Sunday, July 12, 2009

Apple #396: That Asparagus Smell

(Image from Grocery Outlet)

I was going to begin this post by saying simply, "You know what I'm talking about." But when I did a little research into this, I discovered that not everyone does, in fact, experience the asparagus smell.

  • For roughly 40% of the population, in less than an hour of eating asparagus, our urine will acquire an odor like soggy eggs inside of old socks, and perhaps even take on a greenish tinge.
  • The rest of the population has no idea what I'm talking about.
  • The reason that some people have it and some people don't is genetics.

A gene is a bunch of DNA, some of which is active (exons), some of which is inactive (introns). Thousands of genes make up a chromosome, which is that X-shaped thing on the right.
(Diagram from Wikipedia)

  • Scientists discovered back in the 1950s that a certain dominant gene is at work in the 40% whose urine changes smell after eating asparagus.
  • There is some debate about whether the 60% who don't experience The Asparagus Effect can't produce the odor or that they can't smell it. Recent research suggests that everyone's urine gets stinky from asparagus, but not everyone can smell it.
  • I'm very surprised that fewer people have it than those who don't. Because everyone I've ever discussed this phenomenon with knows exactly what I'm talking about. My sample must not be random enough. Population sample, I mean.

(Image from NEDARC)

  • While scientists know that it's a dominant gene that makes you capable of doing this, they disagree about what compound produces that distinctive odor.
  • Generally, they agree that the compound is a combination of something present in the asparagus plus some digestive enzyme in the body that breaks down the asparagus.
  • Here is a list of the proposed culprits and a rough explanation of what they are:
  1. Methyl mercaptan - same type of thing that makes skunks smelly.
  2. Several S-methyl thioesters - results of sulfur reacting with an acid. The S stands for sulfur. Very smelly.
  3. Asparagine - non-essential amino acid produced by asparagus reacting with other amino acids, and which is apparently also stinky.
  • Whichever of these compounds is responsible, they each involve sulfur or methane, or both. Sulfur smells like rotten eggs or burnt matches. Methane is the stinky stuff in the gas we all pass. So the stuff in the asparagus-laced urine is definitely stinky, whatever it is.

A hunk of sulfur, which came from a natural gas plant in North Dakota. It even looks stinky, doesn't it?
(Image from the North Dakota Geological Survey)

  • In terms of health, if you're smelling this stuff in your urine, it's a good thing. Your kidneys filter out things your body doesn't need into your urine. Whatever this stinky compound may be, your body doesn't need it.
  • So if your urine smells stinky after eating asparagus, be glad because it means your kidneys are working fine.
  • If you don't smell it, don't worry; you're probably one of the 60% who don't have that dominant stink-smelling gene.

P.S. I've been thinking about this some more, and I'm wondering if that initial research that came up with that 40% yes/60% no split was flawed. Because it sure seems like way more people say they smell it than don't. So I'm wondering if at least some of those 60% lied and said they couldn't smell it. Maybe they didn't want to admit their urine was stinky.

So, dear reader, if you have no idea what I'm talking about, if you have never experienced this at all before, would you be kind enough to post a comment to this entry and let the rest of us know that you really do exist? You would be advancing the cause of science!

Wise Geek, Why Does Asparagus Make Some People's Urine Smell Funny?
The Straight Dope, Why does aparagus make your pee smell funny?, Why does asparagus make your wee smell?
The Guardian, Notes and Queries, Why does asparagus make urine smell?
Blogs at Howstuffworks, Why does asparagus make my urine smell funny?
Nicole Solis, CHOW, Urinalysis
Breakfast at Nancy's, Asparagus - 30 Minutes to Stinkville
Springboard4Health, The Nutrition Notebook, Asparagine


  1. My friend Samantha can't smell it!

  2. I do know what you're talking about and have wondered about it for years! Thanks for finally clearing that up. I also agree that 40% seems a bit low, since everyone I know is our camp.

  3. Yes, yes, I'm one of the 40%.

    But what I really want to know is: what about Super Sugar Crisp? (Now known as Super Golden Crisp, I believe, since sugar is evil.)

    Do you think it's the same 60/40 split?

    I must go apply for grants immediately!

  4. First time I have been on your site. I think it is awesome. I will be bookmarking it. Thank many questions, so little time.

  5. My partner and I both produce and can smell the unpleasant urine odour after eating asparagus (15 minutes after eating). However, my partner's mother can't smell it (but produces it) nor can her husband (my partner's step-father) smell it, but he sure produces it! So, with a sample size of four, 100% produce the smell, but only 50% can detect the odour!

  6. Thanks for the data, Anonymous. I don't want to know how it is that you know the scent of the urine of your partner's mother and her husband, but I'll take your word for it that you do.

  7. My family (two adults, four kids) has never noticed any urine smell; we don't know if we produce it or not!

  8. The plural of anecdote is not data. One cannot simply ignore self-selection bias and declare it scientific.

    Writing a vaguely science-sounding blog is fine, but you seem to suffer from notions of grandeur with regards to your own, erm, "research".

  9. I certainly don't feel grand at all. I mean, look at the subject. I am also definitely not declaring this blog to be scientific. Maybe instead of pointing fingers and frowning, cracking a smile or two would be better.


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