Why does asparagus make your pee smell funky?


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You know what we’re talking about: You chow down on those tasty green stalks with dinner and then get a wafting reminder of the spring veggie the next time you hit the head. While asparagus has plenty of benefits — it’s a very good source of protein; vitamins A, C, E and K; thiamin; riboflavin; niacin; folate; iron; phosphorus; potassium; copper; manganese; and selenium — it has a reputation for causing seriously pungent pee as soon as 15 minutes after consuming it.


Do you smell that?
Surprisingly, not everyone has this reaction — between 22% and 50% of the population report having unusually smelly urine after eating asparagus, according to WebMD. Research has provided conflicting explanations for the discrepancy: Those who do not report the odor either digest it in a different way that doesn’t produce the smell, or they’re unable to smell it.

A 2010 study conducted by the Monell Center and published in Chemical Sense discovered that both explanations might be correct. Researchers found that 8% of subjects did not produce the odor, and 6% produced the odor but were unable to detect it. DNA samples pointed to genetic differences within a group of olfactory receptors.

In case this isn’t confusing enough for you: According to Mental Floss, some people who produce the odor can’t smell it in their own urine but can smell it in other people’s pee. (We really hope research participants were paid well in these studies.)


The science behind the stink
Research has generated various theories for the smell, but the general consensus is that the odor comes from methanethiol and other sulfur compounds that are produced when the body breaks down asparagusic acid.

How do we know, you ask? Marceli Nencki, an apparently sadistic scientist, fed four men about 3.5 POUNDS of asparagus each in 1891, and then tested their urine. He found high amounts of methanethiol, which has a lovely rotten-cabbage fragrance due to its chemical makeup of sulfur mixed with hydrogen and carbon.

Another study supported the asparagusic acid theory by having stinky-pee-producing participants take asparagusic acid orally. Turns out that does the job too — their urine smelled the same as it did when they ate asparagus, and it contained the same compounds.