I have a restrictive diet, instead of asking people to cater for it I just fetch my own food and enough for anyone who might want it.
I did this at a gathering last week and a family member says its embarrassing and I should just eat what's given.
The only thing on the buffet that I could eat was the lettuce garnish even tho the host knows my restrictions, she forgot which is not a problem at all, I know it's a pain.