Very similar set of circumstances for me as well, toomuch. I have a few techniques that I use, all to varying degrees of success:
1) The straight-forward approach. I simply say, "I quit for health reasons". Since most people aren't comfortable asking others about their health issues, this usually ends it.
2) Throw some off-color humor at them. My favorite is the old Robin Williams joke: "Yeah, I got tired of waking up...laying under the bumper of my car....keys in my ass....". The shock of that joke is usually enough to end the discussion.
3) If I feel that it's someone who is close enough to let into my circle, I'll say, "because I like myself better this way", and leave it at that.