FoundationalConceptsNeuroscience

Many have enjoyed eating asparagus and have recognized the char-
acteristic “asparagus pee” smell in their urine shortly after having
consumed the vegetable. However, others who are reading this have
not been aware of any unusual smelling urine after eating this spear-
like vegetable. There are two reasons for this. First, there is variability
in the amount of stinky thiols and sulfides generated during digestion
after eating asparagus. Some people produce very little; some produce
lots. Second, a substantial number of people lack olfactory sensitivity
to this asparagus-pee aroma, or at least have a much lower sensitivity
for detecting it. This is an example of what is called a specific anosmia
—loss of sensitivity to a specific kind of smell.
The most likely cause of a specific anosmia is genetic variation in
one of the 350 olfactory GPCRs. Because there are so many different
olfactory receptor genes, it is highly likely many specific anosmias are
present among people. Usually such anosmias would go unnoticed. It
is only because of the particular circumstance of asparagus pee—such
a novel and distinctive aroma, so obvious to some and yet unnoticed
by others at the same dinner gathering—that this anosmia has drawn
particular attention. The asparagus-pee anosmia has been associated
with variability in a single identified olfactory receptor gene.
Specific anosmias refer to loss of sensitivity to specific categories
of aroma. A general anosmia is loss of sensitivity to a large variety
of aromas, in some cases even a complete lack of olfactory sensitiv-
ity. General anosmias have a variety of causes, ranging from nasal
congestion to unknown developmental factors to head trauma to
degenerative brain disease. Hyperosmias are also possible, in which
there is an increased sensitivity to odors. Hyperosmias often appear
transiently, in association with a migraine headache, for example.
Women sometimes report hyperosmias during pregnancy.