To conserve water, members of my household abide by the old aphorism “If it's yellow, let it mellow.” You're in a state of ignorance about that wizened phrase? If so, it recommends that one not flush the toilet after each relatively innocent act of micturition. But there's one exception to the rule: after asparagus, it's one and done—because those delicious stalks make urine smell like hell. To me and mine, anyway.
The digestion of asparagus produces methanethiol and S-methyl thioesters, chemical compounds containing stinky sulfur, also known as brimstone. Hey, when I said that postasparagus urine smells like hell, I meant it literally.
Methanethiol is the major culprit in halitosis and flatus, which covers both ends of that discussion. And although thioesters can also grab your nostrils by the throat, they might have played a key role in the origin of life. So be glad they were there stinking up the abiotic Earth.
But does a compound reek if nobody is there to sniff it? Less philosophically, does it reek if you personally can't smell it? For only some of us are genetically gifted enough to fully appreciate the distinctive scents of postasparagus urine. The rest wander around unaware of their own olfactory offenses.
Recently researchers dove deep into our DNA to determine, although we've all dealt it, exactly who smelt it. Their findings can be found in a paper entitled “Sniffing Out Significant ‘Pee Values’: Genome Wide Association Study of Asparagus Anosmia.” Asparagus anosmia refers to the inability “to smell the metabolites of asparagus in urine,” the authors helpfully explain. They don't bother to note that their bathroom humor plays on the ubiquity in research papers of the p-value, a statistical evaluation of the data that assesses whether said data look robust or are more likely the stuff that should never be allowed to mellow.
The findings appeared in the notorious Christmas issue, which always features screwball scholarship, of the BMJ (known as the British Medical Journal from 1857 to 1988—that is, two decades after Queen Victoria first sat on the throne until midway in the reign of Elizabeth II). No need to buy the volume, as the urinary tract can be streamed online.
“This study,” the authors write, “was conceived during a scientific meeting attended by several of the coauthors in bucolic Sweden, where it became apparent that some of us were unable to detect any unusual odor in our urine after consuming new spring asparagus.” One could thus say that asparagus itself spearheaded the research.
Our intrepid investigators took advantage of two large, long-term epidemiological studies—the Nurses' Health Study and the Health Professionals Follow-up Study—that provided genomic data. They then recruited almost 7,000 people in those studies to rank the rankness of their postasparagus urine.
“Participants were characterised as asparagus smellers if they strongly agreed with the prompt ‘after eating asparagus, you notice a strong characteristic odor in your urine.’” Any other answer got one rated anosmic. The authors helpfully note, “Those who responded ‘I don't eat asparagus’ were excluded from the analysis.”
The responses indicated that 58 percent of men and 61.5 percent of women could not smell the sulfur. “It is possible that women are less likely than men to notice an unusual odor in their urine,” the scientists say, “because their position during urination might reduce their exposure to volatile odorants.” In this case, men must face the facts.
The genomic analysis revealed three apparently important genetic constructs—all in a region on human chromosome 1 that contains various genes in the olfactory receptor 2 family—related to the ability to smell asparapiss. The researchers, tongues briefly removed from cheeks, point out that their “findings present candidate genes of interest for future research on the structure and function of olfactory receptors [that] ... might shed light more generally on the relation between the molecular structure of an odorant and its perceived odor.”
In contrast to that brief trespass into seriousness, they warn, “Future replication studies are necessary before considering targeted therapies to help anosmic people discover what they are missing.” As long as they don't miss the bowl.