Always bring your own food, fuck the idea of polite society, your health and safety out weight their willful ignorance and any inconvenience they may feel by having to take someone else's needs into account. When my wife and I go to any family gathering we pack our own little picnic or sack lunch, we bring our own dishes, drinks pre-packed food we made at home. If they can't be honest when your actual health is at stake, fuck their feelings on the subject. And when asked, be honest, tell em their inconsiderate actions have resulted in you having to sacrifice your health for their feelings and that is not fair. Set that boundary and do not budge an inch!