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Urine luck? Genetics may explain why you can (or can't) smell 'asparagus pee'

Asparagus
REUTERS/Leonhard Foeger
French botanist and chemist Louis Lemerey in 1702: “[Asparagus spears] cause a filthy and disagreeable smell in the urine, as everybody knows.”

I thought I’d end this year’s Second Opinion column with a report on one of the studies from the BMJ’s “Christmas issue,” now an annual tradition at that prestigious, 176-year-old medical journal (formerly known as the British Medical Journal).

All the research studies in this special two-week edition are “real” in the sense that their authors used accepted scientific methods and submitted them to peer review. But the studies are conceived, written and published with tongue planted firmly in cheek. They are meant to make you smile.

Heaven knows we need a bit of humor in our lives these days. So, here is an account of one of the more quirky studies from this year’s issue.

‘Pee values’ and ‘asparagus anosmia’

“[Asparagus spears] cause a filthy and disagreeable smell in the urine, as everybody knows,” wrote the French botanist and chemist Louis Lemerey in 1702. Eighty years later, Benjamin Franklin made a similar comment. “A few stems of asparagus eaten, shall give our urine a disagreeable odour,” he noted.

Well, maybe. Or maybe not. For, as plenty of other less erudite people have observed in the ensuing years, not everyone is able to smell the chemical metabolites — methanethiol and S-methyl thioesters — produced in urine when asparagus is eaten.

A team of U.S. and European scientists decided to try to figure out why that is so — specifically whether genetic factors are involved. They surveyed 6,909 men and women who were participating in two long-term studies. They found that 40 percent of the respondents strongly agreed that they could perceive a “distinct odor” in their urine after eating asparagus, while 60 percent said they could not.

The researchers then linked the survey responses with genetic data. They found an association between “asparagus anosmia” (the inability to notice the smell) and 871 gene variants on chromosome 1. The variants were all located on genes responsible for the sense of smell, although the researchers were unable to identify which of the 871 mutations are actually responsible for the anosmia.

Women were slightly more likely than men to report having asparagus anosmia (62 percent versus 58 percent), which was surprising, say the researchers, as women have been shown in other studies to be more accurate and consistent at identifying odors. This unexpected gender difference may be due, the researchers write, to “underreporting by a few modest women who are loathe to admit they can smell the distinctive odor in their urine.”

It is also possible, they add, “that women are less likely than men to notice an unusual odor in their urine because their position during urination might reduce their exposure to volatile odorants."

Limitations, implications … and culinary tips

The study has several limitations. For example, the information about asparagus anosmia was self-reported through a survey, and therefore may not have been accurate. Also, everybody in the study was of European descent, so it’s unclear if the same genetic variants would predict asparagus anosmia in people with other ethnic backgrounds.

Still, the study’s results are interesting, including scientifically, as the researchers point out in one of more “serious” sections of their paper.

“Our findings present candidate genes of interest for future research on the structure and function of olfactory receptors and on the compounds responsible for the distinctive odor produced by asparagus metabolites,” they write. “Answering these questions might shed light more generally on the relation between the molecular structure of an odorant and its perceived odor.”

The researchers, though, quickly return to the holiday spirit of the BMJ’s Christmas issue, and end their paper with a culinary tip.  “This holiday season,” they write, “try [Marcus Gavius] Apicius’s asparagus recipe and generate a provocative discussion with your loved ones about the ‘filthy and disagreeable smell in the urine.’”

“Asparagus is not all contentious,” they add. “Make sure to serve the leaves as well to protect the liver against toxic insults so that you can enjoy your holiday nights and potentially alleviate that hangover the next day.”

Ho, ho, ho.

For more info: You can read the full study, “Sniffing out significant 'Pee values': genome wide association study of asparagus anosmia,” on the BMJ website.

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Comments (3)

Asparagusic Acid

It is so much fun to say... asparagusic acid. I thought I would share the joy with all this holiday season.

Yes, it is a real thing.

It's the sulfur that (some of) you smell.

From Wikipedia:

Asparagusic acid is an organosulfur compound with the molecular formula C4H6O2S2 and is systematically named 1,2-dithiolane-4-carboxylic acid. The molecule consists of a cyclic disulfide functional group (a 1,2-dithiolane) with a carboxylic acid side chain. It is found in asparagus and is believed to be the metabolic precursor to odorous sulfur compounds responsible for the distinctive smell of urine which has long been associated with eating asparagus.

Asparagus in polite conversation

One thing I've wondered about is if one doesn't detect the "...filthy and disagreeable smell in the urine..." after eating asparagus, is it possible that their personal metabolism is such that their own urine doesn't in fact smell? Has that ever been studied?

This year

Minnpost wasn't much of a factor in this years news coverage, and I wonder if it was because of too many stories like this. It's kind of crude, and certainly unreadable. It didn't engender much interest in the comments section, yet it's been featured on the front page for what seems like forever.

Maybe it's time for Minnpost to review some of it's editorial choices, and maybe get back to some basics.