Sounds kinda like an addict: gaslighting, blame shifting, rationalizing, excuses..... Is she maybe addicted to food and feels defensive? I'm probably pretty addicted to food, and feel some of this myself around thin and or healthy people. However, I do my best to realize that I am actually feeling conviction to change myself and don't want to admit it at that moment.
I agree with the above poster; as long as you're not being obnoxious and preachy, you aren't really doing anything wrong.
Having said that, I just thought of something that may help. Why not cook her a healthy candle light romantic dinner and maybe rent her favorite movie, when the moment's right try taking to her and relate your concerns about having to walk on eggshells, and that you just want to be healthy. Maybe suggest a way that you could both be healthy together. That can be tricky, but if you do it in a sensitive, gentle way that conveys your love for her and your desire to grow old together and not have health problems get in the way of your golden years.