Why your pee smells after eating asparagus — and why not everyone can smell it

The nutritional benefits of asparagus are undeniable: It’s full of vitamins, minerals, and antioxidants. It’s also a good source of fiber and protein. From a health standpoint, asparagus is a clear winner. However, as most asparagus-lovers can tell you there’s one, albeit superficial, downside to consuming the vegetable. Shortly after chowing down on a healthy, asparagus-rich dinner, you might notice a striking and unpleasant odor in the toilet bowl. It’s asparagus pee and it stinks.

Asparagus’ connection to stinky pee has been documented and discussed for centuries. In 1781, Ben Franklin penned a letter to the Royal Academy of Brussels, writing that “a few stems of asparagus eaten shall give our urine a disagreeable odor.” Here’s why it happens.

What makes asparagus pee so stinky?

Researchers believe the stinky pee culprit in asparagus is the aptly named aspargusic acid, which is found exclusively in asparagus. That aspargusic acid gets broken down into sulfur byproducts during the digestive process. (These byproducts, according to a 2013 study, include “methanethiol, dimethyl disulfide, dimethyl sulfide, dimethyl sulfone, and dimethyl trisulfide.”) And as anyone who has ever smelled a rotten egg can tell you, sulfur is a pungent and generally unpleasant scent. As you pee, these aspargusic acid-turned sulfur byproducts evaporate almost immediately, causing you to smell that distinctive “asparagus pee” scent.

While science hasn’t been able to definitively determine if one compound is responsible for the smell, or if a combination of several cause it, “methanethiol” appears in much of the scientific literature.

Methanethiol, also known as methyl mercaptan, is a known culprit of stinkiness, including fecal odor, bad breath, and flatulence. So its prevalence in asparagus pee would certainly make sense.

How long does the asparagus pee smell last?

A 2019 study published in the journal CPT: Pharmacometrics Systems Pharmacology suggests that the smell can appear as soon as 15 minutes after eating the vegetable and may last for a few hours. How long the scent lasts depends on the amount of asparagus consumed: More asparagus means stinky pee for a longer time. In the 2019 study, the participants who ate the most asparagus noted the smell could last as many as 14 hours after eating.

Why are some people unable to smell asparagus pee?

While the odor may be unpleasant, having stinky pee after eating asparagus is normal. Interestingly, however, some people aren’t able to smell the odor. It is unclear whether this is because they don’t produce the odor or because they are unable to smell it. Likely, it is some combination of the two: Some people may produce the smell, but not be able to smell it, and others might not produce the smell, or they may produce it in such small quantities that it’s not detectable.

Researchers who conducted a 2011 study published in Chemical Senses ultimately concluded that “individual differences exist in both odorant production and odor perception. The biological basis for the inability to produce the metabolite in detectable quantities is unknown, but the inability to smell the odor is associated with a single nucleotide polymorphism (rs4481887) within a 50-gene cluster of olfactory receptors.”

In the latter case, the inability to smell asparagus pee is likely genetic. A 2016 study published in the journal BMJ concluded, “Genetic variation near multiple olfactory receptor genes is associated with the ability of an individual to smell the metabolites of asparagus in urine.” The researchers conclude with the objectively hilarious line, “Future replication studies are necessary before considering targeted therapies to help anosmic people discover what they are missing.”

Hint: If your anosmia is specific to asparagus pee, you’re not missing much.