Here’s a fascinating piece on the sources of food myths, and the scientific truth about MSG: How MSG Got A Bad Rap: Flawed Science And Xenophobia, written by Anna Maria Barry-Jester for FiveThirtyEightScience.

MSG was developed in Japan as a flavour enhancer, and was popular in North America for more than half a century.
But then…

In 1968, the New England Journal of Medicine published a letter from a doctor complaining about radiating pain in his arms, weakness and heart palpitations after eating at Chinese restaurants. He mused that cooking wine, MSG or excessive salt might be to blame. Reader responses poured in with similar complaints…

In other words, anecdotes and confirmation bias. Early research seemed to support the anecdotes. But…

Subsequent research has found that the vast majority of people, even those claiming a sensitivity to MSG, don’t have any reaction when they don’t know they are eating it….
…a small subset of people do have negative reactions that are directly due to glutamate, but the science to date shows that is likely to be a rare phenomenon.

and

“It was the misfortune of Chinese cooks to be caught with the white powder by their stoves when the once-praised flavor enhancer suddenly became a chemical additive.” [emphasis added]