Can You Smell It?


It’s not exactly something one talks about at the dinner table, but it does start with something one eats at the dinner table. Marcel Proust put it finely when he recorded in his diary, “[asparagus]…played at transforming my humble chamberpot into a bower of aromatic perfume”. Although Proust found it delightful, not everyone likes the smell of asparagus in their urine and as it turns out, not everyone can smell it. If you’re following me so far, you’ve probably inherited the gene for a chemical receptor in your brain that recognizes the chemical compound mercaptan (officially known as methanethiol). If you’re totally weirded out by now, you’re probably lacking the gene—and where’s your sense of scientific curiosity?!

Since being able to detect an asparagus-like scent in your urine after you’ve eaten the vegetable isn’t necessarily life-saving stuff, not much research has gone into exploring it. But a team of curious scientists figured ‘why the heck not?’ and delved into the issue in a study published in Chemical Senses, a sub-branch of Oxford Journals. Apparently when asparagus undergoes the human digestive process, almost all of it gets broken down–except for what scientists think is something similar to mercaptan, a compound closely related to sulfur. (This is the compound that is also responsible for the scent of bad breath and flatulence, so thanks a bunch, mercaptan). The exact compound that produces the asparagus-in-urine scent hasn’t been identified though, shrouding the whole issue in a touch of alluring mystery…if you’re into that kind of thing.

Since this molecule comes out the other end in the same shape in which it entered, it is discernible by some as different from the scent of ‘normal’ urine. The ability to detect the scent is associated with a certain allele, or version, of the gene that codes for a scent receptor in the brain—those who have a certain allele of the gene can detect the scent and those without it, cannot.

The study also revealed that even though everybody produces it, some people produce it in very low quantities, making it difficult to detect, and some produce it in very high quantities, making it easier to sniff out. Those who don’t produce the scent as strongly may not be able to recognize it, not necessarily because they lack the gene but because they may never have been exposed to it at high enough concentrations to learn to recognize it. About 6% of the subjects in the study could not detect the scent because of their genetic profile. There was also an interesting, though unrelated, racial difference—the allele had many different variations in Caucasian participants but there was not any genetic diversity found in the allele of subjects of African descent. The whole asparagus situation has now provided a basis for understanding why some people may be able to smell odors, like coffee or certain types of alcohol, in their urine and some may not. (Oh, to have the power!). The scent is stronger when one is dehydrated, and is only possible with substances whose molecular structure remains unchanged after the digestive process.

Fascinating, I know. But you really never know what information may be useful in your exploration of the rest of the world. Or, you know, at parties.