Bitter receptors reveal surprising roles beyond toxin detection
A Bitter Taste Has Long Served as a Warning—But Not All Bitter Compounds Are Toxic
Traditionally, a bitter taste has signaled the presence of potentially poisonous substances. Yet not all bitter compounds are harmful. Some peptides and free amino acids taste bitter despite being non-toxic, nutritious, and even essential for human survival. Now, a new study by the Leibniz Institute for Food Systems Biology at the Technical University of Munich offers the first explanation for this seemingly paradoxical phenomenon.
How Taste Guides Our Food Choices
Of the five basic tastes, sweetness and umami indicate that a food is energy-rich and nutritious. Our sense of salt helps maintain electrolyte balance, while sour notes can warn us of unripe or spoiled food, and bitterness alerts us to potentially toxic substances.
Given the many poisonous plant compounds—such as strychnine from the nux vomica tree or cyanide from cassava—this makes evolutionary sense. It also explains why infants and young children instinctively reject bitter flavors: even small amounts of such toxins can be harmful to them.
Bile-Like Bitterness from Protein Fragments
Yet not everything that tastes bitter is dangerous—some bitter compounds are actually beneficial. An interdisciplinary research team led by molecular biologist Maik Behrens has now investigated the reasons behind this apparent contradiction for the first time.
Using an established cellular test system, the team at the Leibniz Institute discovered that five of the roughly 25 human bitter receptor types respond to both free amino acids and peptides and to the body's own bile acids. The former are produced when proteins break down and are abundant in fermented foods like fresh cheese or protein shakes. Bile acids, on the other hand, play almost no role as a dietary component but instead serve vital functions within the body. This makes them likely candidates for activating endogenous bitter receptors found, for example, on intestinal and blood cells.
Explanation: Structural Similarities
"Interestingly, our modeling experiments show that a specific bitter-tasting peptide can adopt a functionally active 3D shape within the receptor binding pocket that closely resembles that of bile acids," explains bioinformatician Antonella Di Pizio. "This accidental similarity may explain why the same group of bitter receptors responds to both types of compounds."
Lead author Silvia Schäfer adds: "Our genetic analyses further reveal that the ability to detect both bile acids and peptides is highly conserved in three of the bitter receptor types, tracing back as far as amphibians. This suggests that recognizing at least one of these two compound groups has been evolutionarily important across species."
"Bile acids and bitter receptors existed millions of years before the typical bitter compounds found in modern flowering plants—and long before humans, even in fish," says study leader Maik Behrens. "This supports the hypothesis that bitter receptors originally helped regulate key physiological processes, not just warn against toxins."
He continues: "These findings provide new insights into the complex systems of taste perception and suggest that bitter receptors may play additional, as-yet-unknown roles in human health beyond their function in food selection."
Amino acids and peptides: Amino acids are found in all known living organisms. They are the building blocks of proteins and are released when proteins break down. Essential amino acids cannot be synthesized by the body and must therefore be obtained through diet. When amino acids form chains of up to 100 units, they are called peptides. Longer polypeptide chains are classified as proteins.
Bile acids are produced in the liver and are derivatives of cholesterol. They play a crucial role in the digestion of dietary fats.
Bitter taste receptors: Humans have roughly 25 different types of taste receptors responsible for detecting bitter compounds. These bitter receptors are not only found in the mouth but also on cells in other organs and tissues. Researchers—including those at the Leibniz Institute for Food Systems Biology at the Technical University of Munich—are investigating their diverse functions. Current studies suggest that bitter receptors in the airways help defend against pathogens and accelerate the movement of cilia. There is also evidence that endogenous bitter receptors in the gut and blood cells support immune responses or play a role in metabolic regulation.
