Don’t sit at the table.
See if you can help in the kitchen–putting food on plates, washing dishes, etc. Or help serve at the table and clear the table. This way people might not notice that you aren’t eating, because you are still participating in the meal (only in a different way). Even if they know you aren’t eating, at least it won’t seem like you’re rejecting the meal or the occasion.
If the hostess won’t let you help, maybe you can excuse yourself when the meal begins, and go for a walk or sit in another part of the house. Men do this all the time; women seem to have more trouble doing it. You can read or watch TV, or play with a little kid so the mother can enjoy the meal. When people are finished eating, you can join them for tea or coffee or whatever activity is planned for after the meal.
If neither of these is acceptable, make an excuse and arrive late, or leave before the meal begins. I’m learning to socialize without feeling like it needs to include eating together.
I hope that helps. Good luck!