Asparagus Pee Is Real, But Only Some Of Us Can Smell It

In a cruel twist of evolution, some of us can smell it and some of us can't.
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Everyone’s pee smells like asparagus after they digest it. Deny this all you like, but those green stalks contain something called asparagusic acid, among a couple other compounds, that gives urine that unique odor.

As your body digests food, it breaks down different compounds through the enzymatic process. In the case of asparagus, its compounds are volatile and released as a vapor through the urine. That’s the aroma you smell when the compound exits your body, according to HuffPost’s conversation with Dr. Anish Sheth, a gastroenterologist at Princeton Medical Group and author of What’s Your Baby’s Poo Telling You?.

If you insist you’ve never before smelled what’s been informally dubbed “asparagus pee,” it’s because you lack the ability to detect the odor. The smell is there, you just can’t smell it. “The digestive process is pretty constant from person to person, but a person’s ability to detect these odors varies,” Sheth says. This is because our perception of smell — just like our perception of color — is completely personal. “We all have our own idiosyncratic smell perception of the world,” Dr. Ian Davison, a biology professor at Boston University, explains to HuffPost. “Our experience of different smells is completely unique.” The inability to smell this asparagus pee is an instance of specific anosmia, where a specific scent cannot be detected by a specific nose.

Illustration by Eva Hill

There are about 400 different genes for the different receptors in every nose. Some people have a mutation in one of these genes that affects the ability of that receptor to respond to the chemical that makes pee smell funny. The defect seems a bit arbitrary — Sheth chalks it up to “an odd quirk of human evolution;” up to 50 percent of people can detect the smell. Davison guesses the mutation happened over time. He says that for many animals, sense of smell is key for survival. Humans, on the other hand, rely so strongly on their visual sense that smell devolved, and these random errors — like not being able to smell asparagus pee — started creeping through.

Asparagus pee is not the only scent for which people experience specific anosmia. Every two or three people out of 100 have a tough time detecting vanillin, the main compound found in vanilla. Some are particularly sensitive (or insensitive) to androstenone, a component of sweat. “It flips between weak and pleasant to powerful and quite nasty,” Davison says.

We can’t control much of this. But if your asparagus pee is really getting to you, Sheth suggests cutting off the tips of the vegetable before consuming it. “That’s where the bulk of the compounds are found.” He, personally, sees no reason to take this step, as he finds the tips to be most delicious and worth the consequences.

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