Dietary free amino acids and the gastric phase of digestion.

In the stomach, pre-absorptive perception of food constituents is of particular importance in maintaining secretion and motility that matches the quantity and quality of nutrients. Products of food protein hydrolysis, free amino acids and short peptides, are the most potent chemical stimulants of the gastric phase of digestion. They are recognized by a variety of extracellular receptors belonging to the G-protein-coupled receptor superfamily, which are expressed by gastric mucosal exocrine and endocrine cells. Enteroendocrine G and D cells are likely the first level of integration of amino-acid-induced signals influencing a balance of endocrine activation and inhibition of gastric functions. This review focuses mainly on the physiological significance of dietary L-glutamate (Glu) in control of the gastric phase of digestion. The Glu signaling system in the stomach is linked to activation of the vagal afferents. In contrast to other natural amino acids, luminal Glu activates a paracrine cascade led by nitric oxide and followed by serotonin (5-HT), interacting in turn with 5- HT3 receptors on the afferent endings in the sub-mucosal layer. Glu, the only amino acid regularly ingested in a free form, enhances secretory and gastroprokinetic responses to protein- and amino-acid-rich diets but has no effect when applied alone or with carbohydrates. Possible mechanisms are discussed.