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The characteristic smell emanating from the restroom after asparagus was on the menu is familiar to many. However, not everyone is under the spell of "asparagus pee."

Despite its well-known health benefits, asparagus is controversial. Whether green, purple, or white, its ability to cause some people to recoil after their next bathroom visit has fascinated humans for centuries.

In fact, the first mention of the distinctive smell after asparagus consumption dates back to the 11th century. As Stephen C. Mitchell, from the Faculty of Medicine at Imperial College London in the United Kingdom, explained in an article published in Perspectives in Biology and Medicine, "[…] the Ancients thought asparagus had medicinal properties" and took "[…] its odor-producing qualities as proof of its activity."

So, what causes the smell? And why does it cause such grief to some, but not to others?

The world is divided into two classes of people: those who produce asparagus pee and those who do not. What is not known is whether the non-producers actually do secrete the smelly substances in their pee, but the levels are too low to be detected by discerning noses.

At the heart of the issue is the conundrum of which chemical compound is to blame for the smell; no one really knows.

Mitchell explains that chemical structures that contain sulfur are often to blame for unpleasant smells. Rotten eggs are a prime example of this.

Previous studies found the following compounds in urine after a healthy dose of asparagus was consumed: methanethiol, dimethyl sulfide, and dimethyl disulfide. That being said, the first two compounds have also been detected in the air when asparagus is boiled, which indicates that cooking could destroy them.

So, what is the culprit?

In his article, Mitchell points to asparagusic acid. This aptly named compound is also known by its chemical description, 1,2-dithiolane-4-carboxylic acid.

High levels of asparagusic acid are thought to protect the young asparagus shoots from parasites hungry for a tasty snack.

The chemical structure of asparagusic acid is highly resistant to degradation by cooking, leaving our gut in charge of breaking it down.

Whether it is, in fact, asparagusic acid, methanethiol, dimethyl sulfide, dimethyl disulfide, or another compound remains to be seen. Whatever the chemical basis, the reason why some of us find the smell of asparagus pee offensive and some do not lies in our genes.

The story of asparagus pee gets more complex. In addition to the producers and non-producers, some people can smell asparagus pee while some are unable to.

These non-sniffers have asparagus anosmia, which is the technical term for an inability to smell. So, now the world is divided into four classes of people.

Last year, Medical News Today reported on a study published in The BMJ that identified changes in the genes responsible for smell, called olfactory receptor genes, between the sniffers and the non-sniffers.

The study involved 6,909 men and women and showed that a whopping 60 percent had asparagus anosmia.

The question that remains to be answered is whether those with asparagus anosmia are more likely to eat asparagus than those who are subjected to its odor.