Asparagus is can have an odd, but harmless, side effect

You asked: Why does asparagus make your urine smell?

The vitamin filled veggie comes with an odorous side-effect
September 13, 2017

“A few stems of asparagus eaten, shall give our urine a disagreeable odor.” If you thought you’ve noticed a strange phenomenon using the bathroom after you’ve eaten asparagus, you’re not alone. The above quote was from Benjamin Franklin in 1781, proving that even the most brilliant of humans are not immune to the weird bathroom side-effects of asparagus.

Modern science has given us the answers to the questions that everyone is afraid to ask in public (but will Google in private), and an expert from the Texas A&M College of Medicine will answer the awkward question, too.

The science behind it

“We know what causes the odor,” said Gabriel Neal, MD, assistant professor at the Texas A&M College of Medicine. “There’s a couple of different substances that we’ve identified as the cause of it.”

The culprit to the odor is a curious pair of chemicals, methanethiol and S-methyl thioesters.  When asparagus is ingested and broken down in the body, these chemicals give urine its unpleasant sulfuric smell. There isn’t much mystery there. However, there is a mystery about who can and can’t smell the difference.

“Not everybody can smell it,” Neal said. “Now there’s actually research that looks into somebody’s ability or inability to smell it.”

Asparagus Anosmia

In late 2016, a study was published in The BMJ (formerly the British Medical Journal) that sought to determine the factors associated with the ability to smell asparagus metabolites in urine. Almost 7000 people were used in the study and the results were intriguing. About 58 percent of men and 61.5 percent of women were unable to smell the asparagus metabolites and considered asparagus “anosmic”, or, unable to smell.

This raises some questions, because when it comes to sense of smell, one gender is keener than the other.

“In general, women can typically pick up a distinct scent better than men,” Neal said. “For more women to be more asparagus anosmic than men seems rather odd.”

This has become some sort of an anomaly to researchers, and there has been some speculation that subjects’ modesty could influence their response, but that has yet to be proven. There is also research that looks at a particular mutation in our DNA that may be the reason behind asparagus anosmia.

“The BMJ study looked at the DNA sequences on chromosome 1, where our sense of smell is found,” Neal said. “There are over 800 variations of single-nucleotide polymorphisms—that influence our ability to smell that. Everyone produces the chemicals, but it’s genetic mutation on chromosome 1 that determine whether we can smell them, and only 40 percent of people are good at picking it up.”

Add asparagus to your diet

Overall, asparagus is good for you and should definitely find its way into your grocery list. It is also high in folate, a water-soluble B vitamin, which has been linked to improving depression and protecting against certain types of cancer. It’s also filled with vitamins and minerals that make it a good side dish to go along with any meal.

“Asparagus is high in vitamin K, low in fat and calories and it has a lot of fiber that can keep you feeling satiated,” Neal said. “The weird bathroom quirk shouldn’t deter you from keeping asparagus and it’s benefits from your plate.”

— Dominic Hernandez

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