Why does pee smell after eating asparagus?

In the spring period the asparagus: this vegetable finds space in Kitchen in many recipes both for its ease of cooking and preparation and for its characteristic flavour. The gusto typical of asparagus, in fact, is due to different substances solforate contained within them. Unlike other types of vegetables, however, asparagus has a particular characteristic: it confers odor to ours urine. Not everyone smelling their own pee can perceive it. How come? In this article we look from the point of view chemist why our pee smells after eating asparagus and why not everyone is able to smell it.

Because pee stinks

I responsible or, if we like, the “culprits” of the typical smell ofurine after eating asparagus are containing molecules sulfur. Indeed, inside the asparagus sprouts, called turionithere are several sulfur chemicalsas well as potassium, vitamin A, vitamin C and vitamin B₉. One of the main sulfur compounds forerunners of the smell is theasparagusic acida molecule containing two sulfur atoms bonded together.

Once the asparagus has been eaten and digestion has begun, this molecule undergoes transformations through the metabolization process: in particular it is transformed into small molecules always containing sulphur, such as for example the methanethiol (CH₃SH) and the dimethylsofur ((CH₃)₂S). These substances then end up directly in the urine. Being very light and volatilemanage to detach themselves from the surface of the pee reaching ours naso. This explains the origin of the typical smell.

In the study published in Chemical Senses, the whole list of molecules responsible for the smell of asparagus is present, in addition to the two mentioned.

Because not everyone perceives the smell

If we were to ask our friends if their urine stinks after eating asparagus, chances are someone will answer, “No, I’ve never felt that!” This answer has a scientific basis: not everybody they can feel these smells.

Various experiments have been carried out in this regard, but in the study published in Plos Genetics it has been demonstrated that it is all a genetic matter: people who have not been able to perceive the smell of urine after eating asparagus had a genetic mutation on chromosome 1. This mutation causes that non there is a olfactory receptor in their nose; consequently, although these substances were present in their urine, they were unable to perceive them.

Also in the study published in Chemical Senses (we leave you in the link at the bottom of the article), it has also been shown that some people they do not produce odorous substances or has produced them in concentrations that are too low to be perceived. This means that asparagusic acid, or sulfur precursors in general, are not metabolized in the same way by all people.

In conclusion, we can say that it is possible that there are two factors at play: the inability to smell and the inability to produce the molecules responsible for the smell. So we can group people into 4 categories:

• “Producers” of smell and “receivers” of smell;
• “Producers” of odor and “non-perceptors” of odor;
• Odor “non-producers” and odor “receivers”;
• “Non-producers” of odor and “non-perceptors” of odor.