Why Asparagus Makes Your Urine Smell
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.
Why Does Asparagus Make Your Pee Smell?
You may have noticed that after eating asparagus, your pee has a somewhat unpleasant scent.
This usually happens due to the metabolism of asparagusic acid, and the concept is referred to as asparagus pee.
However, this particular side effect of eating asparagus doesn’t happen to everyone, and some may have never smelled such a thing.
This article explains why eating asparagus makes pee smell, and why only some people can smell it.
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Asparagusic acid is a sulfur-containing compound that seems to be found exclusively in asparagus.
It’s a nontoxic substance that produces a sulfurous odor, which some say is similar to rotten cabbage.
Since a strong and pungent smell characterizes many sulfur-containing components, such as rotten eggs, natural gas, or skunk spray, scientists believe that asparagusic acid may be the cause of your pee’s funny scent after eating the vegetable ( 1 , 2 ).
Summary
Asparagusic acid is a nontoxic, sulfur-containing compound that may cause your pee to have a distinct odor after eating asparagus.
Once your body metabolizes asparagusic acid, it produces several sulfurous byproducts that are highly volatile — meaning that they vaporize easily ( 3 ).
When you pee, these compounds evaporate almost immediately, which enables them to travel from the urine up to your nose, allowing you to smell them.
Though scientists have not been able to determine whether one compound is responsible for the smell or if it’s due to the mixture of all of them, a compound called methanethiol is widely mentioned in the literature.
Methanethiol, also known as methyl mercaptan, is characterized by a strong and unpleasant scent that’s often associated with fecal odor and bad breath — and it happens to be the most common odorant found in urine after eating asparagus ( 4 , 5 , 6 ).
How long does the smell last?
Some people notice the rotten-like smell as early as 15–30 minutes after eating asparagus, and studies have determined that within 25 minutes, half of the asparagusic acid consumed has already been absorbed ( 7 ).
The fast absorption rate suggests that the effect of asparagus on urine smell can appear quite quickly, and recent studies also agree that it can last for more than a few hours.
One study in 87 people who ate 3–9 spears of asparagus found that the half-life of the asparagus smell was 4–5 hours ( 3 ).
The half-life of a substance tells you how long it takes for it to reduce to half of its initial amount. Therefore, if the half-life of the asparagus smell was estimated at 4–5 hours, it means that the total effect could last up to 8–10 hours.
Yet, another study in 139 people who also consumed 3–9 asparagus spears reported the half-life of the smell to be 7 hours, meaning that the effect could even last up to 14 hours ( 7 ).
Either way, you can expect your pee to smell for quite a while.
Summary
When your body metabolizes asparagusic acid, it produces numerous smelly, sulfur-based compounds that give your pee a rotten-like smell that can last 8–14 hours.
The effect of asparagus on urine scent is not universal, and a number of hypotheses try to explain this phenomenon.
One hypothesis — called the production hypothesis — suggests that only some individuals are capable of producing the sulfurous compounds responsible for the smell, while others are non-producers.
This hypothesis asserts that non-producers lack a key enzyme that helps metabolize asparagusic acid and are thus unable to produce the smelly byproducts ( 4 ).
For example, a small study in 38 adults determined that about 8% of them either didn’t produce the smell or produced it at concentrations that were too low to be detected ( 4 ).
The other hypothesis — called the perception hypothesis — states that everyone produces the smell, but some are unable to detect or perceive it ( 4 ).
In this case, researchers found a genetic modification that alters one or more of the olfactory receptors that should respond to the asparagus smell, causing what is known as asparagus anosmia, or the inability to smell asparagus pee ( 8 ).
In fact, research suggests that a large percentage of people can’t smell asparagus pee.
One study in 6,909 adults noted that 58% of men and 62% of women had asparagus anosmia, suggesting that this specific genetic modification is quite common ( 8 ).
Summary
Not everyone is familiar with asparagus pee, and researchers believe that it’s because some people either don’t produce the smell or are unable to perceive it.
The asparagusic acid in asparagus produces many sulfurous byproducts that give your pee a rotten-like smell.
The smell can be detected as early as 15 minutes after eating asparagus and may last up to 14 hours.
However, not everyone produces the smell, and the majority of people can’t smell it due to a specific genetic modification.
Last medically reviewed on December 23, 2019
Why Asparagus Makes Your Urine Smell
If you’ve ever noticed a strange, not-entirely-pleasant scent coming from your urine after you eat asparagus, you’re definitely not alone.
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.
Joseph Stromberg was previously a digital reporter for Smithsonian.
