Why Does Asparagus Make Your Pee Smell Funny?

Asparagus has a long and storied history: It was mentioned in the myths and the scholarly writings of ancient Greece, and its cultivation was the subject of a detailed lesson in Cato the Elder’s treatise, On Agriculture. But it wasn’t until the turn of the 18th century that discussion of the link between asparagus and odorous urine emerged.

In 1731, John Arbuthnot, physician to Queen Anne, noted that asparagus “affects the urine with a foetid smell ... and therefore have been suspected by some physicians as not friendly to the kidneys.” Benjamin Franklin also noticed that eating asparagus “shall give our urine a disagreeable odor.”

Since then, there has been debate over what is responsible for the stinky pee phenomenon. Polish chemist and doctor Marceli Nencki identified a compound called methanethiol as the cause in 1891, after a study that involved four men eating about three and a half pounds of asparagus apiece. In 1975, Robert H. White, a chemist at the University of California at San Diego, used gas chromatography to pin down several compounds known as S-methyl thioesters as the culprits. Other researchers have blamed various sulfur-containing metabolites.

What Is Asparagusic Acid?
Why Can't Some People Smell Asparagus Pee?

What Is Asparagusic Acid?

Asparagusic acid, typically found in asparagus, contains sulfur and can sometimes lead to pungent odors. One study demonstrated that asparagusic acid taken orally by subjects known to produce stinky asparagus pee produced odorous urine.

The researchers concluded that asparagusic acid and its derivatives are the precursors of urinary odor (compared, in different scientific papers, to the smell of “rotten cabbage,” “boiling cabbage” and “vegetable soup”). The various compounds that contribute to the distinct smell—and were sometimes blamed as the sole cause in the past—are metabolized from asparagusic acid.

Exactly how these compounds are produced as we digest asparagus remains unclear, so let’s turn to an equally compelling, but more answerable question.

Why Can't Some People Smell Asparagus Pee?

For a 1980 study, subjects whose pee stank sniffed the urine of subjects whose pee supposedly didn’t. Guess what? The pee stank. It turns out, we’re not only divided by the ability to produce odorous asparagus pee, but the ability to smell it.

A 2016 study by the Harvard School of Public Health found that more than 800 genes regulate whether person can detect funky smells in their pee after chomping down on asparagus. After analyzing results from nearly 7000 participants, they determined that roughly 60 percent were asparagus anosmic, meaning they didn’t pick up on any urine odors.

An inability to perceive a smell (anosmia) keeps certain people from smelling the compounds that make up even the most offensive asparagus pee, and like those who produce less stinky pee, they’re in the majority.

Producing and perceiving asparagus pee don’t go hand-in-hand, either. The 1980 study found that some people who don’t produce stinky pee could detect the rotten cabbage smell in another person’s urine. On the flip side, some stink producers aren’t able to pick up the scent in their own urine or the urine of others, either.

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A version of this story was originally published in 2018 and has been updated for 2024.