I equate it to lactose intolerance. Some lactose-intolerant people can have milk but only up to a certain limit.
Similarly, I have slight reaction to gluten (acid reflux), but only if I have a lot of it. So at office parties, I can get away with a square inch of cake or one donut hole and then explain that's all I'm allowed... but oh how it tastes so good. (and actually it does kinda taste good). This tack has worked well with everybody.
solstice, why not bring your own salmon dish to Thanksgiving next year? Then you can avoid the turkey without making demands on or burdening your family... and they can have some salmon too!