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.