Scientists sniff out mystery behind asparagus smell in urine

Asparagus is pictured. (Dani Vincek /shutterstock.com)

Scientists have sniffed out the reason why some people think their pee has a pungent smell after eating asparagus while others do not -- it is all down to genes.

French novelist Marcel Proust once fancifully described the smell as "transforming my humble chamberpot into a bowl of aromatic perfume" but it seems he was in the minority as three in five people are unable to even detect the odour, according to a study published Wednesday.

From data on 6,909 participants, 58 per cent of men and 61.5 per cent of women had asparagus anosmia, or the inability to detect the smell, according to the results in the BMJ medical journal's Christmas edition, traditionally reserved for studies that are quirky but scientifically sound.

Among this majority, researchers from the Harvard School of Public Health found 871 genetic coding variants, or SNPs, on genes associated with smell -- suggesting the inability is inherited.

Professionals have long debated the cause of the pungent aroma that only some people detect in their urine a few hours after eating the vegetable.

Does the difference arise because some people do not produce "asparagus pee," or because they are simply unable to smell it?

The new research goes some way towards answering the question.

"This study was conceived during a scientific meeting attended by several of the co-authors in bucolic Sweden, where it became apparent that some of us were unable to detect any unusual odour in our urine after consuming new spring asparagus," wrote the team.

They went in search of existing studies on the phenomenon, and found two U.S.-based projects in which participants were asked about "asparagus pee" in a broader health questionnaire in 2010.

Using that data for their own analysis, the team classified "asparagus smellers" as people who "strongly agreed" that they discharged a distinctive odour after eating asparagus.

The rest were listed as suffering from asparagus anosmia.

The team called their study: "Sniffing out significant 'Pee values': genome wide association study of asparagus anosmia" -- a play on the term "P value" used to denote statistical significance in scientific studies.

With one question closer to being answered, many still remain, said the authors.

If asparagus is packed so full of nutrients, why would it make some people give off a smell that may put them off ever eating it again?

What drove the evolutionary selection that caused some to lack the asparagus-smelling gene variants?

"And, will scientists take the results of our study and apply gene editing techniques to convert smellers to non-smellers?" asked the team.

More research is needed, they said, "before considering targeted therapies to help anosmic people discover what they are missing."