Q: Why does urine smell so bad after eating asparagus?


A: Before we get to why your urine becomes so pungently perfumed after ingesting a side of asparagus, let’s globetrot for a hot second to Germany where, each spring, Spargelzeit (asparagus season) takes the countryside by storm. During Spargelzeit, numerous farming towns host Spargelfests where a whole lot of the succulent vegetable, particularly of the white variety, is celebrated and consumed in every which way possible (there’s even an entire museum dedicated to Germany’s “royal vegetable” in the Bavarian town of Schrobenhausen. Here's a small summary in English). Not to be crass or anything but can you imagine the smell emitted from the latrines at one of these spargel-scarfing bonanzas? Who knows, maybe the infamous odor is eradicated by a couple glasses of Gewürztraminer but I certainly wouldn’t want to be one stall over from the Spargel Queen of Beelitz after a long day of festivities.


But just wait … there’s a chance that even if you were to share a loo with the exalted, full-bladdered Spargel Queen, the bathroom “bouquet” resulting from her asparagus chow-down may not even be offensive at all … just your run-of-the-mill urine. Or, get this, if the pee in question is indeed pungent, there’s a chance that your nose may not even be able to detect it. Although I’m not a huge asparagus eater, it’s been decided that I’m not immune to producing “asparagus pee” or smelling it. Thank you, genetics!  


Conversations — and scientific studies — revolving around the effect that asparagus has on the smell of urine has been going on for ages now. Even founding father Ben Franklin had something to say on this pressing issue of bodily fluids, noting that, “a few stems of asparagus eaten shall give our urine a disagreeable odor; and a pill of turpentine no bigger than a pea shall bestow upon it the pleasing smell of violets.” Lovely.


Early studies suggested that the smell of everyone’s urine is altered by asparagus (we’re all doomed!) but more recent studies have found that this isn’t always the case. A 2010 study found that some participants failed to develop asparagus pee as they lacked the gene for the digestive enzyme that breaks down the vegetable into its infamously stinky components.


Additionally, researchers concluded that the ability to smell asparagus pee varies from person to person thanks to “genetic differences in olfactory receptors,” according to Marcia Levin Pelchat, a neuroscientist at the Monell Chemical Senses Center in Philadelphia who, along with a team of researchers, was able to identify a single variation in the DNA code responsible for being able to pick up, or not pick up, the scent of an asparagus-laced number one. And again, the ability to both produce and smell asparagus pee isn’t always mutually exclusive. In the Monell study, 8 percent of the 38 roasted asparagus-eating subjects did not produce the offending odor while six percent were unable to smell it. One subject was unable to both produce and smell malodorous urine.


So what is it, you may ask. The jury is still out on what officially is behind the smell but many researchers point to methyl mercaptan, a sulfurous compound found in asparagus that’s, no shocker here, also found in eggs, skunk secretions and, well, flatulence.  After eating asparagus, those possessing the enzyme quickly (within 15 to 30 minutes) metabolize the compound that travels through the kidneys and is eventually excreted in the urine. Voila! A funky odor commonly compared to the stench of rotting cabbage and ammonia is born. But before you completely blame methyl mercaptan for your stinky urine-related woes, also consider asparagine, another asparagus ingredient often fingered as a culprit (either acting alone or with methyl mercaptan and other chemicals) in the great asparagus pee mystery.


I certainly wouldn’t let all of this prevent you from serving up a savory mushroom asparagus frittata or creamy asparagus soup. Asparagus is a nutritious, versatile and mineral- and vitamin-packed veggie (as a member of the lily family, it’s a cousin of leeks, onions and garlic). And while we’re on the topic of asparagus and urine, supplements containing the spears, roots and rhizomes of this miracle veg, when combined with plenty of H20, can increase urine output and treat urinary tract infections and other UT-related ailments. Plus, asparagus supplements can be used to prevent kidney and bladder stones. So eat up. But if your travel plans include frequenting restrooms in rural Germany during April or May (because who’s travel plans don’t?), you may want to proceed with caution. Den spargel kann stinky.




Got a question? Submit a question to Mother Nature and one of our many experts will track down the answer. Plus: Visit our advice archives to see if your question has already been tackled.


Photo: ms.Tea/Flickr; MNN homepage photo: Shutterstock

Matt Hickman ( @mattyhick ) writes about design, architecture and the intersection between the natural world and the built environment.

Why does urine smell so bad after eating asparagus?
Why does urine smell so bad after eating asparagus?