I haven’t seen a study to specifically evaluate why MSG sometimes leads to an undying thirst, but I have experienced that at restaurants on occasion . My guess is that it all comes down to sodium—- you ingest 2000 mg of sodium from table salt, you’ll notice it. Ingest 2000 mg of sodium from a combination of table salt and MSG (or maggi or Braggs liquid aminos or soy sauce), I suspect the more complex sensation might distract you from realizing you’re overdoing it on sodium. Either way, you’re gonna need a lot of water.
My reading of the literature is that there is no scientific evidence to support that MSG has any ill health effects that couldn’t be explained by sodium alone or a placebo effect. Take this study, for example, which concluded that MSG leads to pain compared to a salt (NaCl) placebo, but if you look at their methods the MSG group got twice as much sodium as the placebo group.
Some background food/sensation info…
Setting aside the pressure receptors that respond to Sichuan peppercorns and pain receptors that respond to chili peppers, your mouth has receptors for five tastes – – sweet, sour, salt, bitter, and savory/umami. Umami receptors detect glutamates, proteins that naturally form in everything from tomatoes and mushrooms to fermented Items like soy sauce. Note that taste is different than flavor, which incorporates both taste and smell—- taste conveys presence of an acid; flavor, aside from pH, conveys whether it’s lemon juice or vinegar.
Umami receptors are somewhat of an anomaly for the central nervous system. If you put either sugar, Vinegar, table salt, or quinine on your tongue, your brain will recognize the first four respective tastes. As I understand it, the presence of glutamate alone on your tongue will not, however, elicit a sensation. Glutamate only gets detected by your brain if it’s in the presence of a sweet, sour, salty, or bitter thing in your mouth.