We’ve all heard about how asparagus can make your pee smell.
Personally, I’ve never been able to notice a difference in how my urine smells after eating asparagus. I can’t blame a faulty nose, either, because my urine will tell me when I’ve drunk coffee.
As a result, I assumed I was someone who didn’t produce the odor.
Wrong. One night, years ago, my husband went into the restroom after me and said, “Oh. Stinks like asparagus.”
Apparently I DO produce that odor when I eat the stuff, but I can’t detect the odor in myself or others.
The other thing I’ve always heard is that this asparagus phenomenon has a genetic component. But that only raised more questions:
Does everyone produce that smell when they consume asparagus? Why can’t I smell it? What’s the genetic piece: producing the smell or having the ability to smell it?
In true Rogue Scientist form, I went looking to find out.
Turns out, as often the case with science, the answer was more complex than I expected.
Here are 5 interesting facts about asparagus pee:
Fact #1: Sulphur-Based Compounds Cause the Asparagus Pee Smell
Many scientists believe the odor results from asparagusic acid, a sulfur-based compound found only in asparagus. Often, anything with sulfur can lead to strange smells (rotten-egg smell, anyone?).
However, it’s still unclear which of asparagusic acid’s sulfur-containing metabolites cause the urine odor once the body breaks it down.
We do know these sulfur-containing compounds are “volatile” (they evaporate easily), which makes them easy to smell.
Fact #2: Not Everybody Can Smell It
If you’ve ever wondered what the heck people are talking about when they joke about asparagus pee, you’re probably one of the folks who can’t smell it.
One study in the British Medical Journal found that 58% of men and 61.5% of women (roughly 6 out of 10) could NOT smell the asparagus pee. According to a LiveScience article, other studies examining American populations found that 33-50% of people cannot detect the odor.
However… and this is interesting… studies from other countries found that most everyone in the studies COULD detect the odor. (See Table 3 in this 2011 article for a list of these studies.)
When you get different estimates in different geographical regions or populations for a biological phenomenon, you have to ask the next logical question:
It the ability to smell this stuff genetic?
Fact #3: Your Ability to Smell It (or Not) Has a Genetic Component
Your genes influence a LOT of things, and apparently the ability to smell (or not smell) asparagus pee is one of them.
One of the studies referenced in Fact #2 decided to look into the genetics of detecting asparagus pee smell. Scientists conducted a genome-wide association study (GWAS) to examine genetic factors associated with the ability to smell asparagus metabolites in urine.
A GWAS scans the entire genome of a large sample of people to find genetic markers associated with a particular trait. Think of it as a gene-hunting expedition. This study examined nine million markers (known as single nucleotide polymorphisms, or SNPs).
The study found 871 potential SNPs that showed possible association with the ability to smell asparagus pee. Even more interesting, all the genetic markers are located in a region on chromosome 1 that contains multiple genes for olfactory (smell) receptors.
Fact #4: Not Everybody Produces the Odor After Eating Asparagus
To make things even more interesting, not only can’t everybody smell asparagus byproducts in urine, not everyone produces the odor in the first place.
When I first looked into this particular angle, I found more questions than answers. Some scientists seem to disagree on this.
However, when I dug deeper, I found a review that makes an argument that the ability to produce the odor appears to have a genetic component, acknowledging that more research is needed in this area.
It’s not a crazy notion. Genes play a role in how we break down compounds in our bodies (ever heard of lactose intolerance?), and asparagusic acid is a compound.
Fact #5: Asparagus Pee Smells Differently to Different People (Even Benjamin Franklin)
Recognition of asparagus pee dates back long before modern science. References to it date back hundreds of years.
Benjamin Franklin called the odor “disagreeable,” while two writers from the 1700s described it as “powerful and disagreeable” and “fetid,” respectively. Praise indeed.
Marcel Proust was kinder in his assessment, stating that asparagus “transforms my chamber-pot into a flask of perfume.” Others have described the odor as vegetable-like or grassy.
Chances are, those genes on chromosome 1 play a role in these various reactions.
As for me, I’ll continue in my asparagus-pee-odor-free lifestyle.
Well, now you know all there is to know about asparagus pee. You’re welcome.
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NPR: We Unravel the Science Mysteries of Asparagus Pee