Earlier quoted context omitted.
You want a credible study? Eat a teaspoon of MSG & try not to have a migraine.
That would be approximately the same as trying to eat only a tablespoon of salt. Which would definitely cause headaches, at least from dehydration. You're only supposed to use a half teaspoon to flavor six whole servings of soup. It's an incredibly strong flavor enhancer, and is really not recommended for solo consumption.
In the summers, when I sweat a lot due to running and cycling, I dump that much salt into a bowl of soup. I don't have any ill effects. In fact, beefing up the salt intake wards off muscle cramps and lethargy.
Japan's allowance for daily salt intake for men sets an upper bound of 12 grams (salt, not sodium).
You have it backwards, by the way. MSG has a rather large molecular weight compared to NaCL. A given mass of MSG has less sodium than the same mass of salt. This is very easy to get in precise numbers: the molecular weight of MSG 169.111 g/mol is and of salt 58.44 g/mol. A mole of MSG has the same amount of sodium as a mole of salt: one weighs 169g. the other 54g: close to triple. Secondly, MSG is less dense than salt: the same mass of MSG takes 33% more space than a given mass of salt. This magnifies the difference. Basically if we have equal sized heaps of salt and MSG, the MSG has about 1/4 of the sodium compared to the salt.
In a restaurant, they can easily dump a teaspoon of MSG (about 4g) into your mapotofu or noodle soup or whatever. Manufactured products of good brands are going to be reasonable. Someone holding a shaker MSG over a pot isn't going to be that much. Not consistently so, day in, day out. Consider that some of the ingredients already have MSG (e.g. marinated or processed meats), then there is natural glutamate in some of it, and then they add some half teaspoon (about 2g) of MSG crystals. Things add up.