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 allergy, this onion soup mix keeps making me sick.
So I started checking if onions themselves made me sick. But onions were no problem.
Then I looked at the other ingredients on the onion soup mix. Monosodium glutamate stood out. I remembered hearing that could cause problems. So I started learning more about it, and as I did much of my digestive life came into focus. I remembered moments of intestinal agony ever since I was a teenager that I couldn't explain. I paid attention to what bothered my stomach, and always there was some form of MSG---autolyzed yeast, etc.
Since making an effort to avoid MSG I almost never encounter those symptoms. And when I do it's usually attributable to food prepared by others whose ingredients I can't strictly account for.
People claim MSG intolerance isn't a real thing, but my experience has shown me otherwise.
