I continue to believe that some people must be overly sensitive to MSG and thus susceptible to the unpleasant effects Cecil mentions.
Data point: Myself. MSG sometimes affects me this way. It appears to be only large doses, as Chinese food was often reputed to contain. Foods like Campbell’s canned soup or other such prepared foods have never bothered me, even though they often list MSG among the ingredients.
This cannot be a case of placebo effect. When I was about 15 years old, living in Honolulu, we regularly ate at a certain Chinese restaurant. Every time, I got vaguely sick. (Feeling lightheaded and short of breath mainly.)
I knew nothing about MSG, nor its purported effects at the time. So it couldn’t have been a placebo effect. But the booths had rather deep upholstered bench seats, which caused the table top to be positioned high in front of my chest, which caused my arms (when my elbows or arms were on the table) to be held up high. I always assumed that this posture made breathing difficult.
Eventually, I made it known that I wanted to sit at the tables that weren’t in booths. The posture was better, but the unpleasant effects remained.
At some point during that year, I became aware of MSG and its purported effects. I connected the dots immediately, and concluded that the food there must be full of MSG and that must be the problem.