Traditionally, a bitter taste is considered a warning signal for potentially toxic substances. However, not all bitter compounds are harmful. Some peptides and free amino acids, which are essential and nutritious, also taste bitter.
A groundbreaking study by the Leibniz Institute for Food Systems Biology at the Technical University of Munich offers the first explanation for this paradox.
Our sense of taste helps us make crucial food choices. Of the five basic tastes, sweet and umami indicate nutrient-rich foods, while salty tastes help maintain electrolyte balance. Sour flavors can signal unripe or spoiled food, and bitter flavors typically warn of potential toxins.
This makes sense given the presence of toxic plant substances like strychnine and hydrogen cyanide. Babies and toddlers are particularly sensitive to bitter tastes, as even small amounts of toxins can be harmful to them.
Protein Fragments and Bitter Taste
Interestingly, not all bitter substances are dangerous. Some are actually nutritious. An interdisciplinary team led by molecular biologist Maik Behrens investigated why this is so.
Using a cellular test system, they found that five of the approximately 25 human bitter taste receptors respond to free amino acids and peptides, as well as bile acids. Free amino acids and peptides are common in fermented foods like cream cheese and protein shakes, while bile acids, although not common in food, play important roles in the body.
Background Information
Amino acids, the building blocks of proteins, are essential for all known living organisms. They must be ingested through food as the body cannot produce them. When amino acids form chains of up to 100, they are called peptides. Longer chains are known as proteins.
Bile acids, produced in the liver and derived from cholesterol, are vital for fat digestion. Humans have around 25 different types of bitter taste receptors not only in the mouth but also in other organs and tissues, where they fulfill various functions, including defense mechanisms and metabolism regulation.
Structural Similarities and Taste Receptors
Bioinformatician Antonella Di Pizio explains that their modeling experiments show certain bitter-tasting peptides can mimic the 3D structure of bile acids within the receptor binding pocket. This structural similarity could explain why the same bitter taste receptors respond to both groups of substances.
Genetic analyses by first author Silvia Schäfer reveal that the ability to recognize both bile acids and peptides is highly conserved in three bitter taste receptors, tracing back to amphibians, indicating the importance of recognizing at least one of these groups across species.
Principal investigator Maik Behrens adds that bile acids and bitter taste receptors existed millions of years before modern bitter substances found in flowering plants and long before humans. This suggests that bitter taste receptors originally had important physiological roles beyond warning against toxins.
Their findings offer new insights into taste perception, suggesting that bitter receptors may have additional roles in human health beyond food selection.
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