WHY DOES ASPARAGUS MAKE YOUR PEE SMELL?

If you’ve ever eaten asparagus, you may have noticed that, soon after ingestion, it imparts a strange, somewhat unpleasant scent to your urine. But then again, you may not have noticed anything at all — and there’s a scientific explanation for that too.

The chemicals that cause the effect are all suspected to stem from just one chemical compound: asparagusic acid (which is found naturally only in asparagus—hence its name). It has been pinpointed as the probable source of several organic compounds that have been shown to affect the odor of urine.

When we eat asparagus, the asparagusic acid molecules contained within the vegetable are broken down by digestion into a number of sulfur-containing organic compounds. In studies, a technique known as gas chromatography-mass spectrometry was used to analyze 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 odor-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 likely modified the aroma to give it a “sweet” edge.

The human nose is very sensitive to thiol compounds— a concentration as low as a few parts per billion is enough for us to be able to detect them. To give you an idea of how bad thiol compounds can smell, they’re also found in skunk spray. So, the increase in concentration of these compounds in urine after eating asparagus goes a long way toward explaining why the effect is so potent. The odor is detectable remarkably quickly after eating asparagus, within 15 to 30 minutes.

Interestingly, the ability to smell the aroma of “asparagus pee” is not universal. Research has shown that a proportion of people are unable to detect the change in smell, with one study finding 2 out of 31 people were unable to detect a difference in odor after eating asparagus. It was initially thought that everyone produced the odor, but only some could smell it; however, it has since been suggested, after a range of research, that not all people exhibit the effect after eating asparagus, with another study placing the figure of people who do produce “asparagus pee” at 43 percent.

By Andy Brunning in "Why Does Asparagus Make Your Pee Smell?", Ulysses Press, USA, 2016, excerpts p.37. Adapted and illustrated to be posted by Leopoldo Costa.