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An astonishing amount of research has gone into the question of whether asparagus really makes your urine smell funny. Sci Show explains it all inside!

Hosted by: Hank Green
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Sources:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1379935/pdf/brjclinpharm00087-0116.pdf
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1715705/pdf/brmedj00052-0020.pdf
http://chemse.oxfordjournals.org/content/early/2010/09/27/chemse.bjq081.full
http://udel.edu/~mcdonald/mythasparagusurine.html
http://www.smithsonianmag.com/science-nature/why-asparagus-makes-your-urine-smell-49961252/?no-ist
http://mentalfloss.com/article/24731/why-does-asparagus-make-your-pee-smell-funny
http://www.bbc.com/future/story/20140818-mystery-of-asparagus-and-urine
http://www.webmd.com/food-recipes/features/why-pee-smells-funny-eat-asparagus
[Intro]   You've probably never smelled a skunk and wished that you could reproduce that lovely odor with your own bodily fluids, but if you're among a certain 40% of humans, you actually kind of can.  All you gotta do is eat some asparagus, wait a little bit, and then pee on your enemies.     Asparagus is a weird little vegetable, it's a monocot, meaning that its seeds start out with only one leaf, and it's related to onions and garlic.  Its flowers are actually poisonous to humans, but the stems have plenty of nutrients in them, like vitamin K and vitamin C, and since as far back as the 1890s, scientists have been trying to figure out why it makes peoples' pee smell like, you know, just different, maybe a little worse than usual.  Of course, all sorts of foods, like coffee and garlic, can change the chemistry of your urine, which makes sense; some foods contain strong smelling compounds that your body can't digest completely, and they gotta somewhere.  Sometimes that somewhere is your urine.  And asparagus has its own stinky chemical that doesn't seem to be found anywhere else; it's called asparagusic acid, and when it's digested, it produces sulfur compounds like methanethiol and dimethyl sulfide.  Sulfur, of course, is famous for giving things like skunk spray and rotten eggs their unique aromas.  But the weird thing is that for a lot of people, eating a lot of asparagus doesn't seem to have an effect on their excreta, it just smells totally normal, so when scientists started trying to figure out why that is, they figured that there were two possible explanations.  One is that people who claimed to have non-smelly pee were just non-excretors, that is, they didn't produce urine with the compounds that smell.  But the other possibility was that they were producing the compounds, but they couldn't smell them.  To get to the bottom of the issue, in 1956, two biologists at Oxford conducted a study in which 115 people were fed asparagus and then they had their urine analyzed.  The results showed that some samples did appear to smell while others didn't, and the urine specimens that had the unique scent all contained the same compound, methanethiol.  Since then, other studies have found even more byproducts of asparagusic acid in excretors' urine, like dimethyl sulfide and dimethyl sulfone, so it seemed pretty clear that people were either excretors or non-excretors.     But then, we kept studying it. In 1980, another researcher, this time in Israel, began to wonder if everyone's urine did smell after eating asparagus, but some people just couldn't smell it.  A really astonishing amount of research has gone into answering this question.  Anyway, he managed to get 307 people to agree to sniff different concentrations of one person's smelly asparagus pee.  And it turns out that he was kind of right.  Some people could smell the scent, but others who were given the same specimen couldn't smell a thing.  So.  Everyone was pretty confused for a while.     Then, in 2011, the Monell Chemical Senses Center in Philadelphia tested both possibilities at the same time.  Their subjects gave urine samples before and after eating asparagus, and also had to smell different samples from other people who had eaten asparagus.  The researchers concluded that both hypotheses were essentially correct.  It turned out that people were either excretors or non-excretors, and they could either be smellers or non-smellers.  So some people produce smelly urine but can't smell it, and some people produce the smelly urine and can smell it, I'm one of those, and some people don't produce it but they can detect traces of asparagus in other peoples' pee.  Scientists have gone on to determine that all of these differences are genetic.  Non-smellers have something called a specific anosmia, where one of their 400 or so smell-receptor genes is mutated and effectively turned off.  That same kind of mutation is what makes some people unable to smell things like vanilla or mint.  If you can have one of those turned off, I'd prefer the asparagus pee one than the vanilla one, that's just a bummer.  The non-excretors, meanwhile, have a different genetic mutation that keeps them from excreting the compound in the first place, but no one is really sure where the compounds go if they aren't excreted.  So, it seems that there's even more work to be done into the science of asparagus pee.  Maybe you'll be the one to do it.  French author Marcel Proust once said that asparagus, "as in a Shakespeare fairy-story transforms my chamber-pot into a flask of perfume."  We're all about curiosity here at SciShow, so does asparagus turn your chamber pot into perfume?   Let us know.   To learn how you can help us keep exploring pressing questions like this, you can go to Subbable.com/SciShow, and don't forget to go to YouTube.com/SciShow and subscribe if you just want to keep getting smarter and learnin' weird stuff.  Because that's what we do here.