Tuesday, November 15, 2016

Why Doesn't Rotten Meat Taste Like Cake? The Evolution of Food Preferences and Aversions

by Kitty Bridgewater, Jessica Bowen, Madison Haley, Ryen Beach, and Rebecca Horton

Have you ever wondered why it is that we (humans) typically avoid bitter foods? Or does the answer seem obvious: "Bitter foods taste bad." But why do they taste bad to us? Why does the rancid smell of meat past its due date make us wrinkle our noses, when it make hyenas, vultures, and other scavenging animals lick their chops with anticipation?

What you may not have thought about is that there are deep evolutionary reasons behind the tastes we hate (like bitterness) and the tastes we love (like sweet, meaty, and fatty tastes). Our aversion for bitterness is actually an evolutionary adaptation passed down from our ancestors to help us avoid foods that would be dangerous to us. Our love of flavors like umami, or savory tastes, reflects the fact that foods with these tastes are often rich in calories and amino acids, and thus would have been valuable to our ancestors.

In this post, we'll explore bitterness and umami as two examples of taste preferences that humans have evolved to help us navigate our nutritional environment. We'll also talk a little bit about how learning interacts with our innate preferences, given that even though we generally have an aversion to bitter foods, many people also develop preferences for bitter foods through experience (coffee, alcohol, etc).



Why Do Some Foods Taste Bitter?

Bitterness is present in many plants as the by-product of a defense mechanism to prevent its ingestion by an herbivore or omnivore. Instead of defenses like claws and teeth, these plants have developed an ability to produce various chemicals that are harmful when ingested, such as cyanogenic glycosides, alkaloids, terpenoids, and cardiac glycosides among others (see this paper for a longer discussion). These chemicals, specifically the glycosides, are especially harmful and toxic to humans, and present themselves as bitter when eaten. Humans, as well as many other mammals, all exhibit some level of aversion to bitter foods.

Detection of potentially toxic foods begin immediately. Taste receptors located along the tongue and epithelial (throat) cells of humans that are responsible for the detection of bitterness are classified as T2R receptors, or "taste family two" receptors. These are G-protein coupled receptors (GPCRs), meaning simply that they function as receivers of information outside the cell. In this case, information concerning the chemicals present in the food we eat is picked up by these receptors when a specific signal binds to it. This signal is then sent into the cell, where it is further interpreted and passed along as needed throughout neighboring cells.

It has been suggested that the aversion to bitterness is shaped by natural selection. A study by Glendinning in 1994 examined the prevalence of bitterness sensitivity and aversion in relation to dietary histories of different animal species.

He proposed that animals who consume entirely or mostly vegetation would have a higher threshold for bitterness and thus less aversion due to not being able to afford passing on a potential food source that may not be toxic. (The same study highlighted that bitterness is not a direct, or always accurate, measure of toxicity.)

Carnivores, by the same logic, should have a low threshold and thus a much stronger aversion to bitter foods due to their diet being mostly if not entirely meat, which carries with it a severely lower risk of having these types of toxic substances. Relative levels of reliance on vegetation, and therefore the frequency of exposure to potentially toxic chemicals, should affect the extent to which animals perceive and avoid certain types of tastes.

Glendinning found support for these ideas in a comparison of more than 30 species. His findings were that carnivores were the most sensitive to bitter compounds, and that herbivores were the least sensitive to these same compounds. Importantly, he also found evidence that herbivores were the most tolerant of the compounds that produce bitter tastes, meaning that foods that would be very bitter and also very toxic to a carnivore are less bitter and less toxic to an herbivore.

Evidence for Bitterness Aversion

Humans and many other primates are born disliking bitter tastes--we don't need to be "taught" to avoid them. This avoidance continues to be very strong throughout childhood and into adulthood.

PTC is one example of a bitter-tasting antithyroid compound that tends to create aversive responses in many species. A study done by Shah and Afzal states that although PTC itself has not been found in nature, the ability to taste PTC is correlated with the ability to taste other bitter substances, making it especially useful for research.

Variation in PTC taste sensitivity has been correlated with dietary preferences that may have significant health effects. Much of the existing research suggests that a majority of individuals experience a certain level of bitterness when in contact with PTC, although individuals with PTC "blindness" and with extreme PTC sensitivity (known as "super tasters") also appear fairly regularly.
  Zhou et al found that bats hold bitter taste receptor genes (T2R) in common with cows and dogs. In addition, certain bat-specific bitter taste genes were detected, suggesting that positive selection caused specialization of these receptors to adapt to the bats’ local environment.

If Bitter Foods Are So Toxic and Disgusting, Then Why Do I Love IPAs So Much?

Despite our natural aversion to bitterness, many people (perhaps including you) develop a great liking for the tastes of certain bitter foods and beverages, like coffee and alcohol. How does this process work? 

While we are born with innate likes and dislikes, our taste preferences are also flexible. If we have repeated positive experiences--or a single negative experience!--of a food with a particular flavor, we can learn to like (or dislike) that food. For example, if you enjoy the relaxing effects of alcohol, the bitter taste of ethanol will become less aversive to you over time because you learn to associate that taste with the pleasurable feelings. 

In contrast, if you have learned to enjoy drinking alcohol and you overindulge in a particular kind of alcohol (e.g., Jagermeister) and become sick from it, you're likely to avoid that particular type of alcohol after that bad experience. This is an example of something called "one trial learning" that is observed a lot in acquired taste aversions. It's thought to reflect an adaptation that keeps us from repeating poor (and possibly fatal) food choices. 

Coffee is another example of a bitter flavor that people can come to enjoy. For many people, you may begin drinking coffee with a lot of cream and sugar in it to make it taste less bitter. Over time, though, the actual taste of the coffee will become pleasant to you. This is probably an example of something called flavor-flavor associative learning, in which you come to prefer one taste (coffee) because it's paired with another naturally rewarding taste (sugar, cream).    

Why Do Some Foods Taste Savory?

Umami is the most recently discovered of the five basic tastes that is found in meat and products from animals, as well as plants. This taste is created when the MSG and Ribonucleotides that the food contains combine and create a meaty, or savory taste. The taste buds activated by the taste are identify as T1R1+T1R3. It has been hypothesized that humans identify highly rich tasting umami with an easily disgestable protein, there for, causing less stress on the digested system, and requiring less stored energy for digestion.

It has been suggested that the adaption for the preference towards the umami taste has evolved due to the benefits of cooked meats for one's survival. Animal products especially meats are full in umami taste when cooked, however, meat that is raw, or fresh have very little to no umami taste when consumed. Thus, suggesting that humans have formed an adaptation to cooked meats, due to, the adverse and harmful effects that uncooked foods present, for example, parasites, and bad bacteria. The preference to prefer umami, decreases the risk of consuming contaminated meats, due to, the taste being absent in raw and fresh meats.

Evidence for Umami Preference

Monosodium glutamate, or MSG, which creates a umami taste, has been demonstrated to increase one’s appetite, as well as satiety. In a study conducted by adding MSG to a bowl of soup in which the participants consumed for four days non-consecutively. The participants reported that the soup tasted delectable when the MSG was added to their meal, in which increased satiety, the feeling of being full, as well as, their appetite. Also, in a similar study the participates reported that their emotional health improved after consuming soup that included, MSG. The participants reported that they were happy, content, satisfied, and less disappointed after consuming the MSG soup rather than the control: soup without MSG.

Unlike bitter tastes, we are undeniably attracted to both sweet and umami foods. Umami foods are typically high in fat, calories, and amino acids (protein), and while in excess they could have negative effects on your health, in moderation (which is almost certainly how our ancestors consumed them, given the relative difficulty of obtaining meat) they are quite safe and in fact essential to survival. Thus, this food preference may reflect an adaptation in that it draws you to protein-rich foods. And again, while there foods are typically so plentiful today that they can be eaten to excess, these would have been extremely difficult to come by--and thus extremely valuable--during ancestral times.

Over the years humans have developed strong and ever evolving culinary skills that have allowed them to maximize the flavors that we like the best. There are many food cooking and preparation techniques that specifically help to enhance umami flavor, for instance. As discussed above, the cooking of meat releases amino acids that trigger the umami taste receptors. Fermenting foods can have a similar effect, which is why many cheeses (for instance, Parmesan cheese) have such rich umami flavors.

Image result for pizza cheeseburger meme

Final Thoughts

In closing, our taste preferences are a surprisingly complex business. It isn't as obvious as it might seem that bitter foods are necessarily disgusting or that sweet foods are necessarily delicious. If we were strictly carnivorous, we might think that cake tastes like rotten meat. If we were were scavengers, we might think that rotten meat tastes like cake. The things that we love and hate to eat reflect features of our natural history, and what foods benefited and harmed our ancestors.

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