If you’ve ever noticed a strange, not-entirely-pleasant scent coming from your urine after you eat asparagus, you’re definitely not alone.
Distinguished thinkers as varied as Scottish mathematician and physician John Arbuthnot (who wrote in a 1731 book that “asparagus…affects the urine with a foetid smell”) and Marcel Proust (who wrote how the vegetable “transforms my chamber-pot into a flask of perfume”) have commented on the phenomenon.
Even Benjamin Franklin took note, stating in a 1781 letter to the Royal Academy of Brussels that “A few Stems of Asparagus eaten, shall give our Urine a disagreable Odour” (he was trying to convince the academy to “To discover some Drug…that shall render the natural Discharges of Wind from our Bodies, not only inoffensive, but agreable as Perfumes”—a goal that, alas, modern science has still not achieved).
But modern science has, at least, shed some light on why this one particular vegetable has such an unusual and potent impact on the scent of urine. Scientists tell us that the asparagus-urine link all comes down to one chemical: asparagusic acid.
Asparagusic acid, as the name implies, is (to our knowledge) only found in asparagus. When our bodies digest the vegetable, they break down this chemical into a group of related sulfur-containing compounds with long, complicated names (including dimethyl sulfide, dimethyl disulfide, dimethyl sulfoxide and dimethyl sulfone). As with many other substances that include sulfur—such as garlic, skunk spray and odorized natural gas—these sulfur-containing molecules convey a powerful, typically unpleasant scent.
All of these molecules also share another key characteristic: They’re volatile, meaning that have a low enough boiling point that they can vaporize and enter a gaseous state at room temperature, which allows them to travel from urine into the air and up your nose. Asparagusic acid, on the other hand, isn’t volatile, so asparagus itself doesn’t convey the same rotten smell. But once your body converts asparagusic acid into these volatile, sulfur-bearing compounds, the distinctive aroma can be generated quite quickly—in some cases, it’s been detected in the urine of people who ate asparagus just 15-30 minutes earlier.
Of course, the whole asparagus-urine scent issue is complicated by an entire separate issue: Some people simply don’t smell anything different when urinate after they eat asparagus. Scientists have long been divided into two camps in explaining this issue. Some believe that, for physiological reasons, these people (which constitute anywhere from 20 to 40 percent of the population) don’t produce the aroma in their urine when they digest asparagus, while others think that they produce the exact same scent, but somehow lack the ability to smell it.
On the whole, the evidence is mixed. Initially, a pair of studies conducted in the 1980s with participants from France and Israel found that everyone produced the characteristic scent, and that a minority of people were simply unable to smell it. People with the ability to detect the scent, though, were able to smell it even in the urine of those who couldn’t smell it, indicating that the differences were rooted in perception, not production.
More recent studies, though, suggest the issue is a bit more complicated. The most recent study, from 2010, found that differences existed between individuals in both the production and detection of the scent.
Overall, scientists now conclude that most of the difference is in perception—that is, if your urine doesn’t seem to smell any differently after you eat asparagus, it’s likely that you simply can’t perceive the sulfurous compounds’ foul odor, but there’s a small chance it’s because your body digests asparagus in a way that reduces the concentration of these chemicals in your urine.
It’s still unclear why some people don’t produce the smell, but we do seem to have a clear explanation of why some people don’t perceive it. In 2010, the genetic sequencing company 23andMe conducted a study in which they asked nearly 10,000 customers if they noticed any scent in their urine after eating asparagus, and looked for genetic similarities among those who couldn’t. This peculiarity—which you might consider useful if you eat asparagus frequently—appears to stem from a single genetic mutation, a switched base-pair among a cluster of 50 different genes that code for olfactory receptors.
We’re still waiting for some enterprising team of scientists to attempt gene therapy to convert smellers into non-smellers—but given other priorities to use genetic modification to cure blindness and breast cancer, it seems likely that those suffering from asparagus-scented urine might have to wait a while.