Ok, I'm no featherweight, but I would like to say something anyway.

I would make something up and say that too much food/fat/sugar/salt (whatever you wanna say) gives you a headache/stomachache/heartburn/whatever. Somehow, if people think you will be in physical pain, they tend to back off quicker. They might say "a little won't hurt," to which I would reply, "actually, I must be very sensitive, because all it takes for me is a little and I'm miserable."
Family/friends tend to underestimate how painful it is for us if we over indulge. If we want to, we will eat, but we don't need that added pressure. If they think it's for weight loss, they think one day won't make a difference, but if they think you'll be in actual pain/misery, then maybe they'll lay off. Just a thought.
