Nutritional and Physiological Significance of Luminal Glutamate-sensing in the Gastrointestinal Functions

Abstract

Recent evidence indicates that free amino acids are nutrients as well as acting as chemical transmitters within the gastrointestinal tract. Gut glutamate research is the most advanced among 20 amino acids. Free glutamate carries the umami taste sensation on the tongue and a visceral sensation in the gut, especially the stomach. In theˆeld of taste physiology, the physiological meaning of the glutamate-derived chemical sense, the umami taste, has been proposed to be a marker of protein intake. Experimental evidence in gut glutamate physiology strongly supports this hypothesis. Free glutamate is sensed by the abdominal vagus and regulates gastrointestinal functions such as secretion and emptying to accelerate protein digestion. Clinical application of glutamate has also just begun to treat gastrointestinal disorders such as dyspepsia, ulcer, dry mouth and functional dyspepsia. In this review, we introduce recent advances in gut glutamate research and consider the possible contribution of glutamate to health.

Gastric emptying and duodenal motility upon intake of a liquid meal with monosodium glutamate in healthy subjects

“…The present MRI observations suggest that the effects of glutamate are comparable to those of several GI hormones and neurotransmitters related with the brain-gut interaction (Willie et al 2001;Tsurugizawa et al 2009;Janssen et al 2011a,b;Karnani and Burdakov 2011;Nakayama 2011). As orexin and ghrelin affect both cephalic and gastric functions, L-glutamate contained in food acts as umami taste to evoke appetite through glutamate receptors in the oral cavity, and also promotes antroduodenal transport by organizing gastric and duodenal contractions presumably by facilitating vagal activity (Niijima 1991;Niijima and Meguid 1995;Uneyama 2011). In the small intestine, absorbed nutrients suppress the secretion of these hormones which promote feeding (Janssen et al 2011b).…”

“…,b; Karnani and Burdakov ; Nakayama ). As orexin and ghrelin affect both cephalic and gastric functions, l ‐glutamate contained in food acts as umami taste to evoke appetite through glutamate receptors in the oral cavity, and also promotes antroduodenal transport by organizing gastric and duodenal contractions presumably by facilitating vagal activity (Niijima ; Niijima and Meguid ; Uneyama ). In the small intestine, absorbed nutrients suppress the secretion of these hormones which promote feeding (Janssen et al.…”

“…Among the amino acids, the special role of l ‐glutamate is ascribed to the distinct distribution of amino acid receptors between the stomach and small intestine. Namely, only glutamate receptors are expressed in the stomach, whereas numerous amino acid receptors exist in the small intestine (Uneyama ). Thus, l ‐glutamate alone causes the acceleration of gastroduodenal transport in the stomach, corresponding to its role as umami taste (appetizer) in the oral cavity.…”

“…In the mucous membrane of the gastrointestinal (GI) tract amino acids serve as signaling molecules (Niijima ; Niijima and Meguid ; Uneyama ). In the stomach, proteolytic enzymes digest protein toward peptide, and the gastric mucous membrane contains only glutamate receptors, including metabotropic glutamate receptors (mGluR).…”

An Interesting Tour of New Research Results on Umami and Umami Compounds

“…Health issues, like functional dyspepsia, irritable bowel syndrome and obesity are often related to eating habits. Because of that, the nutritional information from the gastrointestinal tract which affects the regulation of food digestion and nutrient absorption gains more importance as it also has an effect on eating behavior [81]. Other studies also showed that free glutamate in a high protein diet has a positive effect on delayed gastric emptying and abdominal unpleasantness after eating.…”