Wow, this sounds familiar. My co-workers all know about my problem, and yet the last couple of times we've had b-day parties in the office, two of them put the fork to their mouths (with b-day cake) and looked at me and said, "mmmm, this is so good." They know, on top of everything else, that I'm lactose intolerant. I just ignore them. It's way past insensitive in my opinion. Another one continually makes faces when she looks at my lunches.
Oh, and the gym thing. Yep, get tired of people asking why I'm going to the gym, because, they say, I certainly don't need to. That shows their ignorance, as they think gyms are only for people wanting to lose weight.
So try just to ignore the insensitive, ignorant people. It gets easier.