Left out caffeine. Just a small dose and a major migraine in minutes. 'On the floor where I stand' type reaction. I'm so careful that it doesn't happen very often.
They developed after I got older. Used to be impervious to vast quantities of MSG, nitrates, and caffeine. In my late twenties I started having bad headaches, then full blown migraines. Once it started happening, at least the links were pretty easy to make. The time between ingestion and migraine is very short. What is most amazing to me is the minute quantities that it takes. A few bacon bits on a salad may as well be a cyanide pill.
Sugar is more forgiving - which actually makes it a bit riskier because I know that a little is not a problem. It is too much that is a trigger. 'Pure', 'natural' sugars are okay; fruits and such. I can knock 'em down anytime; all I want. It is refined sugars where I can pass a tipping point.
I had a Mexican 7-Up once. Picked it up a local tienda (Mexican grocery or convenience store.) Thought nothing of it, but the recipe is not the same as US 7-Up. It has (or did at the time) nitrates in the ingredients. I was going from there to home and did not think that I would make it. No small dose, it was hours of agony.