Once I've quit eating large amounts of a food for a few months or so, I get sick if I eat even a tiny amount. Other people experience this also, with gluten and other foods.
I got sick from a tablespoon of fructose. It's made from corn. A fructose manufacturer told me it would have no more than 20 ppm corn protein. A tablespoon would have on the order of a milligram. To check that my reaction was from corn, I tried 1/16 of a grain of corn. I sliced it up with a razor blade. I was very sick in bed for 5 days or so, too bleary to do much more than poke my head out the door. That was about 1/2 milligram of corn protein. Now that I'm carefully avoiding corn I'd probably get sick from much tinier quantities. I haven't experimented to see how tiny a trace of corn can make me sick. I've chosen to just avoid all corn products.
I got sick from a teaspoon of guar gum (guar is a legume), one little 200 IU vitamin E capsule made from soy, and an invisible trace of peanut butter on my finger, that I nibbled accidentally;
From a 1-gram fish oil capsule; 1/2 tsp. of D-mannose made from birch trees; a teaspoon of beef chondroitin; a tablespoon of unfiltered honey - yes, the "healthy" natural local honey I can't eat, though filtered clover honey is OK; a tsp. of hazelnut oil; a tsp. of glucosamine made from shellfish; a few drops of sunflower oil.
There are limits to it. I don't get sick from the smell of foods. The baking section of the supermarket, heavy with the smell of wheat, is OK. I was French-kissed by a German Shepherd once that had eaten peanut butter for breakfast, several hours before. I didn't get sick.