Why some people smell asparagus in pee, and others don’t

It has to do with your genes, not your nose, and now researchers know which ones.

Asparagus contains asparagusic acid, which our body quickly converts into sulfur-containing chemicals that stink. But not everyone can smell the foul smell. In fact, in their recent study, researchers at the Harvard T.H. Chan School of Public Health found that of their almost 7000 study participants, 60 percent were spared. Women were less likely to smell asparagus in urine than men.

The researchers discovered 871 variations in the participants’ DNA sequence, known as single nucleotide polymorphisms, on chromosome 1 which were associated with smelling, or not smelling, the vegetable residue. Sarah Markt tells us more about the team’s findings.

ResearchGate: How did you get the idea to study who can smell asparagus in urine?

Sarah Markt: The idea for the study came from a scientific meeting in Sweden. Some researchers from the US and across Europe were eating asparagus at dinner and a conversation about the odor in urine after eating asparagus came up and we realized that many among us could not smell the odor.

ResearchGate: How did you conduct your study? 

Markt: We asked a question about the ability to smell a strong characteristic odor in your urine after eating asparagus in two large prospective cohort studies, the Nurses’ Health Study and the Health Professionals Follow-Up Study. We linked this data with existing genetic data from the same people.

ResearchGate: What did you find?

Markt: We found that variation in olfactory receptor genes was associated with asparagus anosmia, or the inability to smell the distinct odor produced in the urine after eating asparagus.

ResearchGate: What could be the evolutionary benefit of smelling, or not smelling, the vegetable metabolite in urine?

Markt: The evolutionary benefit is a question we had as well and posed in the discussion of our paper - "why does such a delicious delicacy as asparagus result in such a pernicious odor? Why does genetic variation across the olfactory receptor genes exist that leads to susceptibility to asparagus anosmia? What selective pressures – one way or the other – would drive different populations of individuals to smell or not smell?"

ResearchGate: You found that women were less likely to smell asparagus in urine. Do you have any idea why this is the case?

Markt: The prevalence was slightly different in men and women, and we hypothesized that this could be due to underreporting by women, or potentially women are less likely to notice the odor due to their position during urination.

ResearchGate: Should the smell keep people from eating asparagus?

Markt: We did not evaluate whether people were more or less likely to eat asparagus based on their ability to smell in our study. Previous research has shown the general importance of olfactory receptor genes for taste preferences; however, we hope it doesn't prevent people from eating it – asparagus is a nutritious and we think delicious vegetable. Although we did not evaluate this in a scientific manner, anecdotally, we do not know anyone who avoids eating asparagus due to the smell.

ResearchGate: Do you have any asparagus advice for our readers?

Markt: This holiday season, try your favorite asparagus recipe and generate a provocative discussion with your loved ones!

Feature image: Liz West