Friday Weird Science: Why does asparagus make your pee smell?

Mar 19 2010 Published by under Friday Weird Science

Early spring is a good time of year. Sci starts feeling a little more motivated, it's finally warm enough to feel comfortable running outside again (not that Sci ran inside, she was just very uncomfortable outside), and it's asparagus season!

When Sci was wee and her mother would try to feed her asparagus, Sci turned up her little nose at such nonsense. Why on earth would anyone eat something that was that green and looked like it had hair!?

I seem to remember as a child thinking that asparagus tasted over green and like unripe corn and I was very unimpressed. But as an adult I have come a real appreciation of this odd vegetable. You grill it up with some olive oil and spices on...yum. And so now I look eagerly for those little green bunches at the store.
And it was when I started eating asparagus that I heard about the pee thing.

So today's Friday Weird Science is something that REALLY makes Sci curious. I mean, usually I just look for things that make me laugh. But now I'm on a quest, I want to KNOW!
It's time for some weird and Historical (historical enough) science: Waring, et al. "The chemical nature of the urinary odour produced by man after asparagus ingestion." Xenobiotica, 1987.

Most everybody knows that asparagus makes your pee smell funny. Some say it's a high ammonia smell, some say it's more like old socks, and some people...can't smell it at all. What's up with that? Since it's now spring again and Sci is going on a real asparagus bender, she has had more than once wondered: what the heck is the chemical causing that smell?!


These guys set out to analyze what was in your urine following intake of asparagus that produces the weird smell. The researches here describe the smell as "rotten or boiling cabbage" but that doesn't really seem right. Apparently, if people's urine smelled after asparagus but BEFORE the introduction of fertilizer, nobody ever said anything about it or noticed it. So people thought that perhaps it was the sulfur compounds that have been used to improve the growth and flavor of asparagus, which get INTO the asparagus and make your pee smell funny. But the scientists in this paper point out several things:

1) The chemical(s) in charge must be volatile. They have to be able to get from the urine into the air to get to your nose. So just saying what's in the urine isn't going to explain the cause of the smell.

2) Previous studies to extract stuff in urine did things like boil it. That's no good, chemicals that will come out with boiling AREN'T going to be present in urine at normal body temperatures
(unless you're Dr. Manhattan, and I seriously doubt that guy has a problem with asparagus making his pee smell).

And so, the guys in this study took urine from people who ate asparagus (in this case, eight healthy guys and girls from the biochem department at the University of Birmingham, England). They had to make sure that all of them were capable of producing funny smelling pee after eating asparagus. They ALSO got a hold of some people (it happens in about 10% of the population) who DON'T produce the smell, and sampled them.

Anyway, they got these people, took a urine sample, and fed them asparagus for dinner. They then had them bring in their pee at time points from 0 to 24 hours later (I bet most of them were grad students).

They then separated out the urines and analyzed them for the components that were volatile at normal body temperature. To do this, they put the urine at wait level, and took GAS samples at head level, to get the area at which you'd smell the urine, to narrow it down to the right set of volatile compounds. They got these:

Dimethyl sulfide
Dimethyl disufide
Dimethyl sulfoxide
Dimethyl sulfone

These were all compounds that weren't there in the before sample, were there in the asparagus sample, and were ONLY there in the asparagus samples of those who could produce the smell.

They ALSO did something cool to check their work, they reconstituted the urine after they were done with it, and checked with dedicated "smellers" (people who they knew could smell this sort of thing) whether or not it still smelled. Interestingly, their panel of smellers could always tell the difference between two subjects, but not between each urine from a single subject. So each subject probably has varying amounts of each chemical in the urine.

So what's so smelly about these compounds? You'll notice they all (except one) contain the word "sulf" which means they contain sulfur. Methanethiol is known to be really smelly as well. The thing is, most of these compounds would probably be breathed out or broken down in the body after being ingested (when a compound is volatile enough to be a gas at body temperature, lots of things can happen to it). The authors hypothesize that probably the compounds lists above are not present in themselves in the asparagus, but are breakdown products that occur when the asparagus is being digested, which also explains why some people have a mutation that doesn't produce the smell. For their candidate of what compound is responsible, they nominated the aptly named asparagusic acid. Preliminary studies from their laboratory (though this was 1987, and I don't know that they ever published the further results), showed that this compound could be the right one. When asparagusic acid enters the body, it could be broken down, producing the compounds above, and making your urine smell!

Sci thinks this is pretty cool, as it's something she's always wanted to know. She also thinks it's particularly funny that people, when they write about asparagus making your pee smell, go ON and ON about how SIGNIFICANT this is, that it's a quality of LIFE issue! THINK of the HUMILIATION! The HORROR of having your date ruined when your girl finds out your pee SMELLS!!

Sci thinks that if your girl is finding this out on the first date, you're doing it wrong.

Also, Sci thinks this experiment would be pretty cool to do in the lab! Apparently they just did this on a gas chromatograph with mass spec to determine the weights. Sci thinks that this therefore might be a REALLY cool experiment to conduct with an organic chem class in college. Have everyone eat asparagus and see who comes out smelling funny, with what compounds, and how much of each.

Finally, I have a question for you all. When you eat asparagus, what does the resulting urine odor smell like? The compounds would suggest a sulfurous odor (like being in a marsh), but Sci's experience tells her otherwise. Anyone? And also, has anyone suffered quality of life issues from asparagus pee? Anyone?

Waring RH, Mitchell SC, & Fenwick GR (1987). The chemical nature of the urinary odour produced by man after asparagus ingestion. Xenobiotica; the fate of foreign compounds in biological systems, 17 (11), 1363-71 PMID: 3433805

51 responses so far

  • Patrick says:

    Thiol is a sulfur-containing functional group (S=O), so actually, all the compounds they identified contain sulfur.
    Also, I had never before heard that asparagus makes urine smell, which is odd, because normally if something has to do with pee or poo there is a pretty high chance at least one of my friends will tell me about it at some point.
    Friday Weird Science FTW!

  • Patrick says:

    Ack! Thiol is -S-H. My bad.

  • bioephemera says:

    Great post, Sci! My bf and I were just talking about this and wondering why asparagus, which tastes so wonderful, is also cursed with that nasty odor. Nice to know the chemistry behind it.

  • recovering catholic says:

    What about the genetics of this? Does anyone know if it's straightforward Mendelian, or something more complicated? And I wonder if the percentages of smelly pee-ers and non-smelly pee-ers varies between ethnic groups!

  • Ed Darrell says:

    What is the odor? It smells like cooked asparagus.
    Scientists sometimes ask the silliest questions.
    How about coffee-scented urine?

  • Tom Shilson says:

    I have never observed or heard of the phenomenon. Perhaps everyone in my extended family is a non-smelly mutant or they are all too polite (or uptight) to mention it.
    Is there any relation to people liking lima beans? Apparently some people find them unpleasantly sulfurous. I don't.
    Does the phenomenon occur with organic asparagus? That would eliminate fertilizer and various -cides as factors.
    Interesting blog! Thanks!

  • LGRooney says:

    Interesting. My wife and I indulged in crispy prosciutto over asparagus last evening and this morning, upon waking and having to do my AM duty, I was wondering why my pee smelled so strongly like a dive bar, i.e., a mixture of spilled beer and tobacco smoke left over from the night (decades?) before.

  • David Marjanović says:

    I've never eaten asparagus (a rubbery substance of unappetizing smell), but I've noticed that, after eating confit de canard, my pee smells of the freshly served dish for 24 h (and a most lovely perfume it is! Mouth-watering just to think of it!). Apparently the smelling compounds pass through the digestive system, bloodstream, and kidneys unaltered.
    I've noticed this with certain soups, too, but in those cases it's not enough to completely cover the normal smell.

  • LKB says:

    Funny--a few months ago I wrote a blurb about asparagus pee for the science show on my local NPR station. It's still making its way through production (and is taking forEVer), otherwise I'd link to it.
    I think you're right, that 1987 is the last research paper about it--I did find a 2001 paper in Drug Metabolism and Disposition that was a review of both asparagus pee and beet pee, which apparently makes some people pee red, which would be highly disconcerting. It's an interesting review, even though author goes on a bit long about the history of asparagus eating. To her credit, she does mention Pliny the Elder. (IMO, if more papers mentioned Pliny the Elder, the world would be a better place.) But did you see the 1975 Science paper? It references a 1891 study that had test subjects eat *7 kg* of asparagus. I really hope that was not in one day. Because if it was, they might have figured out what asparagus barf smelled like, too.

  • My understanding is that everyone makes the small (perhaps to slightly varying degrees?), but some people lack the ability to detect the smell.

  • Anodyne says:

    I'm not exactly the biggest asparagus fan, but I have, when properly seasoned or drowned in tasty sauce, eaten them on several occasions. I never noticed even a hint of foul odor while visiting the restroom. I was curious to see what this whole fuss about asparagus pee is, and I was never able to find out.
    Reading this, I'm convinced I'm much better off not knowing!

  • Aubergine Kenobi says:

    Sulfur compounds are very smelly -rotten eggs someone? H2S-(for some reason we evolved to be very sensitive to this kind of smells, so we can detect them in very small amounts). Also, sulfur-containing compounds are not rare in plants, they're in garlic an onions (they're responsible for the bad bread associated with garlic and the crying when chopping onions), mustard, horseradish and wasabi (they're the ones responsible for the particular hotness of these plants).

  • DA says:

    I have both asparagus pee and beet pee. The beet pee is always a shocker in the morning when you have forgotten what you had for dinner the previous night.
    Rosie: The study contradicts your understanding, since they had official "smellers." Plus, my wife does NOT have asparagus pee. It will be interesting to see what pee my daughter has -- I'll see if I can convince her to eat asparagus baby food.

  • Jyotsana says:

    Asparagus pee is how we convinced our eight-year-old son to try asparagus in the first place. He didn't want to eat it, but when we told him it would make his pee smell like cooked asparagus he didn't believe us and had to try it for himself. Sure enough, he came out of the bathroom later that evening laughing, holding his nose, and proudly announcing his pee was stinky.

  • Rob Clack says:

    Fascinating post. I used to avoid asparagus for this very reason, then one day realised that was a very stupid reason not to eat something I liked a lot!
    Steam gently until done, serve with butter and a few shavings of parmesan.

  • ENT-TT says:

    Now I'm curious. I've no sense of smell at all, so I guess I'll have to try to convince my wife to whiff it.

  • rork says:

    Re beets. I think everyone pees red afterward, but have noticed that eating fresh stuff, particular from a garden, is much more shocking than canned beets. If you grow them, feed them to your unsuspecting friends, and wait for the report.

  • ad says:

    Asparagus unosmia has been extensively studied. Just a few months back the genetic basis for this was reported (by 23 and me if I recall right). Don't know too much about the non-producers and whether the non-producers are non-smellers as well. Also don't know about digestion since by personal observation the smell is palpable within a half hour of consumptiion.

  • Petra says:

    There is also a wild species of asparagus that is never fertilized and the pee smells the same, so I wouldn't say the fertilizer is the key here.
    I find it quite similar to the smell of pee when you eat some heavy antibiotics. Can you check the correlation?
    Interesting post though.
    Like 😉

  • VJBinCT says:

    Upper-class women from ancient Rome through the Renaissance (and probably later still) used to ingest some turpentine to give their urine a pleasant odor. BTW, the acrid odor and sharp taste of onions (but not vidalia and other varieties which are grown in sulfur-poor soils)are due to sulfur compounds, but do not appear to have an effect on urine odor. Interesting topic!

  • Pietr Hitzig says:

    I proffer the abstract of a 1980 article that states that all our asparagus urine stinks but only a select population can smell it.

    The urinary excretion of (an) odorous substance(s) after eating asparagus is not an inborn error of metabolism as has been supposed. The detection of the odour constitutes a specific smell hypersensitivity. Those who could smell the odour in their own urine could all smell it in the urine of anyone who had eaten asparagus, whether or not that person was able to smell it himself. Thresholds for detecting the odour appeared to be bimodal in distribution, with 10% of 307 subjects tested able to smell it at high dilutions, suggesting a genetically determined specific hypersensitivity.

  • Pietr Hitzig says:

    Babe Ruth, when asked by a wealthy socialite at a dinner party why he had not eaten his portion of asparagus, replied unabashedly,

    "Because it makes my pee stink, ma'am."

  • mercurianferret says:

    I wonder if this is also a reason why my urine smelled odd after - when in Bolivia - I drank coca tea. It didn't smell like asparagus, but it definitely changed the smell of the urine to something quite noticable.

  • Psychointegrator says:

    Why does one smell their urine?
    Oh well, to each their own I believe it's said.

  • Chris Aiken says:

    @8 - If it's rubbery, you're cooking it wrong or it's past prime. It should be wonderfully crisp.

  • John McKay says:

    I've eaten asparagus my whole life (I'm over 50) and crossed through several of the categories discussed. I had heard about the smell, but didn't notice it in my own pee until I was in my late thirties. It's been growing stronger ever since. It seems unlikely that my nose has become more sensitive in middle age, so I assume I didn't make the compounds until then. That points to it being a metabolism thing rather than a genetic thing. When I was young (until I was twelve), we collected wild asparagus. For a few years in my teens we could only get canned. Since then, I have only eaten fresh asparagus. Throughout the year, asparagus comes from different places. Here in Seattle we get our asparagus from the Yakima valley in late summer. During the rest of the year, it comes from California, Mexico, and, I believe, Peru. I cannot believe all of these places use the same combination of pesticides, antifungals, and herbicides, so I'm not inclined to think added chemicals are that important a part in the smell. Maybe someone should just study me.

  • Nora says:

    I'd also heard that it is the ability to detect the distinctive post-asparagus odor, rather than produce it, that is absent in 10% or so of the population. Seems to me it was a biochemist who told me this, but I never did look it up. She did say that it had something to do with the metabolic breakdown of asparagine. And that it was a good way to get children to ingest asparagus, telling them that some could smell it in their pee and some couldn't.
    The latter, at least, I have verified. Kids think it's hilarious (and will admit it). They can also be persuaded to eat beets if you tell them about the, um, excreted evidence of doing so.
    @psycho - I believe the whole point of this is that it makes one's pee so smelly that one can't help but notice the odor. Which is distinctive and not particularly pleasant, but hardly a quality of life issue.

  • David Marjanović says:

    Why does one smell their urine?

    You mean you manage to avoid getting the smell into your nose? (Especially while standing?) Do you smoke? 🙂

  • The Gregarious Misanthrope says:

    I am most definitely in the produce the odor and smell the odor camp. What I find most remarkable is the speed at which the smell appears in the urine. For me, it can be well under an hour after eating asparagus.
    The phenomenon has entered the realm of comedy at least once. I heard a comedian refer to a foul smell as being like a port-a-potty at an asparagus festival.

  • Gary Godfrey says:

    I'm surprised, given the normal lofty level of discussion here, that everyone has been keeping their discussion limited to urine. There are other bodily fluids.

  • Thanks for solving one of life's mysteries. As someone who has never eaten Asparagus, I only heard the rumors from Asparagus loving friends and family.

  • meandmine says:

    Hi Sci - love your blog!
    Although I like asparagus (by choice asparagus & parmesan risotto), I have a personal dislike for all cruciferous vegetables (members of the botanical family Brassicaceae, formerly Cruciferae) - brussels sprouts, broccoli, cauliflowers, cabbage, etc. - which I put down to the taste caused by high sulfur content.
    That said, it is partially this sulfur content that makes them so useful as biofumigants - the glucosinolates ( break down and help control nematodes and other unwanteds in the soil -
    Pest control the organic way!

  • Christopher M says:

    I've noticed that eating a few bowls of Honey Smacks cereal (or its competitor/equivalent Golden Crisp) makes my pee smell remarkably like the cereal itself. (And it's not just me: see Google, Maybe a slightly less interesting puzzle than asparagus, since the smell is so redolent of the original food. But I've yet to see a plausible proposal for the responsible compound(s).

  • lylebot says:

    The smell, to me, is not really like anything else I can name. It's in a class of its own. And it doesn't really bother me to be honest.
    Also, I think it's much more noticeable with steamed asparagus than with sauteed or grilled asparagus.

    (a rubbery substance of unappetizing smell)

    If it's rubbery, you're doing it wrong.

  • eff says:

    I can smell the asparagus smell on the skin of someone who has eaten asparagus, including myself. Once my girlfriend's kid came over from his father's house and I correctly guessed that he had eaten asparagus at his previous meal based on his body odor. I wonder whether those same volatile chemicals could be present in sweat, and what this says about how they are metabolized.
    I assume if you've read these comments this far that you're not currently eating, but be warned;
    All these urine stories remind me of a friend of mine who was really into psychedelics for a while and reported that in order to potentate the psychoactive chemicals in the Amanita Muscaria mushroom one had to eat the mushrooms, wait a couple of hours, and then urinate. It was by drinking the urine that the drug's effects could be felt. Apparently this method was developed by indigenous Siberian Shamans.
    It makes you wonder how many potentially useful medicinal chemicals may be found as metabolites in urine. It is only recently that pharmacologists have realized there is a wealth of knowledge in the supposedly "primitive" cultures of indigenous people.

  • cleek says:

    i can smell the ghosts of asparagus, coffee, honey and onions within an hour of eating them. beets turn it red, honey turns it brown.

  • Mike says:

    I noticed that wild asparagus which is much tastier than commercial asparagus produce no smell at all in your urine.

  • Alice says:

    I used to work for a UK government project promoting science education. My boss there wanted to run a big national project to explain genetics by getting kids to smell their wee after eating asparagus and then sending in the results. Very sadly, they turned it down in funding so we did a forensics thing instead.
    So, anyway, according to her, there was an easy to explain genetic basis to the smelly wee thing. Now idea what her reference was though.

  • Chuck says:

    This may be TMI, but on the subject of other bodily fluids.
    I have seen on Craigslist postings that sometimes men soliciting oral sex will state: No asparagus eaters.
    Apparently it's not just urine.

  • essman says:

    I have a recollection that dimethyl sulfoxide was an ingredient in certain salves that people used to use on their skin, and made them "taste" oysters.

  • Ali says:

    My pee tends to smell like whatever I've been consuming, if it's strongly flavoured. Asparagus pee smells like cooked asparagus. Broccoli, coffee, spinach, even my b complex vitamin (the filler has a distinctive smell)--if it has a strong odour, I'll notice it later. There's no major change in the scent from consumption to urination.
    I do eat unholy amounts of asparagus, though. It's been a favourite since I was a baby.

  • Yana Ray says:

    Asparagus does nothing to my urine, on the other hand coffee and B vitamin pills - I can smell these straight away, especially coffee, though the smell is nothing like the real thing, just something similar and less pleasant, more like weak, watery instant coffee. Eugh.

  • rb says:

    I have noticed that when picked fresh from my garden, asparagus does not have that effect on me (eaten either raw or cooked). When stored for a day before consuming, it produces the smell (and of course asparagus eaten at restaurants or purchased at grocery stores also produces the smell). As I use only leaf compost for garden fertilizer and no commercial products in the yard, I tend to discount the "sulfer in the fertilizer" explanation and favor the hypothesis of an endogenous substance generated in decomposing (no longer vital) asparagus (n=1, but a testable hypothesis).

  • Erin says:

    A friend of mine had a quality of life problem with asparagus. He ate some while on a date, took the girl home with him, and received a blow job. Turns out, it didn't just make his pee smell. It's also changed the smell and taste of his semen, causing the girl to throw up and likely not want to administer any more oral sex in the near future.

  • Thisbe says:

    I am (as a person involved in biological sciences) chronically irritated by the constant repetition of the "fact" that everybody's pee stinks after eating asparagus but some people can't smell it.
    My pee does not smell different after asparagus, but I can definitely smell it when other people's pee has the distinctive asparagus odor. Hypothesis disproved! Thanks, Sci, for an excellently informative post about what might actually be going on.
    (Literally EVERY TIME this comes up, someone will exclaim in horror "But WHY are you smelling anyone else's urine?" - I suppose it's more unusual than I might think to live in multi-person households with a no-flush policy for normal pee.)

  • Ben says:

    Good post. I remember when I was a kid, I always thought that it was something about eating very rich food that caused the pee smell. Since we didn't have asparagus all the time, it seemed more likely that I'd notice the smell after a dinner out at a really nice restaurant or some big holiday meal at home. I think I was in my 20s, in a restroom at a restaurant, and a drunk guy went to do his business at the stall next to mine, and practically yelled, "Hahah, that asparagus man, makes your pee smell, doesn't it?" Sure enough...

  • [...] has blogged before on why asparagus makes your pee smell. It turns out there are sulfurous compounds in asparagus that come out during digestion, making [...]

  • christina Nielsen says:

    I have always had stinky asparagus pee. Mine smells like a cross between a skunk and burning rubber. It is very strong! Makes you wonder how differently we smell everything, depending on our genes...

  • Jay says:

    This is awesome! I've been wondering about this "smell" myself since I have it as well! Glad I decided to "Google" this to find out! But now I know, lol!! Thanks for the article, it was a good read!

  • imdatraw says:

    Well I've discovered dat not only does it makes yr urine smelly it makes my scalp smellweird scalp nva had dis weird smell before ...only happens whn I consume the asparagus.

Leave a Reply