I get what you're saying but that's why I used chicken broth as an example as that has tons of free glutamate as well as sodium to go with it. As for free vs. bound glutamate, if it's bound glutamate it's not going to contribute to taste, if you can taste that umami flavor then it's free glutamate you're tasting. (side note, glutamic acid on it's own will dissociate in water just like any other ionic bond, but I get what you mean) And while glutamine or glutamic acid in a protein isn't digested as quickly as free glutamate, it still gets hydrolyzed and given that "MSG syndrome" supposedly lasts for a while it stands to reason that even a slow release over the period of a day should still trigger symptomes given that there's generally a lot more bound glutamate than free glutamate.
But a little free glutamate goes a long way, there's no chinese restaurants just dumping MSG in by the cup, that would be way too over seasoned. I use MSG on a lot of dishes myself and I'll use amounts that have less free glutamate in them than in some other common foods.
The only reason why I mentioned that is to suggest that I know what I proper amount of MSG to add for seasoning is. The amounts in those studies are very excessive and unrealistic.
"The amounts in the studies don't conform to my culinary preferences" doesn't amount to "MSG doesn't cause headaches".
If the small amounts of MSG that you prefer caused headaches in a significant fraction of the general population, it would not be usable at all, and we wouldn't be here talking about it.
There is a combination of circumstances: most people seem to tolerate larger amounts, whereas a few people don't. And, outside of carefully controlled name-brand processed foods, misuse can happen: the problematic quantities are not that far-fetched. A stray teaspoon here or there can be critical.
It's a recipe, pardon the pun, for a controversial issue.
