A tale of two sugars: How studying ingestive behavior revealed glucose sensing pathways in the taste system

To optimize health, animals must evaluate the nutritional value of foods before digestive and absorptive processes reveal their metabolic consequences. Taste is traditionally viewed as a chemosensory system that categorizes compounds into a small set of perceptual qualities (e.g., sweet, salty, bitter) that guide ingestion or avoidance behaviors. In this framework, sweetness is assumed to signal the presence of carbohydrates, and, thus, energy. However, sweet taste does not uniquely signal caloric or metabolic value. Some sweet-tasting compounds provide little or no energy, whereas others differ in how they are metabolized despite producing similar sweet sensations. Emerging evidence suggests that the taste system may also encode information about the metabolic value of nutrients themselves. Using glucose as a model nutrient, this review highlights behavioral, neural, and molecular findings indicating that oral carbohydrate sensing involves mechanisms beyond canonical sweet receptor signaling. Our studies show that animals can learn to rapidly discriminate glucose from other sweet-tasting sugars that differ in metabolic fate, providing a behavioral foundation for identifying glucose-sensitive pathways in the taste epithelium. Candidate mechanisms include metabolism-dependent sensing via glucokinase, transporter-linked signaling through a sodium-glucose cotransporter, and enzymatic preprocessing of complex carbohydrates at the level of the taste buds. Importantly, the relative contribution of these mechanisms appears to depend on dietary experience, physiological state, and the broader sensory context. Together, these findings support a revised view of taste in which the oral cavity serves not only as a detector of chemical identity but also as an early stage metabolic sensing system that helps organisms predict the postingestive consequences of foods at the moment of ingestion.