Physiological role of dietary free glutamate in the food digestion.
Gustatory and anticipatory cephalic stimuli during a meal yield nutritional information and aid efficient food digestion. Mammals, including humans, can detect the amount of dietary protein and its quality via cephalic relay to initiate proper digestion in the upper gastrointestinal (GI) tract. In addition to gustatory stimuli, visceral sensing by the abdominal vagus conveys primary afferent nutritional information from the digestive system to the brain. Electrophysiological studies indicated that abdominal vagal afferents, which were innervated into the stomach and intestine sending information to the brain, were activated by luminal glutamate. Histochemical analysis also revealed the existence of a glutamate signalling system (metabotrophic glutamate receptors) in the GI tract. Luminal glutamate in the stomach and intestine provides the efferent reflection of the abdominal vagus, supporting the modulation of exocrine and endocrine excretion during digestion. These results strongly indicate that glutamate has regulatory effects on the food digestive processes through the gut nutrient-sensing system. It plays physiological and nutritional roles and initiates digestion in the stomach as well as anticipates subsequent processes in the small intestine and the liver. We reviewed recent studies on glutamate physiology in the gut including our research, and discussed the physiological significance of dietary free glutamate in the regulation of gut function, focusing on the visceral sensation from the stomach.