This morning I chanced upon, via that inexhaustible font of newsly tidbits otherwise known as Twitter, this fresh Scientific American essay by veteran science writer Steve Mirsky expounding upon a commonplace phenomenon that has been a lasting mystery as well as, interestingly, a source of conversation around science —the strong smell in urine following the consumption of Asparagus, that well-loved, delectable vegetable of Le Printemps known to mankind since 3000 BCE.
Of course, this particular topic having a lot to do with the ubiquitous bodily fluid, it is natural and inevitable that such discussions should contain references to potty humor (the more juvenile, the better)—so sayeth me, who has never been known to shy away from such whimsies. (And it seems, I am in good company in that regard; Benjamin Franklin himself once held forth on the subject of various odors of post-digestion flatulence, mentioning how a few stems of asparagus cause urine’s ‘disagreeable odor’.) Imagine my delight, when I found Steve didn’t disappoint, right from the lede onwards!
Using puns that are groan-worthy and witty in equal measure (a reason why I absolutely loved them), Steve clarifies that the sulfur-containing (which explains the stink) volatile chemical substances, a.k.a odorants, are almost* always produced by the digestive breakdown of Asparagusic acid, a substance unique to Asparagus officinalis; however, the ability to detect that stench in post-asparagus urine is genetically determined—meaning that these asparagus-derived urinary odorants are omnipresent, but not all of us are equipped to ‘feel’ them. Which, of course, led Steve to produce such gems as:
But does a compound reek if nobody is there to sniff it? Less philosophically, does it reek if you personally can’t smell it? For only some of us are genetically gifted enough to fully appreciate the distinctive scents of post-asparagus urine. The rest wander around unaware of their own olfactory offenses.
Indeed, it has been found that this ability is related to a type of genetic variations, known as single nucleotide polymorphisms (which means the variant forms differ by one of the four building blocks of DNA). Close to 900 polymorphic variants have been identified within a large cluster of olfactory genes (responsible for various olfactory receptors, which are protein molecules that help us recognize different odors) located on the human chromosome 1, and so far four independent variants or SNPs (pronounced ‘snips’) have been associated with the condition known as ‘asparagus anosmia’—which means that people bearing these genetic variants are unable to smell the sulfur in post-asparagus urine (what Steve delightfully refers to as ‘asparapiss’). However, their prevalence in the population seems highly variable. In the 2016 study Steve refers to in his essay, roughly 6 out of every 10 of the 7000-odd participants of European descent were anosmic for asparagus; earlier studies with non-European populations (summarized in this 2011 paper linked above) have shown widely different proportions.
Do read Steve Mirsky’s SciAm essay. Full disclosure: (a) I loved reading it, thoroughly enjoying his double entendres, and (b) I can smell my own asparapiss within minutes of eating asparagus, a vegetable which I am entirely too fond of (especially when it accompanies steak or salmon). And now, if all that talk of asparagus has made you hungry, here is a New York Times Guide by David Tanis on how to buy, store and cook this versatile vegetable. Enjoy!
* This qualification is necessary because there is a small proportion of people who may not, for multiple reasons, produce these odorants at all, or in sufficiently measurable quantities.