A Gene for Smelling Asparagus in Urine?

by Emma Steppe on November 26, 2013

Throughout this semester, I have enjoyed delving SONY DSCdeeper into the science behind important health research and blogging about it. Most of the time, research provides insight and implications for improving health in the real world. However, sometimes science findings are just, for lack of a better word, weird. For example, in the past several years, new gene technology has reopened research into the sulfurous odor of urine after eating asparagus, often described as smelling like boiling or rotting cabbage. Research suggests that individuals may differ in their ability to both produce and smell this odor, and that there may be a specific gene responsible for this ability to smell asparagus pee.

The direct-to-consumer genetic testing company, 23andMe, allows consumers to spit into a kit, send it in for analysis, and receive their personal genetic information. Researchers sent out additional surveys to 23andMe consumers about specific traits, such as the ability to smell an odor in their urine after eating asparagus (for the rest of this piece, I will refer to the odor as “asparagus odor”).

The question was: “Have you ever noticed a specific odor when you pee after eating asparagus?” and the results were as follows:

  • Yes: 53%
  • No, what are you talking about?: 33%
  • I don’t eat asparagus or have never had asparagus: 14%

By comparing the 9,126 correspondents’ answers to this question with their genetic results, researchers were able to isolate a single nucleotide polymorphism (SNP) that was associated with the likelihood of being able to smell the asparagus odor. The SNP is located in a cluster of olfactory (smell) receptor genes, which are responsible for the ability to detect various odors.

Another group of researchers that same year (2010) decided to perform an experimental study to look further in the identified SNP and to determine whether variation existed only in the ability to smell asparagus odor, or in the actual ability to produce it. Research from the 1980’s is divided on this issue. A 1980 study claims that the difference lies in the ability to smell, while one from 1987 claims it lies in the ability to produce. Since this research was performed decades ago, Pelchat et al. decided to re-visit the debate and perform a study with more rigorous experimental methods and the ability to verify with genetic tests.

38 participants provided urine samples before eating asparagus, after eating asparagus, and after eating bread (on a different day from eating the asparagus). Participants then smelled a series of samples, including their own, either pre-asparagus, post-asparagus, or post-bread. Several of the subjects who provided urine samples ended up not participating in the smelling phase because they had such a strong aversion to smelling urine. The remaining participants were asked if they were able to smell the asparagus odor in each sample. 8.1% of the participants had post-asparagus urine that other participants couldn’t smell the asparagus odor in, indicating that they didn’t produce the odor. 6.4% of the participants weren’t able to distinguish between pre- and post- asparagus samples, indicating the inability to smell the odor. Cheek cells were collected from all participants for genetic testing, and once again, the same SNP from the previous study was found to be associated with the ability (or inability) to smell the asparagus odor.

What do these findings mean? It seems that individuals vary in both their ability to produce and smell the asparagus odor in urine. Genes may be responsible for the ability to smell the odor, but genes haven’t been found to be responsible for producing it (at least not yet). The practical application of this genetic variation is unknown, but the topic has been of interest to humans for centuries, as is evident by a quote from Benjamin Franklin:

“A few stems of asparagus eaten shall give our urine a disagreeable odor.”

While the significance of the results of these studies are as yet undetermined, it is interesting to note the difference in the methods of the two 2010 studies. The first relies on surveys and genetic data from a direct-to-consumer genetic testing company, whose methods of genetic testing have uncertain accuracy. In fact, just this week, the FDA ordered 23andMe to stop sales of its genetic testing kits . The FDA believes that given the uncertain accuracy of the testing procedures and the possibility of false positives, 23andMe cannot claim to identify or prevent any health risks. The second study employs an experimental study design in addition to genetic testing, but has a much smaller sample size (38 vs. 9126). As technology advances, and if accuracy can be proven, data collected from direct-to-consumer genetic companies may provide a new avenue for studying genes on a widespread scale. But it’s important to remember limitations of this, and all research, and to interpret scientific findings with a critical eye. For now, I leave you with these words from the movie Elf.

video source: YouTube user xSPFLDxOHx retrieved from http://www.youtube.com/watch?v=U_ZoEqVfIKU

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