Most people know about the strange smell that asparagus gives off after it has been, ahem, processed by some humans. Yet other humans aren't able to smell the odor at all. That makes asparagus an unusual marker for the intricacies of genetic variation.
Asparagus has a reputation for being a stealth skunk. Although not objectionable while being cooked, it contains sulphur compounds, which are metabolized by the body and come out in the urine -- sometimes. Asparagus pee has become an unusual area of study. Hundreds of people have been fed asparagus on a scientists' dime, and then made to either excrete it, sniff the excretion, or both. What's surprising is this is still a controversial area of study, in part because some people can't smell the compound for which they're being tested.
The first chemical analysis of the subject revealed that methanethiol, CH4S, is the compound at work in people's urine. This introduced a common bit of lore into popular imagination. That 1956 study concluded that methanethiol was either there or it wasn't, so people were either excreters or they were not. Another study showed that 100 per cent of French people were able to smell asparagus urine, although it didn't identify the method by which those results were obtained. Based on these and later studies, people came to believe it was a simple genetic trait.
Still, some scientists questioned whether methanethiol was really the culprit. In 1980 we got another wrinkle. A study done in Israel showed that some people simply couldn't distinguish (by smell) asparagus urine from tap water. Researchers then fed asparagus to the "non-smellers" and had their urine sniffed by "smellers" -- all of whom could smell the asparagus.
So what's going on? Asparagus is letting us in on a more complicated story than we imagined. First of all, there is definitely something about asparagus urine that slips past some people's noses. A 2011 study had people sniff urine spiked with basil odor compared to regular urine. It showed that people who were nose-blind to asparagus urine could distinguish basil odor just fine. About 6 per cent of people couldn't smell asparagus urine when confronted with it, and 8 per cent didn't produce noticeable asparagus odor -- but these two percentages were not the same people.
What's more, the researchers suggest that some people produce a high concentration of the odor compound, while and others produce much low concentrations (perhaps even too low to be detected by any humans). And while some people were sensitive to the smell, others were nearly completely anosmic to the scent, and unable to detect the odor.
It looks like there isn't one simple genetic trait that allows us to sniff or not sniff, produce or not produce. Which means that somewhere out there is someone who produces an ungodly reek whenever they eat asparagus, and is also completely oblivious to it.