If you notice an unusual aroma to your pee after you eat asparagus, you aren’t alone. And if you don’t? Well, it still might stink like fresh-cut-grass-thrown-on-a-garbage-dump...at least to some of us. The culprit is asparagusic acid, a chemical compound that science is still trying to crack.
Dr. Daniel Whitehead, an associate professor of chemistry at Clemson University, describes asparagus pee as "the typical sort of putrid smell that we encounter in our labs. A rotten egg kind of smell." He’s something of an expert in objectionable odors: One of his lab’s main projects is creating nanoparticles that can capture and remove odor-causing molecules from the water and air surrounding offensively foul-smelling areas, like rendering facilities that cook down the "leftovers" from slaughterhouses into pet food and fertilizer.
As the name suggests, asparagusic acid is unique to asparagus, at least among plants that humans eat. But while various studies have theorized that it repels parasites like roundworms and bacteria, or that it inhibits the growth of other nearby plants, nobody’s quite sure what, if any, purpose it serves the plant.
What we do know is that atomically, the key to many bad smells is sulfur, and asparagusic acid comes loaded with a double barrel of stinky sulfur in the form of a disulfide bond—two sulfur atoms attached to one another. "Some sulfur compounds are toxic at high levels, and we’ve naturally evolved so that [sulfur] doesn’t smell good to us, so we stay away from it," Whitehead explains. Sulfur-based molecules are the key to why skunk spray smells bad, why cutting onions makes your eyes tear up, and why deadly mustard gas killed thousands of soldiers during World War I.
"Asparagusic acid itself doesn’t smell," Whitehead clarifies. "But when you eat it, the body breaks it down, and you get smaller molecules with sulfur in them. Those can actually vaporize, and that’s how you can smell them."
Chief among these molecules is methanethiol—one of the six to eight sulfur molecules researchers have identified as likely culprits of smelly asparagus pee. It’s found in some stinky cheeses, bad breath, and farts, but you most likely know it as the molecule you're smelling when the stove burner doesn’t light or there's a gas leak. (Natural gas on its own has no odor, but the highly smelly methanethiol, or sometimes its cousin ethanethiol, is added in tiny quantities so people can detect when there’s a leak.)
Two of the other suspects are dimethyl sulfide and dimethyl disulfide, which reek of cooked cabbage and strong garlic, respectively. At higher concentrations, they smell unbearably awful, and they’re two of the more common objectionable rendering-plant odors Whitehead works to get rid of. "They’re part of a sort of a suite of chemicals that are small enough and stinky enough that they turn up in a lot of these places," he says.
Because asparagusic acid is processed by the kidneys and doesn’t have to pass through the full digestive system, the process also happens extremely quickly: It can take just 15 to 30 minutes to go from chowing down on a grilled asparagus appetizer to releasing a stream of sulfurous sewage before dessert.
Why Some People Don’t Smell Asparagus Pee
What about those of you who’ve been reading this and have no idea what I’m talking about? You tender souls whose post-asparagus bathroom trips have never seemed out of the ordinary are most likely blessed with a genetic condition called asparagus anosmia.
Yes, there is an asparagus-pee gene, and it’s one of thousands of genes that genetic-analysis company 23andMe tests for. If you spit into a tube and send it off to the company, you can get a personalized report about whether your DNA says you can smell asparagus-tainted urine. "We have people who consent to be part of the research program, and we ask them a bunch of questions. Then we compare the genetic variants that we have to the answers," says Dr. Alisa Lehman, manager and senior product scientist for 23andMe. One of those questions was about whether subjects could smell asparagus pee, and the company found a single nucleotide polymorphism—a one-letter change in the genetic code—strongly associated with whether people can smell asparagus pee or not. "This variant is near a number of different genes for olfactory receptors," Lehman says, but so far nobody’s narrowed down exactly what that particular stretch of DNA does. (According to a 2014 study in Science, humans have about 400 different smell receptors, which together can theoretically discriminate between more than a trillion individual scents.)
About a quarter of 23andMe’s subjects say they can’t smell asparagus pee at all, and Lehman estimates that somewhere between 20-40% of people have the associated genetic variation for asparagus anosmia. A similar study run by public health researchers at Harvard in 2016 that used a stricter definition of being able to smell the odor found that slightly more than half of people don’t detect the aroma of asparagus pee. (That study also looked at statistical associations between genes and people’s answers to a wide variety of questions, and I’m only mentioning it here because it has the wonderfully nerdy stats-joke title of "Sniffing out significant ‘Pee-values’".)
And there's more: There’s an even smaller percentage of people who actually don't have smelly asparagus pee—and it's unrelated to whether they can smell it in others. We know these people aren’t just asparagus anosmiacs thanks to the wonders of urine-sniffing for science. There are only a handful of small studies, but scientists have indeed fed people asparagus, asked them to pee in cups, and had other people smell it. The results revealed that about eight percent of people just don’t make stinky pee after they eat asparagus, likely because they can’t process asparagusic acid or don’t break down enough of it to create the smell. That is likely also related to genes, but it hasn’t been pinned down like the odor-detection gene. (And it’s a completely separate process; smellers might not produce the odor, non-smellers might produce it, and vice-versa.)
For Lehman, 23andMe’s reports on genes like the one for asparagus pee (as opposed to its reports on less-frivolous genes, like those that raise your risk for cancer or other diseases) "just exposes how genetics can affect everyday life. Your genes are part of what makes you, you." A few of the other more unusual genetic associations 23andMe has found include the photic sneeze reflex ("some people sneeze when they look at bright light—it’s a similar thing to asparagus pee where you either know it, or it doesn’t make any sense," Lehman says), as well as individual genes associated with misophonia (a strong emotional reaction to the sound of people chewing), your sweet-or-salty preference, and even your favorite ice cream flavor. "There’s a plethora of ways your genetics impact what you like to eat," Lehman explains.
And that’s more than you ever needed to know about asparagus and urine. As you’ve probably guessed, I am both an odor-maker and an odor-smeller, and every time I eat asparagus it’s like a reminder that at a base level, we’re part of the natural world: just sacks of goo that take in chemicals and push out other chemicals, some of which smell like natural gas and garlicky cabbage farts.