Monosodium glutamate: the same glutamate found in tomatoes, parmesan and mushrooms. 'Chinese restaurant syndrome' hasn't held up in controlled studies.
Chemistry
Your gut can't tell glutamate from MSG apart from glutamate in food.
History
Fears trace to a 1968 letter. Double-blind studies haven't consistently reproduced symptoms at normal intakes.
Sodium
MSG has about a third of the sodium of table salt.
Evidence & sources
FDA considers MSG generally recognized as safe; studies have not been able to consistently trigger reactions in people who report sensitivity. Evidence: regulatory. Known: FDA review and controlled trials. Unknown: Some people may have mild, short-lived symptoms after large doses without food. Source: US FDA, Questions and Answers on Monosodium glutamate (MSG) (Current revision).