Earlier quoted context omitted.
Doesn't this mean that eating any meat, or your own body chemistry, would also cause headaches?
You would think so, but somehow the artificial stuff is different or in greater concentration. Here's how I discovered my MSG intolerance: I made a bunch of onion soup (from onion soup mix), and ate a ton of it. I then spent the next 24 hours feeling terrible, downing pepto, etc. A few weeks later I made a recipe that included onion soup mix as an ingredient. Again I felt terrible. I though, Aha, I must have an onion…
When I was little, back in the 70's, my mom used these bouillons: Knorr and whatnot. We had no idea what is MSG, but somehow I clued in to the idea that I was feeling sick from that soup and was dreading the "headache soup".
In the late 80's, I once so ill from a cup of instant noodles that I had to go home.
Not long after that, I finally learned about MSG from a book I stumbled upon in the library (circa 1989, I think). It all clicked.
I understood mom's soup and the instant noodles incident.
I started avoiding everything with MSG and have scarcely had any apparently food-related headaches since: all the ones I've had over have been from eating out.
One particular series fell into a clear pattern. A new Chinese restaurant opened near where I work. I ate there on three separate occasions over the course of six months, around lunch time. Each time, I felt some discomfort in the abdomen toward the evening and headache/nausea the following morning lasting into the afternoon. Each time, exactly the same repetition.