If you smell a distinct whiff when you wee after eating asparagus, blame your genes.
A new study has identified the genetic origin that determines whether one can smell the strong, characteristic odour – because not everyone can.
Researchers studied multiple genes involved in our sense of smell, finding hundreds of variants in the DNA sequence that are strongly associated with the ability to pick up the odour.
But, contrary to what you may believe, it’s not the asparagus that you can smell.
The distinct odour is in fact the smell of a metabolite, a substance produced during metabolism.
Not everyone can detect the odour of metabolites produced by consumption of asparagus.
The researchers, led by Sarah Markt and Lorelei Mucci at the Harvard TH Chan School of Public Health, set out to determine whether genetic factors are important in the ability to smell the odour.
Their study involved more than 6,000 men and women from two cohorts.
Among them, 40% of participants agreed that they could smell a distinct odour in their urine after eating asparagus, while 60% said they could not and were labelled as “asparagus anosmic”.
The researchers discovered 871 particular variations in DNA sequence which were associated with being asparagus anosmic.
These genetic variants were found in several different genes responsible for sense of smell.
They also found that a higher proportion of women reported they were unable to detect the odour, compared to men, despite women being known to more accurately and consistently identify smells.
The researchers suggested that this unexpected result might be due to under-reporting by a few modest women, or because they might be less likely to notice an unusual odour because of their position during urination.
The authors explained: “Our findings present candidate genes of interest for future research on the structure and function of olfactory (sense of smell) receptors and on the compounds responsible for the distinctive odour produced by asparagus metabolites.”
They said further research is needed to determine why some people have these genetic variants.
They also noted that asparagus provides a rich source of iron, fibre, zinc, folate, and vitamins A, E and C and is thought to reduce risk of cancer, cognitive impairment and cardiovascular related disease – so don’t let a little smell put you off eating it.