I obviously had too much time on my hands today. I investigated the odd sulfurous odor of our urine after eating asparagus after a discussion of this at the dinner table last night. This asparagus urine odor is anticipated for most of us when we enjoy this vegetable. For years it has been known that a small minority of people don’t smell this odor when they eat asparagus, but there has been some debate as to whether this is due to not producing the odor, or not being able to smell the odor.
In 2010 a report in Chemical Senses, a journal I had never heard of before I prepared for this post answers this question. Both lack of olfactory awareness of the odor of asparagus urine and not producing the sulfurous odor are relatively common. In this study, with a control for ability to smell roses to exclude people who just cannot smell at all, 8% of participants did not produce the sulfurous odor urine after eating asparagus, whereas 6% of people could not detect the odor when it was present.
The lead author Marcia Levin Pelchat PhD as quoted in Science Daily concludes, “Although seemingly just a curiosity, the individual differences in metabolism could be important in other realms. Additional studies are needed to determine whether the inability to produce the odor is associated with other metabolic traits or disorders.”
The article states that the chemical basis of not producing the urine odor from asparagus is not known, but the genetic cause of not smelling the odor is a polymorphism in a 50 gene cluster of olfactory receptors in a genetic code area. Polymorphism is when there are more than one version of genes that code for a demonstrable trait. Eye color, hair color, male pattern baldness, and earlobe shape are other examples of polymorphism.
For the chemists or botanists among you the unique component of asparagus that is metabolized to the odiferous urine components is probably asparagusic acid, as it is the primary unique compound in asparagus not noted in other foods that don’t lead to this odor in the urine. It has been shown that the presence of the urine odor is remarkably fast, often noted in the first 15-30 minutes after asparagus ingestion, and that young asparagus contains much higher concentrations of asparagusic acid than more mature vegetable sprouts.