bitter taste perception

introduction

Taste is one of the senses through which humans perceive their environment. Most families have at least one fussy eater, so we know from experience that taste perception varies considerably between individuals and populations. These differences in perception depend to some extent on the kind of foods we become accustomed to during childhood. However, there is more to taste than meets the eye (or tongue!). Scientists think that much of taste perception is genetically determined.

There are essentially five main categories of taste; sweet, sour, salty, bitter, and umami (a Japanese term for the savory taste sensation triggered for example by the amino acid glutamate, which is naturally present in meat, poultry, seafood and vegetables).

Taste is perceived in taste receptors on the tongue surface, commonly known as taste buds. From an evolutionary perspective, the ability to distinguish between different chemicals in food and drink is crucial for the survival of humans and all other animals. Thus, individuals who perceive an unpleasant taste when attempting to ingest harmful foods are more likely to survive and reproduce, and even more so if they are also drawn to highly nutritious food because of a perception of pleasant taste. Accordingly, foods that have a pleasant taste tend to be nutritious and contain substances that are good for us, such as sugars, salts and proteins. Salty and sour detection helps to control the salt and acid balance of the body, our positive perception of sweetness ensures that we consume food that is rich in calories, and umami is thought to attract us to protein-rich foods. Conversely, things with an unpleasant taste tend to contain substances that are detrimental to our health or dangerous concentrations of useful substances.

A key element of unpleasant taste is the perception of bitterness. It is likely that this category of taste evolved in animals to help them avoid eating plants and other foods containing toxins and other harmful chemicals. It is a testament to the power of natural selection that such substances are typically perceived as bitter-tasting by humans.

However, not all humans have the same perception of bitterness for some substances. An intriguing example is the case of substances that are chemically similar to phenylthiocarbamide (PTC) and propyl-thiouracil (PROP). Such compounds are for instance found in cabbage and rapeseed. Some people perceive no particular taste of these compounds ("non-tasters"), whereas others experience an extremely unpleasant bitter taste ("tasters"). Among tasters there is also variation, in that some tasters (so-called "supertasters") are extra sensitive to bitterness. The frequency of tasters and non-tasters varies considerably among human populations. Thus, the frequency of non-tasters ranges from 3% in West Africa; 6-23% in China, 40% in India and is estimated to be around 30% in people of European descent.

The cause of differences in the perception of PTC-like compounds among humans has been traced to genetic variants in the TAS2R38 gene on chromosome 7. If you are a "taster" of bitterness, you are likely to carry the C allele of the SNP rs1726866. The C allele is dominant, so having one copy is enough to have the perception of a bitter taste. On the other hand, if you have the T allele of this SNP on both copies of chromosome 7, then you have about an 80% chance of being a "non-taster" of bitterness in response to PTC-like compounds. This means that foods that may taste bitter to others taste far less bitter to you. It is thought that about 20% of the variation in bitter taste perception of these compounds is explained by other genetic variants.

The deCODEme Genetic Scan identifies the SNP rs1726866 in the TAS2R38 tasting gene on chromosome 7 and gives an interpretation of the associated likelihood for being a "taster" or "non-taster" of bitterness in reaction to PTC-like compounds.

1. [1] The Taste Science Laboratory, Division of Nutritional Sciences, Cornell University.

risk factors

Understanding the diversity of the bitter-taste perception and the genetics of taste is a growing area of research. It has implications beyond the physiology of taste itself and increased understanding of human evolution is one of the goals.

Taste perception and the genetically determined human response to bitter-tasting foods may also have a considerable effect on nutrition and health. Studies have for example found that the non-taster genotype is a predictor of increased alcohol consumption in adults and also associated with lower preferences for sweetness in children and may therefore reduce their likelihood of dental decay.

Studies have also found that "supertasters" find some foods too bitter to enjoy, for example grapefruit, coffee and tea, brussel sprouts and cabbage. They may also be more sensitive to sweetness and much less likely to tolerate hot and spicy foods. Future studies will improve our understanding of the origin and the implications of these various taste perceptions for nutrition and health.

more information

You can find out more information about bitter taste perception by talking to your doctor and visiting these Web sites:
The Taste Science Laboratory at Cornell University
Wikipedia article about taste perception
Wikipedia article about genetic variations in taste

scientific references

1. Drayna, D. (2005). Human taste genetics. Annual Review in Genomics and Human Genetics. 6(1), 217-235.
2. Duffy, V.B., Davidson, A.C., Kidd, J.R., Kidd, K.K., Speed, W.C., Pakstis, A.J., Reed, D.R., Snyder, D.J., & Bartoshuk, L.M. (2004). Bitter receptor gene (TAS2R38), 6-n-propylthiouracil (PROP) bitterness and alcohol intake. Alcoholism: Clinical and Experimental Research, 8(11), 1629-1637.
3. Guo, S.W. & Reed, D.R.(2001). The genetics of phenylthiocarbamide perception. Annals of Human Biology, 28(2), 111-142.
4. Kim, U.K., Jorgenson, E., Coon, H,, Leppert, M., Risch, N.,& Drayna, D. (2003). Positional cloning of the human quantitative trait locus underlying taste sensitivity to phenylthiocarbamide. Science, 299(5610), 1221-1225.
5. Mennella, J.A., Pepino, M.Y., Reed, D.R. (2005). Genetic and environmental determinants of bitter perception and sweet preferences. Pediatrics, 115(2), e216-222.
6. Timpson, N.J., Christensen, M., Lawlor, D.A., Gaunt, T.R., Day, I.N., Ebrahim, S., & Davey Smith, G. (2005). TAS2R38 (phenylthiocarbamide) haplotypes, coronary heart disease traits, and eating behavior in the British Women's Heart and Health Study. American Journal of Clinical Nutrition, 81(5), 1005-1011.
7. Timpson, N.J., Heron, J., Day, I.N.M., Ring, S.M., Bartoshuk, L.M., Horwood, J., Emmett, P., & Davey-Smith, G.(2007). Refining associations between TAS2R38 diplotypes and the 6-n-propylthiouracil (PROP) taste test: findings from the Avon Longitudinal Study of Parents and Children. BMC Genetics, 8(1),51
8. Wooding, S.(2006). Phenylthiocarbamide: A 75-Year adventure in genetics and natural selection. Genetics, 172(4), 2015-2023
9. Wooding, S., Kim, U.K., Bamshad, M.J., Larsen, J., Jorde, L.B., & Drayna, D. (2004). Natural selection and molecular evolution in PTC, a bitter-taste receptor gene. American Journal of Human Genetics, 74(4), 637-646.