Why Does Asparagus Make Urine Smell? – The Chemistry of Asparagus

02/10/2014

Asparagus chemistry

The latest food chemistry graphic looks at the chemistry of asparagus – specifically, why it causes the urine of some (but not all) people to smell. Over the past forty years several papers have been published on the subject, and several studies undertaken, to try and determine the chemical compounds responsible, and though there is still no definitive verdict as to the manner in which these compounds are formed, it has been suggested that they all form from asparagusic acid.

Asparagusic acid is, unsurprisingly considering the name, a chemical found exclusively in asparagus, and absent in other related vegetables. This has made it an obvious candidate for being the origin of the peculiar effect that asparagus has on urine. It has been suggested by recent studies that it could be metabolised in the body to produce the volatile compounds found in the urine after consuming the vegetable.

Gas chromatography-mass spectrometry was used to analyse the ‘headspace’ of urine produced after consumption of asparagus. The headspace is the gas space immediately above the liquid surface, which is occupied by light, volatile compounds in the liquid, and analysis of this is useful in identifying odour-causing compounds. The analysis of the post-asparagus urine showed the presence of several compounds that were not present, or present in negligible amounts, in normal urine. The primary compounds present, in quantities a thousand times greater than in normal urine, were methanethiol and dimethyl sulfide. The compounds dimethyl sulfoxide and dimethyl sulfone were also present and it was suggested that they modify the aroma to give it a ‘sweet’ edge.

Interestingly, the ability to smell the aroma of asparagus-influenced urine is not universal. Research has shown that a proportion of people are unable to detect the smell; in one study, 2 out of 31 people were unable to detect a difference in aroma. As well as this, it has also been discovered that not all people produce smelly urine after eating asparagus, with one study placing the figure of people who do produce ‘asparagus urine’ at 43%. Research in this area is still patchy, and is yet to provide a genetic link to these two phenomena; however, it has been suggested that ability to excrete asparagus-influenced urine may be coded for by a single gene.

To download the graphic as a high resolution PDF, click here. If you want to read more on the various research surrounding ‘asparagus urine’, a number of links are provided below.

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References & Further Reading