Why Does Urine Smell After Eating Asparagus?

There are two kinds of people in this world: those who can eat asparagus without worrying about pungent urinary after-effects … and those who can’t. Put more simply, some people will notice that their own urine smells a little “funny” after ingesting asparagus, while some people won’t. To further complicate matters, there are also those who are more adept at detecting the abnormal smell than others, meaning that just because you can’t smell the weird odour in your own urine after eating it, doesn’t mean it’s not there.

How do we solve this age old question? 

Well, stopping short of asking a neighbour or colleague to take a whiff of your wee – always an unpleasant subject to raise – the amazing world of science has come up with an amazing way of differentiating between urine samples which contain the tell-tale smell. Not only that, the innovative technique aims to shed further light on the topic, not only identifying the asparagus-scented urine but also the reasons behind its scent. 

Chromatography, or the act of separating out a compound into its various components, has been used in the realm of the laboratory for years now. By converting a substance from a solid or liquid into gaseous form, scientists are able to analyse the conversion, and from this information, extrapolate information about its constitutional makeup. To learn more about the specific terms that are employed when discussing this process, check out the article: A Guide to Basic Chromatography Terminology.

Back in 1987, a study by the University of Birmingham decided to try and isolate the pungent element in asparagus-urine using the sophisticated techniques of chromatography. By vaporising the asparagus-tainted urine sample, they broke it down into its various parts and noted that the likeliest culprits behind the smell were methanethiol, dimethyl sulphide, dimethyl disulphide, bis-(methylthio)methane, dimethyl sulphoxide and dimethyl sulphone - which all have the faint smell of aging cabbage. Interestingly, these components are all volatile, meaning that they reach boiling point at room temperature, hence their ability to vaporise in your toilet bowl and find their way into your nostrils. 

Introducing Asparagusic Acid

Even more interestingly, all of these elements are unlikely to have been present in the asparagus prior to ingestion, since cooking would almost certainly have destroyed their delicate properties. Therefore, the element that is present in the asparagus to begin with must be hardy enough to withstand boiling or roasting, but flimsy enough to be broken down by the body’s digestion system. Step forward the red-herring-sounding “asparagusic acid”, a chemical which to the best of our knowledge is only present in asparagus. After being broken down by our bodies, this combines to release those compounds, which in turn return to the outside world as “interesting” odours.

As for those who don’t seem to produce the smell, the evidence gleaned from investigations is mixed. Whilst it is possible that some people don’t produce the odour altogether, the likelihood is very small. Much more plausible is the idea that they do produce, but simply can’t detect the asparagus musk, an anomaly which has been traced down to a simple genetic mutation in the olfactory receptors.

So now we know the answer to why (most) urine smells like asparagus after consumption. Whilst interesting, it is hardly the most pressing issue in the world – but fear not, chromatography is also addressing much weightier issues. In the same unpleasant realm of human waste, recent breakthroughs have led to the detection of ailments via the analysis of excrement. For more information, see the article: Stool Sample Smells Used to Identify Bowel Diseases. 

Image Source

• Asparagus via John