Sci has blogged before on why asparagus makes your pee smell. It turns out there are sulfurous compounds in asparagus that come out during digestion, making your pee smell significantly musty and weird for many hours after.
Unless, of course, you can't smell it at all.
I have to admit, I thought EVERYONE could smell asparagus pee. It was only when I first posted about it that I found out that other people can't! And of course, I immediately have to wonder why? Am I special or something? Maybe other people spent too much time in Bed, Bath, and Beyond?
But it is none of these things, of course. It's genetics.
But if you're going to prove it's genetics, well you have to test it. Everyone ready to sniff someone's else's pee?! Wait, where are you going?! It's for SCIENCE!!
Lison et al. "A polymorphism of the ability to smell urinary metabolites of asparagus" British Medical Journal, 1980.
(Oh sure, it looks so innocent. Source)
Reading the introductions to papers can be a really enlightening experience. For example, some people can smell asparagus pee, some people can't smell it, and some don't produce it at all! Apparently only about 40% of us lucky asparagus noshing humans produce methylmercaptan, one of the most pungent of asparagus' metabolites.
But forget producing, what about smelling? Who can smell it and who can't?
To determine this, the authors of this study took one guy. And they fed him a TON of asparagus. 450g of canned. I pity him. That's a LOT of asparagus, and you have to figure it got old after a while. Maybe they allowed spices?
Anyway, they gave him this pile of asparagus, and then collected urine samples full of the good stuff about 6 hours later. They then diluted it out to various concentrations, and presented it to a bunch of test subjects (who thankfully did not know they were being presented with pee), and asked them if they smelled anything funny. They gathered over 300 of these unfortunates, and gave them a whiff.
One of the important things about this is that the study took place in Israel, where most of the people at the time, regardless of origin (they had a large number of Ashkenazi Jews, some Yemenites and some Moroccans) had never had asparagus. Ever. So they probably wouldn't have any idea what they were smelling if they did smell anything funny, and thus would be pretty blind to the conditions. They also had two groups of American students, 10 of whom were self-identified asparagus sniffers, and 11 of whom were not.
When it came to the thawed straight urine, undiluted, everyone could tell they smelled something funny. The American asparagus sniffers (which, by the way, will be the name of my next band) immediately identified the smell as being asparagus. But when it came to the dilutions, they got a rather interesting result.
What you can see here is a bimodal distribution. Most people are non-sniffers, the ones to the left of the graph. They can smell the asparagus pee, but only at higher concentrations (the low dilution number means a higher concentration of pee). But there's another small lump over there, a lump of sniffers, who can smell the pee even at pretty low concentrations (high dilution number is low concentration of pee). The result was pretty consistent, and the authors believe that this means that there's a genetic trait that creates a small number of people who are much more sensitive to the asparagus smell than everyone else. Of course, they don't know what this gene is. Now that we can sequence for such things (which we could not when this paper was published) it'd be very interesting to see what it is, and if it's important in smelling other things. Are people who can smell asparagus in urine more sensitive sniffers in general? Is it just specifically for this type of smell? And what does that mean?
So it appears that the asparagus pee smell isn't as universal as I thought! First you have to pee it, and then you have to smell it! For those who can't...pity us poor sniffers. We smell what you did there.
M LISON, S H BLONDHEIM, R N MELMED (1980). A polymorphism of the ability to smell urinary metabolites of asparagus BRITISH MEDICAL JOURNAL, 221 DOI: 10.1136/bmj.281.6256.1676