I’ve got a pretty severe sensitivity to – of all things – sugar. (I know, “sugar” isn’t very precise, but I’m pretty sure it’s either glucose, fructose, or sucrose.) I virtually never eat anything with added sugar or anything with any significant amount of natural sugar. And I’ve eaten that way for like 20 years now. I’m practically blind to half the produce department (any “sweet” fruits like apples, pears, cherries, grapes, oranges, etc) at the grocery store, let alone the candy isle.
You might want to look into fodmaps. They are sugar alcohols that people can have intolerance too (lactose is common) frustose in excess of glucose can be a problem. There are lots of others. Monash University does studies on how much is in what fruits and vegetables so you can exclude all of them, then test by reintroduction. Once you know which you are sensitive to, there are lists of which foods have which and safe portions.