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!


Sources
Wise Geek, Why Does Asparagus Make Some People's Urine Smell Funny?
The Straight Dope, Why does aparagus make your pee smell funny?
ilovebacteria.com, 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