Why asparagus makes your urine smell

On the whole, the evidence is mixed. Initially, a pair of studies conducted in the s with participants from France and Israel found that everyone produced the characteristic scent, and that a minority of people were simply unable to smell it.

More recent studies, though, suggest the issue is a bit more complicated. The most recent study , from , found that differences existed between individuals in both the production and detection of the scent. This peculiarity—which you might consider useful if you eat asparagus frequently—appears to stem from a single genetic mutation, a switched base-pair among a cluster of 50 different genes that code for olfactory receptors.

This is made up of methanethiol and S-methyl thioesters. It is thought that everyone who eats asparagus produces this asparagus-tainted urine. 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. 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 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.

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.

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? When your body metabolizes asparagusic acid, it produces numerous smelly, sulfur-based compounds that give your pee a rotten-like smell that can last 8—14 hours.

The effect of asparagus on urine scent is not universal, and a number of hypotheses try to explain this phenomenon. One hypothesis — called the production hypothesis — suggests that only some individuals are capable of producing the sulfurous compounds responsible for the smell, while others are non-producers.

This hypothesis asserts that non-producers lack a key enzyme that helps metabolize asparagusic acid and are thus unable to produce the smelly byproducts 4. The other hypothesis — called the perception hypothesis — states that everyone produces the smell, but some are unable to detect or perceive it 4. In this case, researchers found a genetic modification that alters one or more of the olfactory receptors that should respond to the asparagus smell, causing what is known as asparagus anosmia, or the inability to smell asparagus pee 8.

The asparagusic acid in asparagus produces many sulfurous byproducts that give your pee a rotten-like smell. The smell can be detected as early as 15 minutes after eating asparagus and may last up to 14 hours. Asparagus is low in calories yet rich in nutrients. The health benefits of asparagus include better digestive health, weight loss, healthy pregnancy….

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