I'm a blunt person, I admit I don't even bother anymore with so-called 'politeness' that I'd be the only one to take into account anyway (obviously... since if other people did it, they wouldn't question my food choices at first). So while I don't necessarily want to go into the weight loss discussion--after all, there's much more than weight to healthy eating--I just say something along the lines of "sorry, I have problems digesting this. Do you want me to give you the whole details, or shall I leave you enjoy your food just like I'd like to enjoy mine?" The prospect of hearing about potential gastric and intestinal explosions not being very appealing to most people, especially not during a lunch, the subject is then quickly dropped.

(And it's not a lame excuse, by the way; junk food IS okay until it reaches my stomach, but the next two days after a meal at, say, McDonald's are... uh, rough. The body is a beautiful thing; once you start feeding it with healthy foods, it doesn't tolerate the junk ones as it did before. At least mine doesn't.)