Genetics and Supertasting: A Pattern with Hidden Depths

By Zara Haider

Two years ago, researchers in Japan conducted a study to replicate the results of the original study that introduced the concept of “supertasting”, using a more refined method of measuring subjects’ taste sensitivity. 

The researchers decided to employ a conjunction of two methods: the modified two-alternative forced-choice (2AFC) procedure and the Quick Estimation by Sequential Testing (QUEST) method. 2AFC presents participants with two samples–one with a solution of a bitter tastant PROP and a nontasting solution–and tasks them to identify the tasting sample, after which the next set of samples’ concentrations are adjusted for researchers to identify their threshold at which they can taste a certain flavor. QUEST is a statistical, psychophysical tool that recommends adjustments to the concentration of the sample based on input data from previous samples to identify an individual’s threshold. While 2AFC can narrow down the range in which a person’s tongue can perceive a taste, QUEST expedites the conclusion. By this combination method, researchers have the support of a tool to verify/compare results.

The study starts by procuring saliva samples from the 118 participants to be used to identify their genotype of the TAS2R38 gene responsible for bitterness perception. After that, they start the test: tasting samples–making sure to rinse their mouths before sampling another solution– and having researchers use their hybrid method to dictate their next sample’s concentration.  

The study yielded a consistent pattern amongst the 3 common genotypes: PAV/PAV, PAV/AVI/, and AVI/AVI. Those with PAV/PAV are the most sensitive, having the lowest threshold; those with AVI/AVI have the highest threshold, making them insensitive to PROP; those with PAV/AVI have a threshold slightly higher than PAV/PAV. This confirms the results of the discoverers of the association between the TAS2438 gene and PROP sensitivity

Those same discoverers also found a relation between PROP sensitivity and fungiform papillae (FP) density–bitter-sensing taste buds. They’ve concluded that “nontasting” (high threshold) individuals have a low density of FP and that “supertasters” (low threshold) have a high FP density.

While the pioneers of the linkage are credited with expanding our knowledge of the science of tasting, a 2008 study (just 14 years prior) that revisits their work sheds new light: other unknown factors play into determining an individual's PROP bitterness threshold. 

Hayes et al. collected their data in two rounds. They first used 2AFC, and in the second round of testing, they repeated the 2AFC method but included duplicates of the solutions and used sound in between tasting new sets of solutions to “reset” the subjects' taste buds. 

The researchers found that FP density wasn’t correlated to genotype, with AVI/AVI and PAV/PAV capable of high and low density. 

A graph that plots each subject’s genotype and their FP density

Additionally, the relation between FP density and PROP bitterness threshold didn’t hold for heterozygotes. The authors of the 2008 study did mention that the original study in 1994 excluded “unusual” nontasters (heterozygotes), those who had a nontaster threshold but a high PROP ratio (bitterness of PROP to saltiness of NaCl). Those individuals were likely to have a high density of FP, thus skewing results to show a pattern of FP and PROP bitterness in the first place. 

The FP density vs. PROP bitterness graph shows no correlation amongst heterozygotes 

Lastly, while PROP bitterness threshold and quinine (another bitter tastant) are correlated, the genotypes show no pattern. Some PAV/PAV weren’t “supertasters”, some AVI/AVI were medium-level tasters. 

  Quinine vs PROP bitterness (3.2mM) graph, plot each subject with their respective genotype

While we have a foundational understanding of the role of genetics and our dietary preferences, it’s not to be confused as the sole determinant. Evidence has shown that there are exceptions to the commonly established pattern. Even if there wasn’t any information that challenged the notion, our cultural and environmental upbringings–what we grew up eating–have a significant role in how we perceive new tastes. So maybe we’re far from understanding the true cause of supertasting. 

Sources:

1. Aoki, K., Mori, K., Iijima, S., Sakon, M., Matsuura, N., Kobayashi, T., Takanashi, M., Yoshimura, T., Mori, N., & Katayama, T. (2023). Association between Genetic Variation in the TAS2R38 Bitter Taste Receptor and Propylthiouracil Bitter Taste Thresholds among Adults Living in Japan Using the Modified 2AFC Procedure with the Quest Method. Nutrients, 15(10), 2415. https://doi.org/10.3390/nu15102415 

2. Garneau, N. L., Nuessle, T. M., Sloan, M. M., Santorico, S. A., Coughlin, B. C., & Hayes, J. E. (2014). Crowdsourcing taste research: genetic and phenotypic predictors of bitter taste perception as a model. Frontiers in Integrative Neuroscience, 8. https://doi.org/10.3389/fnint.2014.00033 

3. Hayes, J. E., Bartoshuk, L. M., Kidd, J. R., & Duffy, V. B. (2008). Supertasting and PROP Bitterness Depends on More Than the TAS2R38 Gene. Chemical Senses, 33(3), 255–265. https://doi.org/10.1093/chemse/bjm084