Asparagus is considered a delicacy, but it's also known to produce a
distinctive odor in urine. Not everyone can detect the odor of
metabolites (methanethiol and S-methyl thioesters) produced by
consumption of asparagus.
In The BMJ
's Christmas edition, a study identifies
the genetic origin of the ability to smell the strong, characteristic
odor in human urine produced after eating asparagus.
‘Hundreds of variants in the DNA sequence across multiple genes are involved in sense of smell that are strongly associated with the ability to detect asparagus metabolites in urine.’
A team of U.S. and European researchers found hundreds of variants
in the DNA sequence across multiple genes involved in sense of smell
that are strongly associated with the ability to detect asparagus
metabolites in urine.
They say more research is needed to understand why such food results
in a particular odor, and what selective pressures would result in such
a significant genetic predisposition to be able to smell - or not smell
- the metabolites.
The researchers, led by Sarah Markt and Lorelei Mucci at the Harvard
T.H. Chan School of Public Health, set out to determine whether genetic
factors are important in the ability to smell the odor.
Their study involved 6,909 men and women of European-American
descent from two cohorts: the Nurses' Health Study and Health
Professionals Follow-up Study.
Findings show that 40% (2,748/6,909) of participants agreed that
they could smell a distinct odor in their urine after eating asparagus,
and 60% (4,161/6,909) said they could not and were labelled as
The researchers linked information from genome wide association
studies on over nine million genetic variants with the asparagus anosmia
They discovered 871 particular variations in DNA sequence, known as
single nucleotide polymorphisms, on chromosome 1 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 odor, compared to men, despite women being known
to more accurately and consistently identify smells.
The researchers suggest 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 odor because of their position during
Study limitations include self reporting of odor, rather than an
objective measurement, although this is unlikely to explain their
findings, and the sample focusing on people of European descent, so it's
unknown whether the same genetic variants predict asparagus anosmia in
The authors explain that "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 odor produced by asparagus metabolites."
"Future replication studies are necessary before considering
targeted therapies to help anosmic people discover what they are
They also note that asparagus provides a rich source of iron, fiber,
zinc, folate, and vitamins A, E and C, and consumption is thought to
reduce risk of cancer, cognitive impairment, and cardiovascular related
Therefore, they call for research to "consider using these
identified single nucleotide polymorphisms to better understand how a
lifetime of eating asparagus might protect people from developing