Asparagus odor detection
After ingesting asparagus, some people can detect an unmistakable aroma in their urine. Not all people have this ability, which is influenced by genetics.
The nasal epithelium contains a large number of olfactory receptors specialized in the detection of odor molecules. Using different combinations of receptors, humans can detect thousands of different odors. It is the availability of the different types of receptors that determines which odors can be detected or not.
When we eat asparagus, sulfur compounds (methanethiol and S-methyl thioesters) are produced in the body and excreted in the urine, giving it its characteristic odor. However, the recognition of the smell of asparagus in urine varies among people, some being unable to detect it. The most likely explanation is that these individual differences are due to a specific anosmia (loss of the sense of smell). Specific anosmias are common for biologically important odors, such as volatile steroid hormones, sweat and the odor of human urine in different nutritional states.
In some people, the receptors involved in detecting this type of odor are less functional and this may be genetically determined. However, specific anosmias are not all-or-none traits, but rather there are continuous ranges of odor detection and it is possible that this is also the case for the detection of asparagus odor. Some people are more sensitive than others, detecting the odor more easily while others are almost unable to detect it.
It appears that the ability to detect asparagus odor in urine, produced by chemical compounds metabolized after ingestion, is related to a single nucleotide polymorphism near the OR2M7 gene, which codes for an olfactory receptor. This genotype-phenotype relationship is similar to that of other olfactory receptor alleles that reduce the ability to smell androstenone and isovaleric acid. The polymorphism is found within a large group of olfactory receptors on chromosomal region 1q44. OR2M7 itself responds to other odorant compounds such as geraniol and cintrone.
This result is important because it informs us about a possible anosmia, i.e., a specific loss of the sense of smell due to loss of function in one or more olfactory receptors.
13.5 million variants
Pelchat ML, Bykowski C, Duke FF, Reed DR. Excretion and Perception of a Characteristic Odor in Urine after Asparagus Ingestion: a Psychophysical and Genetic Study. Chem Senses, 2011; 36(1):9–17.
Eriksson, Nicholas et al. Web-based, participant-driven studies yield novel genetic associations for common traits. PLoS genetics vol. 6,6 e1000993. 24 Jun. 2010.