This question has baffled people for years. Why does asparagus make our wee smell strange, but also why does the phenomenon only affect some people, with others unable to tell any difference?
Asparagus, or Asparagus officinalis, is a spring vegetable that used to be regarded as a huge delicacy because of a short growing season. We can of course now buy asparagus all year round because of our global economy, but in the UK it grows locally between April and June. People only tend to eat the young shoots, as the older asparagus can become quite woody.
So why does this vegetable cause so much controversy? It’s all down to the rather off-putting odour that some of us experience after eating asparagus then going for a wee.
- Mmm. Tasty. Just make sure you open a window after a “comfort break”.
The answer is actually two-fold. Firstly, asparagus contains asparagusic acid. When we digest the vegetable, we break this acid down into sulphur-containing compounds that are excreted through our urine and that give off a very unpleasant smell.
These compounds are volatile. This means they can vaporise and enter a gaseous state at room temperature, which allows them to travel from a person’s urine into the air and up into the receptors in their nose.
The ability to produce these volatile compounds, to a level high enough for detection, seems to vary amongst individuals and might suggest why some people can detect the unpleasant sulphurous smell in their own or others’ urine and others cannot.
However, it’s not as simple as that. Not everyone can smell the compounds released into the urine. A recent study published this month in the BMJ (Sniffing out significant “Pee values”: genome wide association study of asparagus anosmia) suggests that the ability to smell asparagus in urine may be down to genetics. In a study of nearly 7,000 people, 60% were unable to smell the asparagus (a condition known as asparagus anosmia, which was slightly more common among men than women).
The scientists believe the reason could be due to “genetic variations near multiple olfactory receptor genes”. This essentially means there’s variation in olfactory receptor DNA which might mean functionality is reduced and hence an individual may be unable to detect the compounds, leading to some people being able smell the compounds after going to the loo and others unable to.
If you want the detail, scientists identified “missense single nucleotide polymorphisms” or mutations in DNA that were marginally associated with asparagus. Of these, three single nucleotide polymorphisms were classified as “probably damaging” to the individual's ability to detect the smell of asparagus.
Further research is needed to learn more about how these differences in our DNA inhibit the ability to smell to asparagus. But for the time being, it appears to be due to our genetic ability to smell the odour – and potentially our ability to produce it.
Alexis Poole is Company Nutritionist for Spoon Guru, which finds foods and recipes to match your unique dietary needs.