Our sense of taste is a complex puzzle, and researchers have recently uncovered a new piece.
Sweet, salty, savory, sour, and bitter — these are the five tastes that allow us to enjoy daily meals, motivate us to seek the best restaurants and markets, and cook imaginative dishes.
They also have a more practical role tied to our evolution as a species — that is, to keep us safe from harm by giving us clues as to which plants may be toxic, as well as which ingredients are harmful or unlikely to provide the necessary nutrients.
Although we know much about how our sense of taste works, every new study seems to uncover fresh and previously unsuspected insights about this crucial mechanism.
One such discovery is linked to our ability to distinguish sweet, bitter, and savory, or umami, flavors. So far, it has been thought that one protein — called transient receptor potential 5 (TRPM5) — was primarily responsible for "encod[ing] sweet, umami (amino acid), and bitter taste sensations."
Now, Kathryn Medler — at the University at Buffalo College of Arts and Sciences in New York — in collaboration with colleagues from international institutions, has conducted research on mice that uncovered the role played by another protein in perceiving the sweet, savory, and bitter taste trio.
"Our research shows that there is redundancy in the taste system. This is important because taste is actually central to our survival. If you can't taste something bitter, you might gobble up something that's poisonous without ever knowing that it could be harmful."
She also notes that "[t]aste, in general, is one of our underappreciated senses," which makes it all the more important that we should pay more attention to the underpinnings of this mechanism.
Medler and colleagues' findings were published in the Proceedings of the National Academy of Sciences Early Edition.
'Study challenges a central dogma'
Working with mice in which the production of TRPM5 was suppressed, the researchers noted that the animals could still detect savory, sweet, and bitter flavors in the presence of another protein that has so far been largely ignored: TRPM4.
The mice were able to enjoy sugar water and umami treats and avoided the bitter-tasting quinine. At the same time, however, animals that missed both TRPM4 and TRPM5 proteins found it much more difficult to distinguish these three tastes.
"Our study changes a central dogma in the field — that detecting bitter, sweet, and umami stimuli is dependent on the presence of TRPM5 alone," says first author Debarghya Dutta Banik, a doctoral researcher from the University at Buffalo.
"This research helps us understand how the taste system works," he adds.
Both TRPM4 and TRPM5 make up chemical channels that help to pass on the information about specific flavors to the brain, where it can be decoded and processed.
The experiments undertaken in the current study led the researchers to conclude that mice are most receptive to bitter, sweet, and savory flavors when both TRPM4 and TRPM5 are present, stressing the proteins' equal importance to taste perception.
This finding, Medler explains, is probably also applicable to humans; TRPM5 has been found to play a crucial role in taste in both humans and mice. Additionally, both humans and mice normally have TRPM5 and TRPM4 in their taste cells.
Why we should understand how taste works
One reason why Medler and colleagues are interested in studying the mechanisms related to taste is due to their relevance to the regulation of appetite and its influence on our overall health.
In a study from 2013, Medler and her team at the time noticed that overweight mice had a reduced sensitivity to a range of appetite-inducing taste stimuli, including sweetness.
The researcher conjectures that this lack of sensitivity to normally emphatic stimuli may induce overweight animals to continue overeating in order to reach that pleasurable sense of reward that regular-weight mice achieve more readily.
In addition to the potential link between taste, appetite, and obesity, the researchers also note the problem of appetite loss among seniors. In older adults, taste cells also tend to lose sensitivity to various flavors.
If older adults don't get the same pleasurable sensations from foods that younger individuals do, this can cause them to eat less, affecting health and possibly causing malnutrition.
Thus, Medler says, "It's important for us to understand how the taste system works," because "[t]he more we know, the easier it will be to find solutions to problems when the system isn't working correctly."