When stuff like this happens I always like to play dumb.
"Wait...did you bring a pound of bacon and eggs in Tupperware for lunch?"
Me: "Yeah, but there's also steak and sour cream in there. Also some spinach."
"Aren't you worried about [fill in the blank]?"
Me: [Long blank stare] "...Huh?"
"It's so much [fat/cholesterol/bad for you, etc]"
Me: "I don't understand what you're getting at. You've seen my raging abs, right?" [shrug, walk away]