As It Happens

Asparagus pee: Harvard scientists sniff out a genetic variation

Some people smell something funny in their urine after they eat asparagus, but not everyone can detect the odour. Harvard epidemiologist Sarah Markt was part of a team that attempted to unravel the mystery.
A new study in BMJ examines how some can detect a funny smell in their pee after they eat asparagus, when others cannot. (Julie Van Rosendaal)

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The genes of thousands of asparagus-eaters have been tested by Harvard epidemiologists. They wanted to find out why some people are graced with a total inability to smell the stench the vegetables leave in our pee. The study, called Sniffing out Significant "Pee Values": Genome Wide Association Study of Asparagus Anosmia was published this week in the BMJ. 
Researchers asked nearly 7000 people if they noticed a distinct smell in their urine after eating asparagus. About 40 per cent said yes, they could. (Cal Tobin/CBC)

Every year around Christmas, the prestigious journal publishes a few quirky scientific studies. This year, it was Pokémon Go ... and you guessed it, stinky pee.

One of the researchers in the latter study is Sarah Markt. She spoke with As It Happens host Carol Off.

Carol Off: Ms. Markt, how does a cancer epidemiologist wind up studying asparagus pee?

Sarah Markt: Yes, that's an interesting question! The idea for the study actually came up from a scientific meeting we were having in Sweden. We were having a delicious dinner of asparagus, and you know, kind of started chatting, and a conversation about the odour in urine that's produced after eating asparagus came up. And we realized that there were many amongst us that actually could not smell the odour produced.

CO: I think only medical people would have the smell of their pee come up during dinner. 

SM: Yes, it was an interesting conversation.

CO: So what is the smell, what does it smell like?

SM: To me, being a non-anosmic person, it's sort of a sulfury smell. 

CO: Ok, explain what anosmia means. 

SM: Anosmia means that you're unable to detect an odour. There are different anosmias to different compounds and so one of them is to asparagus, specifically the metabolites produced in the urine after eating asparagus. 

CO: How many people did you ask to sniff their pee for this study?

SM: We had information on just over 6900 men and women, and we found that 60 per cent of them reported that they could not smell the odour. 

CO: Was there a difference between the genders?

SM: In the men we found that 58 per cent of them reported that they could not. And, in the women we found 62 per cent.
Elna Edgar shows off some freshly-picked asparagus at the farm near Innisfail, Alberta in May, 2016. (Julie Van Rosendaal)

CO: Now, you also examined these people at a genetic level, and this is what's new in this, because people who have talked about the ability to smell asparagus pee before. What did you discover?

SM: We have genome-wide data on these same people who answered the question, and so we were able to link that data with their questionnaire data. We found there was genetic variation in the ability to smell asparagus. Specifically, we found there were over 800 single-nucleotide polymorphisms, or SNPS, which were associated with the inability to smell.

CO: What does that tell you?

SM: They were all on a similar region on chromosome 1 and most of them were in olfactory receptor genes. Basically, we showed there was genetic variation between people that leads to asparagus and anosmia.

CO: What would be the value to us, as animals, being able to detect this odour in our urine?

SM: I think that's a really good question. We pose that in our discussion. Why would, what I think is a delicious vegetable, and is a nutritious vegetable, asparagus, result in such a pernicious odour? And what are some of the selective pressures driving genetic variations that lead to asparagus and anosmia? I think that's sort of an open question.

This interview has been edited for length and clarity. For more on this story, listen to our full interview with Sarah Markt and read her study here.

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