Here's what I know:
1. I occasionally get symptoms often associated with MSG. Terrible headache, confusion, nasal drip, nausea, and so on. It can put me out of action for about 12 hours.
2. I invariably get these symptoms about 24 hours after I've eaten processed foods with an unusually rich flavour and which have, somewhere in their ingredients, either MSG or something that might conceal MSG (e.g. "flavourings").
I can't guarantee that it's MSG, because I'm too chicken to test it directly (that headache really sucks), but it certainly fits the profile.
Now some people point out, "It occurs naturally in all kinds of foods. The original MSG was derived from miso soup, which you eat every day."
And my answer is, "I don't know. Maybe it's a subtle variant of MSG that only occurs in the synthetic variety? Maybe the naturally occurring MSG is bound up with other things that render it safe for me to eat? But something's making me ill, and it's only in synthetic ingredients."