Nonetheless, glutamate by itself does not elicit an intense umami taste. The mixing of glutamate with nucleotides inosine-5-monophosphate (IMP) or guanosine-5-monophosphate (GMP) enhances the taste of umami; T1R1 and T1R3 respond primarily to mixtures of glutamate and nucleotides. While research has shown that this synergism occurs in some animal species with other amino acids, studies of human taste receptors show that the same reaction only occurs between glutamate and the selected nucleotides. Moreover, sodium in monosodium glutamate may activate glutamate to produce a stronger umami taste.Two hypotheses for the explanation of umami taste transduction have been introduced: the first posits that the umami taste is transduced by an N-methyl-D-aspartate (NMDA) type glutamate ion channel receptor; the second posits that the taste is transduced by a metabotropic type glutamate receptor (taste-mGluR4). The metabotropic glutamate receptors such as mGluR4 and mGluR1 can be easily activated at glutamate concentration levels found in food. Sources
Natural occurrence
Glutamate is ubiquitous in biological life. It is found naturally in all living cells, primarily in the bound form as a constituent of proteins. Only a fraction of the glutamate in foods is in its "free" form, and only free glutamate produces an umami flavor in foods. The savory flavor of tomatoes, fermented soy products, yeast extracts, certain sharp cheeses, and fermented or hydrolyzed protein products (such as soy sauce and fermented bean paste) is partially due to the presence of free glutamate ions. Asia
Japanese cuisine originally used broth made from kombu (kelp) to produce the umami taste in soups.
Even though there are also reports of MSG sensitivity among a subset of the population, this has not been demonstrated in placebo‐controlled trials. Controversy
Origin
The controversy surrounding the safety of MSG started on 4 April 1968, when Dr. Robert Ho Man Kwok wrote a correspondence letter to the New England Journal of Medicine, coining the term "Chinese restaurant syndrome". In his letter, Kwok suggested several possible causes before he nominated MSG for his symptoms. This letter was initially met with insider satirical responses, often using race as prop for humorous effect, within the medical community. Some claimed that during the discursive uptake in media, the conversations were recontextualized as legitimate while the supposed race-based motivations of the humor were not parsed.In January 2018, Dr. Howard Steel came forth claiming that it was actually a prank submission by him under a pseudonym. However, it turned out that there was a Dr. Robert Ho Man Kwok who worked at the National Biomedical Research Foundation, both names Steel claimed to have invented. Kwoks children, his colleague at the research foundation, and the son of his boss there confirmed that Dr. Robert Ho Man Kwok, who had died in 2014, wrote this letter. After hearing about Kwoks family, Steels daughter Anna came to believe that the admission that the letter was a prank was itself one of the last pranks by her late father.The claims of Chinese restaurant syndrome have the same symptoms as hypernatremia, so it may actually be salt poisoning.