Why Asparagus Makes Your Urine Smell and Why Some People Cannot Detect It

Asparagus can make some people’s urine smell. Photo credit: liz west//CC BY.

If you have ever noticed a strange sulfurous smell coming from your urine after eating asparagus, you are not alone. But if you have not, you are not alone either. Those who produce this odor usually assume that everyone does, and those who do not have no idea what the others are talking about.

Recently, scientists have discovered that while some people produce compounds that give their urine a characteristic scent similar to that of boiled or rotten cabbage, others do not or to a much lower extend. The ability to smell the odor also differs. While the inability to produce these compounds is unknown, the inability to smell the odor is associated with a mutation in a cluster of genes coding for olfactory receptors which are responsible for the detection of smell.
All in all, if your urine does not smell any different after you eat asparagus, your body either do not produce the sulfurous compounds or you are not able to perceive the odor.

People who produce this strange smelling urine, do so because certain odor-causing molecules are produced when the body metabolizes asparagus. It does not take long for these molecules to be produced, the odor has been detected within 15-30 min after eating asparagus.

Scientists have not yet been able to precisely identify the odor-causing molecules but an acid present in asparagus and its metabolites may be responsible. Asparagusic acid is a sulfur-containing acid found in asparagus. When it is digested, it breaks down into several chemicals that have two things in common; they have a somewhat unpleasant odor and they are volatile, which means that they are in a gaseous state at room temperature. This is why they are able to travel from the urine into the air and up your nose. Asparagusic acid, on the other hand, is not a gas, so it is not until you ingest it and your body converts it into its metabolites that the smell occurs.

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References:

Mitchel, SC. 2001. Food Idiosyncrasies: Beetroot and Asparagus. Drug Metabolism and Disposition. Vol. 29, No. 4, Part 2.

Pelchat ML et al. 2011. Excretion and perception of a characterisitc odor in urine after asperagus ingestion: a psychological and genetic study. Chemical Senses. Volume 36, pages 9-17.

Somers E. 2000. Eau D’Asparagus. WebMD.

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