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Q: Why does my pee smell when I eat asparagus? – D.T., Rutland, VT

A: OK, maybe this isn’t an important scientific question, but it’s kinda of interesting. So here’s the science behind asparagus pee.

People have known for a long time that asparagus causes the urine of many (but not all) people to smell pungent. Benjamin Franklin famously wrote about it, stating that,” a few stems of asparagus shall give our urine a disagreeable odor.” Asparagus pee stinks, but asparagus itself doesn’t have a similar smell, even when cooked, which suggests that the chemical responsible for the smell is a metabolite – something made in our bodies out of something in the asparagus. This is definitely true, asparagus contains a chemical boringly-named “asparagusic acid.” Asparagusic acid contains sulfur, which is a stinky element, and is responsible for the rotten egg smell (among others). Once most people eat asparagus, they metabolize aspargusic acid to several small volatile chemicals, one of which, methanethiol, is believed to be the major source of the smell. Methanethiol, being a volatile chemical, has a low boiling point and therefore is freely released as a gas into the air, hence the strong smell even several feet away from your toilet. This metabolism occurs rapidly – you can generally detect the smell about 30 minutes after eating asparagus. Besides methanethiol, there are several other stinky sulfur-containing metabolites found at lower levels, including dimethyl disulfide, dimethyl sulfone, and 2,4-dithiapentane.

For many years it was believed that a large percent of the population did not metabolize aspargusic acid to methanethiol (and the other sulfur compounds) and hence did not experience the strong smell. It turns out this was only partially true. There are definitely many people who don’t produce the smell at all, likely due to currently-unidentified differences in their metabolism. However, there are also a significant number of people who simply cannot smell the metabolites of asparagusic acid even when they are present. This was traced to a single mutation in a gene involved in olfactory function, identified by the genetic testing company 23andMe. This breakthrough was confirmed in a 2011 study that involved asking people to smell not only their own urine, but that of other people who had eaten asparagus. Science isn’t always sexy, folks.

The exact percent of people that don’t experience the smell, either because they don’t metabolize asparagusic acid or are incapable of smelling the metabolites is a bit unclear at this time. In the 2011 study, only 8% of subjects failed to produce the odor, while only 6% failed to detect it. Other older studies have reported much higher numbers of people who don’t experience the smell – up to 50% in one study. Based on the results across populations with different ethnicities, it seems likely that there is quite a bit of variation depending on your genetic background (in particular, people from China or Israel almost all experienced the smell, while people from England or the United States were less likely to notice it. The higher rate of people who don’t get the smell in these populations is likely due to a combination of the single gene variation (for smell) and the unidentified genetic factors affecting the metabolism of asparagusic acid.

Now, if you or someone you know doesn’t experience the smell, you can easily find out if you have the genetic variation. The simplest (and cheapest) way would be to find a friend who does experience the smell and ask to smell their urine. If you still smell nothing, it’s because you are incapable to experiencing the smell. If, however, you do smell the metabolites, then your metabolism is responsible. For those unwilling to smell someone else’s urine, you could just get the DNA test through 23andMe. The variation is on chromosome 1 and is officially termed “rs4481887.” The variation appears to be autosomal recessive, which means you need two copies of the variant gene in order to lose the ability to smell these chemicals. This means that if both you and your spouse are “non-smellers”, then your children will be too. If only one of you is, then your children have a 50/50 chance if your spouse is a carrier, and zero chance if they are not. Again, you’d have to get the genetic test in order to determine if you or your spouse is a carrier. That is, if knowing the chances of your children enduring the stench of asparagus metabolites for their entire lives is something that is important to you. Which would be kinda weird.