First of all, I would never say, “I can’t have…” whatever is offered to me. I would simply say “Thanks anyway.”
We’re just not junk food eaters in our family, so the food isn’t a problem; and if new guests are present, they learn pretty quickly that our whole family is more health orientated than ‘what’s for dessert?’ Sure, there are times when we’re at parties, etc. when we just have to say, “No thank you” to cakes and other sugary foods, but the people close to us have come to know that this is who we are. If it bothers them in the beginning, they either get past it or leave, it’s their choice just like our food preferences are our choice. It’s really all about individual free choices and no one else’s concern. Or so it should be.