S-T-B Glutamic Acid is an amino acid that is actually produced in our bodies when we don't
have enough of it (thus it's not an "essential" aa) - MSG is just one of a number of salts of the amino acid, which is also produced naturally within our bodies. However, back in the 70s (or before?), bad info came out that MSG was causing everything from diabetes, to high blood pressure, to scizophrenia, to just about every other disorder, and supposedly was the cause of the infamous "Chinese Restaurant Syndrome". Later, it was found that these were untrue, but restaurants to this day have signs "No MSG"!
As taxlady noted, fish sauce is a natural source, as are many other naturally fermented products. Soy sauce is another, as well as many other fermented soy products. And cheese is another, with one of the highest in glutamates being parmesan - no wonder that stuff is good on so many things! I'm sure they could overfeed mice with parmesan in a study, and say that's harmfull, due to all the sodium in it!
As for that fish sauce, stay away from what I call "fake" fish sauces - same thing with soy sauces. In the fake versions you will see hydrolyzed vegetable protein, plus other things. With the versions that have been naturally fermented you will just see listed simply Fish,salt, or with soy, simply Water, soy, salt, and often wheat, and a natural color, for dark types. The fake sauces are made quickly, by adding acids and enzymes to the fish, plus whatever those vegetable proteins are, and they get the sauce in under a week, while the natural fermentation takes a couple of years.
This is my theory for that "Chinese Restaurant Syndrome" many people used to come down with. I got a terrible headache one time, when I made a Thai meal, using a new fish sauce, that was one of those fake brands, recommended by a well known CB author. A friend that also had the meal with me also got a headache, but wasn't even going to mention it, because he would get headaches on a regular basis, but I never get them! We figured out that the fish sauce was the only new item in the meal, and we each ate a little more than 2 tb of it. The headache took about 3 days to totally go away, and a couple weeks later I took about 1/2 tsp of the sauce by itself, and got a slight headache from it. And, while I never got a headache from a Chinese restaurant, I'm sure there are many that use similar "fake" soy sauce, and with someone who is sensitive to similar ingredients, it could trigger a headache. While it's not necessarily the hydrolyzed vegetable proteins causing this, seeing that listed in the ingredients will tell you that it's not natural fish sauce or soy sauce - two good sources for MSG.
