How can I say no to food without offending?

As a Scottish revert, I find it hard to politely refuse food and invites in traditional Muslim households. I'm autistic, so large gatherings overwhelm me, and I have bad heartburn-spicy or oily dishes can make me sick for days. I always tell them the food looks amazing but I can't eat due to health, yet 'no' often isn't accepted, and I end up eating after repeated offers. Now I'm pregnant too, so I can't skip meals for hours. Sometimes I need to decline an invite to go home and rest. I know offering food is from kindness and I don't want to upset anyone, but I need a firm 'no' that is respected. Any tips?