Humans have known for a long time that asparagus makes our pee smell. While ancient Romans certainly ate (and loved) their asparagus, the first to record the vegetable's tendency to add a stench to urine was an 18th century physician to the French royal family. After running several experiments, he came to the conclusion that asparagus, “Eaten to Excess …causes filthy and disagreeable smell in the urine.” In 1731, the physician to Queen Anne of England (also mathematician and writer of satire) John Arbuthnot also noted a similar phenomenon in his essay about diet and the human body, writing that asparagus “affects the urine with a [fetid] smell...and therefore have been suspected by some physicians as not friendly to the kidneys." Founding Father Ben Franklin drafted a 1781 letter to the Royal Academy of Brussels in which he stated that eating asparagus gives “urine a disagreeable odor” (the letter’s real purpose was asking for a drug that would “render the natural discharges of wind from our bodies... as agreeable as perfumes”). Then, there was the French novelist Marcel Proust who, perhaps sarcastically, claimed to like the smell of his asparagus-tinged urine by proclaiming in 1913’s Swann’s Way that it transformed his “humble chamber into a bower of aromatic perfume."
All of this is to say, the after effects of asparagus have been a well-documented phenomenon through history. But why exactly does asparagus make your pee smell?
Scientists in general agree that the odor likely comes from asparagusic acid, a compound that’s only found in asparagus (as the name implies). When the acid goes through our bodies’ digestive tracts, it gets broken down in several sulfur-based compounds. The volatile nature of these compounds means that they evaporate and turn into gas at room temperature. This can happen very quickly, perhaps only 15 to 30 minutes after one has eaten the vegetable. While sulfur gas tends to smell like rotten eggs, the sulfuric compounds found in asparagusic acid have been described as more akin to old cabbage.
But what makes this asparagus-smelly urine discussion fascinating is that there’s evidence that not everyone may have a nose for the smell. Two studies were done in the 1980s, one in France and one in Israel, that determined a percentage of the population could not detect the odors. This lead some to believe it was a genetic trait, found in some ethnicities but not others. A more recent study concluded that it possibly stems from a single genetic mutation, but it probably isn't tied to one particular ethnicity. As Boston University biology professor Dr. Ian Davison explained to Huffington Post in 2014, it all may simply be arbitrary and "an odd quirk of human evolution.”
To add to the mystery, a 2010 study found that a small percentage of people’s bodies may breakdown asparagusic acid differently than the rest of us, producing a smaller concentration of the odorous compound. It is conceivable that, in these people, the concentrations are so small that they would be undetectable to the human nose, even if the person possessed the genetic mutation to detect the smell.
In short, everyone is pretty sure why asparagus makes your pee smell. On the other hand, no one is totally positive about who can smell it and why. In a world where we’ve proven Einstein’s theory correct, scientists still haven’t figured out why you can smell your stinky asparagus pee and the guy in the next urinal can’t.