Gastrointestinal nutrient chemosensing and the gut‐brain axis: Significance of glutamate signaling for normal digestion

Gustatory and visceral information as chemical senses during meal are cue to recognize food ingestion and to determine its composition of nutrients when these chemical substances stimulate sensory organs. The brain realizes food intake to predict efficient digestion and to control each nutrient concentration in blood as well as brain within normal limits; viseral senses of each nutrient by digested products in alimentary tract as intake signal of nutrients notify into the brain through inner body sensory organs. Electrolytes and 20 kinds of amino acids in concentration of body fluid are strictly regulated all day long without species difference in mammals including humans, regardless of food intake. On the other hand, changes of glucose and free fatty acid concentration in blood and related hormones released from specific endocrine tissues can also primarily be occurred by chemical senses for control of appetite and satiety for foods as major marker for energy balance in living body. Sense of taste profile adapts to intake particular group of nutrients and forms boundary concentration to feel specific basic taste such as sweetness for energy, saltiness for electrolytes, and umami taste for protein. Dietary protein is composed of 20 kinds of L-amino acids, and both moiety of glutamic acid and glutamine are dominant. Typical umami taste substance is L-glutamate (Glu) salts. In other words, in order to feel the viseral senses of each nutrient compositiion of free nutrients, in parallel with composition of nutrients, must be more concentrated rather than those included in the saliva, which is the reason that foods rich in required nutrients. However, foods without smell and/or taste are hard either to stimulate appetite or to reach satiety. Therefore, people have been seeking for palatable seasonings for foods. Now, monosodium L-glutamate (MSG) as umami taste seasoning is globally used regardless of different food culture because of a basic taste. Recent research has exposed that L-glutamate (Glu), an amino acid, has multiple physiological functions in our body, particularly alimentary tracts during and after meal. As an essential substrate in the intermediary metabolism, free Glu is present in most organs and tissues (skeletal muscles, brain, kidneys, and liver) in substantial concentrations to operate physiological roles independently. Glu plays an important role for the energy metabolism and the substrate for other amino acids, glutathione, and body proteins. In the brain, Glu, which is locally produced completely de novo from glucose, acts as a major excitatory neurotransmitter, and its activity regulates synaptic plasticity, learning, memory, motor activity, and neural development. However, dietary Glu is almost impermeable into the circulating blood from the alimentary tract. Also, there is the blood–brain barrier against free Glu in between blood and brain never to incorporate into the brain. These Glus play to operate independently to dietary Glu in the alimentary tracts as Glu signaling alone. Major portion of Glu more than 95% of dietary intake is catabolized for energy sources and partially remaining substrate for protein synthesis. In addition to the gustatory roles of Glu likely to umami taste perception orally, recent studies have unveiled the postingestive significances of Glu signaling on various physiological functions such as process of digestion, yielded nutrient absorption and metabolism, and their homeostasis via brain activation. These effects might be mediated via luminal gut Glu sensors functionally linked either to the afferent branches of the vagus nerve as visceral sensations or partially the afferent sensory nerves in the oral cavity as taste sensation. Moreover, Glu acts as a reinforcer after ingestion via vagal afferent activation in the gut. For example, we recently observed that an intragastric infusion of 60 mM Glu aqueous solution induced conditioned flavor preference in rats. It is strongly suggested that the sense of Glu nortified food intake into the brain to operate efficient digestion in parallel with memory functions because of expectation for normal appetite for similar foods previously and maintenance for homeostasis without any disorder of nutrient levels. Thus, this review was conducted to summarize recent progresses of efficacies of Glu signaling in the alimentary tracts including gut–brain communication for healthier life. doi:10.1111/jgh.12408 bs_bs_banner