, on becoming pregnant, started to notice asparagus pee. This could well mean that the women were smellers, but not stinkers, while the unborn children were stinkers1. Also, not everyone describes the smell in the same way: to some it is hideous enough to stop them eating asparagus, to others it is not unpleasant, just a bit strange.
I’ve still not answered my question though, what are the chemicals involved? Asparagusic acid (on the left above) is looking like a likely culprit as subjects fed that substance produced pee smelling suspiciously asparagus like. The current thinking is that asparagusic acid is broken down into a variety of sulfur containing compounds that then go on to break down into a collection of volatile and pungent sulfur compounds in urine. These intermediates are not known, and characterising them is probably difficult due to the hazards of trying to extract them from urine without altering them chemically. The final compounds must be volatile for the odour to be detected. These include methanethiol, dimethyl sulfide and bis(methothio)methane, which is reported to be reminiscent of cheese, horseraddish, onions, garlic, truffles, with earthy and spicy notes (on the right above).
I suppose that the precise nature of asparagus pee is not a burning research question because that was about all the information I could find. Still, I think I’m mostly satisfied with a tenuous explanation involving stinkers and smellers, aparagusic acid and breakdown products. After all, asparagus is still good eating!
 Akers et al., Food & Foodways, 1997 (2) 131
2 Mitchell, Drug Metabolism and Disposition, 2001, (29), 539
3 Lison et al., British Medical Journal, 1980 (281) 1676
PS – I think I’ve got the images working before. For some reason NN didn’t like displaying images saved in Picasa.