In 2002, umami receptors were discovered
Since long ago, Japanese people have been using broth made from kelp, dried bonito flakes, and dried shiitake mushrooms for cooking. Japanese people have known from experience that the components in broth affect the taste since there is a big difference in taste between cooking with and without broth.
In 1907, chemist Ikeda Kikunae discovered that glutamic acid contained in kelp broth was the component of broth and named it umami.
This umami is one of the basic tastes for Japanese people, but in fact, in other countries, it was not recognized as a basic taste such as sweet, bitter, sour and salty.
I have often been told at academic conferences overseas that it was just a mixture of saltiness and other tastes. In other words, umami was not recognized as a single taste.
But this started to change after the discovery of taste receptors around 2000.
In humans, the taste is mainly sensed by the taste buds on the tongue. The taste buds, which consist of dozens of taste cells to sense the taste, work as taste sensors because they have taste receptors, which are protein molecules.
These taste receptors are molecules that respond to the components of taste, but a single taste receptor does not respond to all different tastes. Each receptor has a specific role, that is, sweet receptors respond to sweetness, and bitter receptors respond to bitterness.
As such analyses advanced, in 2002, it was revealed that the taste receptors called T1R1/T1R3 in humans are strongly activated by glutamic acid, which is an umami component. In other words, umami was not a mixture of tastes such as saltiness, but a basic taste to which specific umami receptors responded.
Today, umami is recognized as one of the five basic tastes such as sweet.
More interestingly, Japanese people have been adding broth made of dried bonito flakes or dried shiitake mushrooms to kelp broth.
Glutamic acid, which is an umami component of kelp, is a kind of amino acid, and umami components of dried bonito flakes and dried shiitake mushrooms are inosinic acid and guanylic acid respectively. These are chemical substances called nucleotides, which are nucleic acid umami components.
It turned out that these nucleic acid umami components bind to different locations in the umami receptor from amino acids such as glutamic acid and enhance the activation of the receptor.
That means umami receptors react more strongly and receive umami of glutamic acid, an amino acid, more strongly. As a result, people feel umami more strongly.
As a matter of fact, it was only 2008 that the mechanism by which glutamic acid and the nucleic acid umami components synergistically activate umami receptors was revealed. However, Japanese people have known this phenomenon from experience and have used a cooking method called awasedashi (blended broth) since long ago.
Of course, it is not just Japanese people who feel umami. Umami receptors work in the same way for people all over the world.
For example, bouillon and consommé contain umami components, and cheese and tomato contain glutamic acid. That is, Western people also like umami and use it for cooking.
In addition, cheese, tomato and soy sauce contain a flavor component called methional, and our research revealed that this flavor component activates umami receptors.
Just as Japanese people frequently use soy sauce as a seasoning, Western people often use cheese and tomatoes for cooking. Without doubt, I think Western people also recognized umami as a good taste.
Changes in the feeding habits of birds induced by umami receptors
The discovery of taste receptors and their genetic information enabled the introduction of the genes into cultured cells, accelerating research into what substances taste receptors respond to. This has led to the rapid elucidation of the mechanism of taste over the past 20 years.
Furthermore, the elucidation of umami receptors not only adds umami to the basic tastes, but also leads to the understanding of the evolution of living creatures.
For example, birds have a broken gene for the formation of sweet receptor. In other words, birds cannot feel sweetness.
However, hummingbirds in the Americas feed chiefly on flower nectar. They intake sugar by doing so, however, it is strange that the birds have a liking for feeding on flower nectar even though they are not able to feel sweetness.
But, in 2014, we analyzed umami receptors of hummingbirds and found that their umami receptors have varied to respond to sugar components.
The reason why such changes occurred is yet to be clarified.
The swift, which is a close relative of the hummingbird, is an insectivore, and its umami receptors do not respond to sugars.
Probably, the ancestors of hummingbirds were also insectivores, but they had flower nectar by chance when they caught insects in flowers. Then they learned that it could provide them with energy.
After that, the umami receptor mutated, and the species that were able to enjoy flower nectar appeared, which may have helped them survive in the battle for existence.
The same phenomenon has happened to songbirds, which make up about half of bird species. We announced that in the summer 2021.
Their ancestors, which originally inhabited Australia, became able to eat flower nectar and increased their ability to survive and were then able to spread all over the world. This includes white-eyes and bulbuls in Japan.
Umami receptors that promote upsizing of primates
Changes in taste receptors vary what animals eat, and consequently, their ability to survive or prosperity improves. This phenomenon applies to humans as well.
When primates were a small-sized species weighing less than 1kg, they chiefly fed on insects.
Insects contain plenty of the umami component of nucleotides, and the umami receptors of small-sized primates are activated by these nucleotides. Thus, they had a liking for insects and insects were their source of protein.
Even today, small-sized primates such as squirrel monkeys and marmosets have nucleotide-responsive umami receptors.
However, as some primates increased in size, others began to feed on plant leaves. As a matter of fact, plant leaves contain almost no nucleotides and are bitter. Therefore, they are not considered a tasty food for primates.
However, leaves contain glutamic acid. Accordingly, they evolved to have umami receptors which were activated by glutamic acid, an amino acid, rather than by nucleotides.
As a result of being able to get protein from the leaves of plants that were abundant on the ground and could be found without much effort, primates grew larger and evolved in many ways. During the course of this evolution, human beings emerged.
The reason why primates, which used to be insectivores, changed their umami receptors as they grew larger, and why they could promote their upsizing consequently, is yet to be clarified.
However, it is certain that umami, which was not considered as important as other basic tastes in the past, played a significant role in the diet of living creatures.
In addition, humans have developed cooking techniques and made good use of umami and others to eat bitterness and sourness, which are essentially to be avoided. In that sense, the eating habits of humans are unique.
But it can be said that the human way of eating is to utilize these unique eating habits to enjoy various tastes without feeding exclusively on a specific taste.
Studies on taste have advanced rapidly over the past 20 years. Because of this, conversely, we now have more things that need to be clarified.
Why did primate ancestors like nucleotides? Why do we feel the taste in the first place? What is the origin of taste? We intend to pursue further research to answer these questions.
I hope you will be interested in the fact that eating, the most important habit of living creatures, is therefore based on various mechanisms.
As we learn that taste is not just for preference, we may be able to prevent lifestyle-related diseases that are derived from eating habits.
* The information contained herein is current as of March 2022.
* The contents of articles on Meiji.net are based on the personal ideas and opinions of the author and do not indicate the official opinion of Meiji University.
* I work to achieve SDGs related to the educational and research themes that I am currently engaged in.
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