Asparagus odor detection
The skin of the nose contains a lot of nerve cells covered with molecular sensors, called olfactory receptors. These receptors are specialized in detecting odor molecules. Using different combinations of more than 300 olfactory receptors, humans can detect thousands of different scents. The types of olfactory receptors determine what odors you can or cannot smell.
When humans eat asparagus, some people report a distinct odor afterward from their urine. However, the recognition of the asparagus odor in urine is not an all-or-nothing phenomenon; some people produce an asparagus odor that is easy to detect, and the presumption is that some people produce more odorant. Odorant production varies from individual to individual, and people with urine that does not have a detectable odor may produce it, albeit at a low concentration. Part of the difference in odorant production could also be due to the production by some people of less volatile variants. Also, some individuals are unable to detect asparagus odor.
Therefore, the most likely explanation is that these individual differences in odor detection are specific anosmia (loss of odor sense). Specific anosmias are common for biologically important odors, such as volatile steroid hormones, musk, and sweat, and the smell of human urine in different nutritional states, for example, after asparagus consumption.
One or more olfactory receptors respond to the asparagus odor but these receptors are less functional in some people. Differences in olfactory ability can be due to heritable variation in olfactory receptors. Although specific anosmias are often thought of as being all-or-none traits, thresholds are on a continuum and that is likely to be the case here. Some people are more sensitive than others, detecting the asparagus as if it were in higher concentrations than those normally found in human urine and, conversely, other people are almost unable to detect it.
Gene or region studied