The Asparagus Pee Smell Gene (Chemistry Behind the Scenes)

You probably have experienced yourself, or at least heard of a funny smell present on your urine after eating asparagus. Then you must know that the so-called asparagus pee smell gene is behind this.

I hear the same discussion pop up form time to time among both chemists and non-chemists: why does some people’s urine smell funny after eating asparagus?

A lot of people claim to never have experienced this phenomenon. Then, they incorrectly assume that it is because they do not metabolize the chemical components of asparagus the same way. This is not completely true.

But for starters, what are the chemical components of asparagus?

The Chemical Composition of Asparagus

Chemophobia is present all over the place these days. People don’t realize that everything is made up of chemicals. A lot of badly-informed people would be shocked if they took a quick look at the actual chemical composition of completely natural foods, such as asparagus.

Check out the ingredients list of asparagus! This picture was taken out of the book called “Molecules: The Elements and the Architecture of Everything“. This is a great chemistry reading, by Theodore Gray and Nick Mann.

But out of that huge list, we actually just need to look at one ingredient: asparagusic acid.

What is Asparagusic Acid, and Does it Make our Urine Smell?

Asparagusic acid is a carboxylic acid which has a disulfide group as a part of a five membered heterocyclic ring.

Even though asparagusic acid is a organosulfur compound, it doesn’t really smell bad. As a matter of fact, asparagusic acid is just the precursor of different sulfur-containing volatile metabolites.

Usually, compounds such as carboxylic acids have low volatilities. For us to detect a smell, we need the responsible compound to be airborne.

The boiling point of asparagusic acid is 324 ºC, so even if it really smelled bad, it would have a hard time reaching our olfactory receptors. Actually, this compound was both isolated and prepared synthetically in a chemistry lab, and it does not show the typical asparagus pee smell.

Total Synthesis of Asparagusic Acid

A convenient laboratory total synthesis of asparagusic acid was reported by Yanagawa and co-workers in 1973. They started by treating a readily available diethyl malonate diol derivative with hydrogen iodide. This allows carrying out, at the same time, iodination of both alcohols, and a single decarboxylation.

After hydrolysis of the resulting monoester, the first intermediate with two iodines is obtained. Reaction of this intermediate with sodium trithiocarbonate (Na₂CS₃) gives the corresponding dithiol after treatment with sulfuric acid.

Finally, oxidation using DMSO at high temperature gives asparagusic acid. This organic synthesis, leads you to the desired compound in just three steps.

We have confirmed that asparagusic acid is not smelly. So, what does actually cause the asparagus pee smell?

When we eat stuff, we metabolize its chemical components, and the (bio)chemical transformations that take place in our body, generate new chemical compounds. The compounds generated after eating and processing asparagusic acid, together with the asparagus pee smell gene, are the two players that cause this unpleasant sensation for many people.

What is the Asparagus Pee Smell Gene?

So, our bodies break down asparagusic acid into smaller sulfur-containing molecules, which are smelly? That’s it?

Reality is, there is a gene that controls if each human being is able to smell or detect the metabolites that make asparagus urine smell. This is what we call the asparagus pee smell gene.

We have already covered this briefly in our list of explanations to 100 fun chemistry facts, but we felt that an entire post for this one may be useful and interesting.

This gene makes the whole story complete. Not all of us can smell the asparagusic acid metabolites. So even though it seems like not all of us produce them, we actually all do! It is just that some of us cannot smell them.

You can read further in this study published in Chemical Senses. The summary is that, actually, there are individual differences in both the production AND detection of the cabbage-like smell of asparagus-pee. The biology associated with metabolite production is mostly unknown. However, the inability to smell these metabolites is identified in association with an individual nucleotide polymorphism. This is what we refer to as the “asparagus pee smell gene”.

If you look really closely, it seems that there are many variations of these two traits: some people actually produce very small amount of these compounds, which can be enough to be detected by some, but not by others. So it is not a matter of black or white, there are intermediate situations.

A recent online survey with almost 3000 participants, showed that around 70% of the population can detect the smell in their own urine after eating asparagus.

Which Are the Chemicals that Actually Smell Bad in Our Urine?

A lot of small organic molecules containing sulfur atoms have a particular unpleasant smell.

For example, tert-butylthiol is used in very small concentrations to give natural gas (which is odorless by itself) a detectable smell. But in higher concentrations, I can tell you that it is extremely unpleasant. I’ve seen labs evacuated for the day after a few microliters of the stuff were dropped out of a fumehood.

Not tert-butylthiol, but different volatile compounds are produced while metabolizing asparagusic acid. Many structures have been proposed over the years. As far as 1891, it was proposed that methanethiol was the responsible of this smell. More recently, gas chromatography analysis allowed detection of different volatile organo-sulfur compounds arising from the metabolism of asparagusic acid:

Some compounds showing the functional groups in the table above, smell extremely bad in high concentrations. However, our ability to detect them in the amounts in which are metabolically produced by our bodies, depends on this asparagus urine smell gene.

I Got the Asparagus Pee Smell Gene: Can I Neutralize Asparagus Smell?

Maybe you enjoy eating asparagus. But also maybe, thanks to the asparagus pee smell gene, you are unlucky enough to be able to detect very low concentration of the sulfur metabolites.

To be fair, most people report the fetid odor going away quite quickly: you will naturally pee once or twice with the smell before it disappears.

Disappointingly, there is no clear way of making the smell go away quicker. The rate at which you get rid of metabolites through your urine depends on your glomerular activity (kidney filtration rate). This is not really accelerated by drinking more water or by other home-remedies.

However, drinking a lot of water will definitely dilute your urine and turn the smelly sensation milder.

But there is no big short-cut other than not eating asparagus!

Are There Other Foods that Make Your Urine Smell?

Asparagusic acid is a compound that is found almost only in asparagus. But other foods, such as Brussels sprouts, also have sulfur-contaning precursors that give rise to smelly metabolites.

Other foods can act as dyes for pee, making your urine colored. For example, don’t be surprised if your pee turns pink/red after eating beets or blackberries!

If you are interesting in other food-related stories such as these, you really need to get a copy of “Why Does Asparagus Make Your Wee Smell? And 57 Other Curious Food and Drink Questions” by Andy Brunning.

This is a great read for any science student or enthusiast. It can be enjoyed by all audiences: from kids at school to professional chemists. We were really inspired by that book for writing this post.

Asparagusic Acid and the Asparagus Pee Smell Gene: Conclusion

In conclusion, there is a combination of two factors that allow you to smell this annoying “asparagus urine smell” or not:

Asparagusic acid gets metabolized to different extent by different individuals, giving smelly volatile organo-sulfur compounds. On the other hand, there is a gene that determines our detection threshold for these compounds. Some people detect very low concentrations, and others are just lucky and simply can’t detect them!