I'm the same as you. i just explain that seed oils make me ill (in my case they do - I was very ill for years before I discovered it was the seed oils) and sorry but I can't eat your food. I've stopped caring if they are offended, i'm not rude just being honest. I have been surprised how many people also don't eat seed oil that I otherwise would never know.
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It’s true for me as well. I mistakenly ate something once that I didn’t know had seed oils in it and got diarrhea. I definitely notice the effects food have on my stomach now.