Recent research has identified the genetic origin of the ability to smell the odor in human urine produced after consuming asparagus.
Previous studies have shed some light on the issue of who can and cannot detect the distinctive sulfurous odor of asparagus pee.
Researchers were initially unsure of the reason why some people are unaffected by the smell. They hypothesized that certain individuals might lack the ability to smell or produce the odor, or that absence of aroma could be down to a specific loss of the sense of smell - known as "asparagus anosmia."
The substance produced and expelled in urine through metabolizing the vegetable is called asparagus metabolites. This is made up of methanethiol and S-methyl thioesters.
It is thought that everyone who eats asparagus produces this asparagus-tainted urine. Researchers say that these metabolites create "a rather malodorous bouquet."
People who are unable to smell asparagus metabolites in their own urine are also unable to detect it in the urine of individuals that are known producers of asparagus metabolites. This suggests that anosmia may be the most likely explanation.
In a bid to find out if there are genetic factors involved in the ability to either smell or not smell asparagus metabolites, a team of researchers from the United States and Europe conducted a new study and published the results in the Christmas edition of The BMJ.
Link between genetic variation, asparagus anosmia explored
Led by Sarah Markt and Lorelei Mucci - at the Harvard T.H. Chan School of Public Health, MA - the research team carried out an analysis of 6,909 men and women of European-American descent who were involved in two long-term research studies: the Nurses' Health Study and the Health Professionals Follow-up Study.
Participants were asked to respond to the prompt: "After eating asparagus, you notice a strong characteristic odor in your urine."
People who responded with "Strongly agree" were categorized as being able to smell asparagus and those who responded "Moderately agree," "Slightly agree," "Slightly disagree," "Moderately disagree," and "Strongly disagree" were categorized as having asparagus anosmia.
The researchers then explored the association between genetic variation and the asparagus anosmia trait in more than 9 million genetic variants.
Markt, Mucci, and colleagues identified hundreds of variants in the DNA sequence - across multiple genes involved in the sense of smell - that are strongly linked with the ability to detect asparagus metabolites.
The investigation revealed 871 variations in DNA sequence particularly associated with being asparagus anosmic. These variations, known as single nucleotide polymorphisms (SNPs), were located on chromosome 1 - a chromosomal region that contains multiple genes connected to the sense of smell.
The study authors point out that the discovery of these SNPs provides scientists with future research routes that may uncover the genetic structure and function of the overall sense of smell. "Future replication studies are necessary before considering targeted therapies to help anosmic people discover what they are missing," they note.
More women than men noseblind to asparagus metabolites
The findings show that 40 percent of participants strongly agreed that they could smell a distinct odor in their urine after eating asparagus, whereas 60 percent could not and were therefore branded "asparagus anosmic."
A higher proportion of women (62 percent) than men (58 percent) reported that they could not smell the odor. The researchers are dubious of this result as they say that women are known to more accurately and consistently identify smells.
The team proposes that this unexpected result might be due to a few modest women who did not want to admit that they could smell the odor, or that due to the female position during urination, they may be less likely to notice an unusual scent.
"Outstanding questions on this topic remain. First and foremost perhaps is: why such a delicious delicacy as asparagus results in such a pernicious odor, and what are the selective pressures driving genetic variations that lead to asparagus anosmia?"
Despite the disagreeable odor, the researchers urge people to continue feasting on spears of crisp-tender asparagus this holiday season due to the vegetable's potential health benefits, and to generate a provocative discussion with your loved ones about the "filthy and disagreeable smell in the urine."
They note that asparagus provides a rich source of iron, fiber, zinc, folate, and vitamins A, E, and C, and that consumption may reduce the risk of cancer, cognitive impairment, and cardiovascular-related diseases.
Harvard T.H. Chan researchers call for studies to "consider using these identified single nucleotide polymorphisms to better understand how a lifetime of eating asparagus might protect people from developing chronic conditions."
The team concludes by advising to serve the asparagus leaves as well, in order "to protect the liver against toxic insults so that you can enjoy your holiday nights and potentially alleviate that hangover the next day."