The evolutionary background of bitterness

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1. Bitter taste as an evolutionary tool Link to heading

In human evolutionary biology, bitter taste receptors play an important protective role, as they can detect potentially noxious edibles (Logue, 2015, p. 48). A great amount of natural bitter compounds found in plants are in fact toxic. Therefore, the signalling of poisonous food with a sharp and repulsive taste was crucial for survival, in contrast with “useful” food components often perceived sweet or salty (Witt, 2019). The importance of harmful food detection is demonstrated by the fact that for salty, sour, and sweet we have only one type of receptor for each, while for bitter, there are no less than 25 different kinds (Lee & Cohen, 2016). It is our most sensitive taste primary in both senses of the word, for bitter taste receptors are damaged easily (due to viral tracheal and ear infections), and are also the first to fade with aging (L. M. Bartoshuk et al., 2012).

Unpleasant bitter taste triggers a rejection response, and this effect can be observed across the animal kingdom as well. Herbivore primates prefer the younger, immature leaves lower in toxins, as many plants have developed chemical defence. Insecthunting bats learn very fast that the greenish glowing of bioluminescent fireflies means: “Don’t eat me, I taste terrible!”. During an experiment involving some unfortunate bats, the nocturnal animals immediately discarded the (previously unfamiliar) chemically protected insects after tasting them, showing strong aversive behaviour that can be familiar to us: they salivated, coughed, and rapidly shook their heads in a response to the gnarly taste of the toxic fireflies which they never attempted to catch again (Leavell et al., 2018).

Interestingly, a general negative response persists even if the bitter taste does not pair with any level of toxicity, meaning that animals keep away from bitter tasting nutrients even when they are not dangerous (Glendinning, 1994), while us humans can voluntarily enjoy olives or broccoli, because we know they are safe to consume.

2. Human taste perception of bitterness Link to heading

Human perception of bitterness since evolved due to the use of fire, various cooking techniques, our overall change in diet, and the fact that we do not rely on this particular method of toxin avoidance anymore. In connection to this, scientists are reporting evidence on the loss of function of bitter taste receptor genes (Wang et al., 2004). Bitter substances are almost entirely organic, such as alkaloids used in drug and medicine production, e.g. caffeine (coffee), quinine (tonic), nicotine (tobacco), and cocaine (coca plant). The ethanol in alcoholic drinks carries a base bitter taste as well (Hall & Guyton, 2016, p. 685).

Besides well documented genetic variation in bitter taste perception, it is generally true that most people find bitter taste unattractive (Duffy & Bartoshuk, 2000). However, in some cases, it is indeed desired and consumed intentionally. Most importantly, human societies consume medicinal herbs and herbal potions for thousands of years. Traditional herbalism – what we now consider folk medicine – marks the dawn of the history of medicine, and gave the basis for many modern remedies (Petrovska, 2012). Hence the notion that bitter medicine is hard to swallow however beneficial, and the related philosophical twist about bitter truths.

Popular bitter tasting foods are also in abundance: e.g. vegetables usually deemed healthy, such as artichokes, kale, dandelion greens, and bitter gourd (bitter melon); fruits, such as grapefruit, citrus peel, cocoa, and therefore (dark) chocolate; spices, such as turmeric and galangal (both rhizomes of two related plants). As for beverages besides ale brewed with hops: tonic water, gin, campari, ouzo, pastis, and other herbal bitters are being perfected for hundreds of years now, some of them originally marketed as health drinks, evolving into many of the apéritifs and digestifs of today (ToussaintSamat, 2009, pp. 166, 636). And we must not forget the magic potion of our time: coffee.

By anecdotal evidence we all know that young children tend to follow the primal evolutionary response of rejection, they usually loathe the taste of coffee and beer – not to mention the infamous Brussel sprouts, triggering grimaces worldwide¹. We might also assume that as we age, many of us –for the lack of a better word – learn to enjoy bitter food. The fact is that we gradually lose the ability to taste bitter, as taste buds change overtime (Prutkin et al., 2000). Nerve fibres that mediate bitter taste are sensitive and easily damaged by viruses, therefore our perception of taste diminishes when we are sick, say, catch a cold (L. Bartoshuk et al., 1996). With aging, these harmful experiences accumulate enough so that it results in the deterioration of bitter tasting capabilities, resulting in desensitization and diminishing our perception for it. In turn, the flavour profile of various food items translates to our tongue differently, making some of them enjoyable for adults. Furthermore, our preferences change over time through conditioning as well. Our brain knows what our body needs (sodium, sucrose, proteins, etc.); and if our intake fulfils its need, the dish would be more palatable in the future. Ergo, we like some food more and more over time (and very often these are unhealthy foods containing large amounts of fat, salt, sugars).

References Link to heading

• Bartoshuk, L. M., Catalanotto, F., Hoffman, H., Logan, H., & Snyder, D. J. (2012). Taste damage (otitis media, tonsillectomy and head and neck cancer), oral sensations and bmi. Physiology & Behavior, 107(4), 516–526. https://doi.org/https://doi.org/10.1016/j.physbeh.2012.06.013

• Bartoshuk, L., Duffy, V., Reed, D., & Williams, A. (1996). Supertasting, earaches and head injury: Genetics and pathology alter our taste worlds. Neuroscience & Biobehavioral Reviews, 20(1), 79–87.

• Duffy, V. B., & Bartoshuk, L. M. (2000). Food acceptance and genetic variation in taste. Journal of the American Dietetic Association, 100(6), 647–655. https://doi.org/https://doi.org/10.1016/S00028223(00)001917

• Glendinning, J. I. (1994). Is the bitter rejection response always adaptive? Physiology & behavior, 56(6), 1217–1227.

• Hall, J. E., & Guyton, A. C. (2016). Guyton and hall textbook of medical physiology (13th ed.). Elsevier.

• Leavell, B. C., Rubin, J. J., McClure, C. J. W., Miner, K. A., Branham, M. A., & Barber, J. R. (2018). Fireflies thwart bat attack with multisensory warnings. Science Advances, 4(8), eaat6601. https://doi.org/10.1126/sciadv.aat6601

• Lee, R. J., & Cohen, N. A. (2016). Bitter taste bodyguards. Scientific American, 314(2), 38–43. Logue, A. W. (2015). The psychology of eating and drinking (4th ed.). Routledge.

• Petrovska, B. B. (2012). Historical review of medicinal plants’ usage. Pharmacogn Rev, 6(11), 1–5. https://doi.org/10.4103/09737847.95849

• Pilcher, J. M. (2012). The oxford handbook of food history. Oxford University Press.

• Prutkin, J., Duffy, V., Etter, L., Fast, K., Gardner, E., Lucchina, L., Snyder, D., Tie, K., Weiffenbach, J., & Bartoshuk, L. (2000). Genetic variation and inferences about perceived taste intensity in mice and men.

• Physiology & behavior, 69(12), 161–173. Toussaint-Samat, M. (2009). A history of food (2nd ed.). WileyBlackwell.

• Wang, X., Thomas, S. D., & Zhang, J. (2004). Relaxation of selective constraint and loss of function in the evolution of human bitter taste receptor genes. Human Molecular Genetics, 13(21), 2671–2678. https://doi.org/10.1093/hmg/ddh289

• Williams, J. A., Bartoshuk, L. M., Fillingim, R. B., & Dotson, C. D. (2016). Exploring ethnic differences in taste perception. Chemical Senses, 41(5), 449–456. https://doi.org/10.1093/chemse/bjw021

• Witt, M. (2019). Anatomy and development of the human taste system. Handbook of clinical neurology, 164, 147–171.