My first thought is that if she didn't respond at all, you would probably take it as confirmation that you're as fat as you fear you are. That's based on my own experience of having an eating disorder, though, and maybe you would feel different.

Maybe you could ask her to bring up when she has concerns in a different way without mentioning how you look? I don't think it would be a good idea for her to collude with your ED and just ignore the issue. You're not seeing her for her to watch you starve yourself to death.