The malodorous mysteries of asparagus pee

Everyone’s pee smells. But you may have noticed it takes on a more pungent, kind of rotten-egg quality after you’ve munched on a few spears of asparagus.

This smell stems from the aptly named asparagusic acid, a chemical present only in asparagus. In the plant, this chemical is thought to act as a natural pesticide to protect the vegetable’s tender young shoots from attack by predatory parasites.

Curiously enough, some people never even notice.

Scientists have two theories to explain this olfactory antipathy. The first theory suggests that some people simply don’t produce any malodorous molecules when they digest asparagus.

A 2011 study by the Monell Chemical Sciences Center in Philadelphia suggests both possibilities are true.

The researchers asked 38 men and women to pee in a cup before and after noshing on grilled asparagus. Then they asked the study volunteers to come back for a smell-test of their own samples, as well as samples from the other donors. The study ran into a bit of trouble when several of the recruits dropped out due to “unanticipated aversions to urine.” Still, the researchers were able to gather enough data to show that 8 percent of people couldn’t produce the asparagus odorant and 6 percent of people failed to detect it.

Another study traced the latter olfactory disability to a single genetic change, amid a cluster of genes that code for the proteins that help process and identify smells.

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By Marla Vacek Broadfoot. Illustrated by Jaime Van Wart. 

Marla Vacek Broadfoot is a geneticist-turned science writer. She currently serves as a contributing editor at American Scientist and a columnist at the Raleigh, NC, USA News & Observer and Charlotte Observer. When she isn’t writing, Marla enjoys sharing gross science facts with her family, often at the dinner table. Learn more about Marla here and follow her on Twitter @mvbroadfoot.

Jaime Van Wart is a graphic designer and illustrator based on Los Angeles. Previously a User Experience Designer at IBM, Jaime is now studying motion design as a second-year MFA in the Graphic Design program at CalArts.See her work here and follow her on Twitter @saucebomb.

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