This question has baffled scientists for over half a century. In 1956, British researchers divided the population into two categories: excretors (those whose urine smells after they eat asparagus) and nonexcretors (asparagus eaters who remain odor free).
Since there’s no documentation of the asparagus-pee phenomenon before the 1700s, about the time farmers began using sulfur to fertilize soil, this and subsequent studies hypothesized that a particular gene allows people to process a sulfur-containing compound in asparagus (most likely asparagusic acid). The theory was that if you have that gene, your pee won’t stink. However, they were relying on the test subjects’ own reports and weren’t considering the subjects’ ability to smell.
In 1980, Israeli researchers performed a similar experiment but asked the nonexcretors to smell the excretors’ urine. Shockingly, they found that everyone’s urine smells after eating asparagus; it’s just that some people can’t smell it. So they, too, divided the world into two camps: perceivers and nonperceivers.
This story was originally published in January 2007.