Asparagus pee smell

How it works

Asparagus contains asparagusic acid, a sulfur chemical that seems to occur only in asparagus⁵. The body probably turns it into the smelly sulfur compounds in the urine⁵. One lab team compares the smell to cooked cabbage¹.

Two separate traits decide whether a person notices it¹. Some people do not make enough of the compounds for anyone to detect. Nobody knows why¹. Other people cannot smell the odour at all, even in urine from someone who makes it¹.

This second trait is a specific anosmia. A specific anosmia means a person cannot smell one odour, while the rest of their sense of smell works normally¹. In a 1980 study, everyone who smelled the odour in their own urine could also smell it in anyone else’s⁴.

In 2010 the gene-testing company 23andMe found a variant, rs4481887, near a smell receptor gene called OR2M7³. A 2011 lab study linked it to smelling the odour, not to making it¹. A 2016 study found 871 linked variants, all in one region of chromosome 1 full of smell receptor genes². The exact receptor is not proven¹.

How common it is

The answer depends on the test. In a lab test with paired urine samples, only 2 of 31 people (6%) could not smell it, and 3 of 37 (8%) did not make it¹. Both figures rest on very few people.

A 2016 survey of 6,909 European-American adults found 60.3% anosmic². But anyone who did not “strongly agree” that they notice a strong odour counted as anosmic². One self-report question also cannot separate not smelling it from not making it⁷.

Try it

1. Eat a normal portion of asparagus.
2. Notice the smell of your urine over the next few hours.
3. If you notice nothing, you may not make the smell, or you may not smell it. A lab test can tell which¹.

Is it a problem?

No. Both traits are normal differences between people¹. Making the smell is not a sign of a metabolic disorder, as was once supposed⁴. In the lab test, the people who could not smell asparagus urine could smell a rose odour normally¹. So a specific anosmia is not general smell loss. A new, general loss of smell is a different thing, and worth mentioning to a doctor.

History and culture

Around 1781, Benjamin Franklin wrote that “a few stems of asparagus eaten, shall give our urine a disagreeable odour”². People who make the smell tend to assume that everyone does⁶. The 2016 study ran in the BMJ’s Christmas issue. Its authors wrote that “the non-anosmic wish to remain anonymous”². The 2010 23andMe study also found gene links for photic sneezing³.