This sounds pretty typical. I agree with "Catslave" below that it is a shared lane. I guess it depends on your view of marriage, but I always felt that we should be concerned for each other's health and lovingly help each other overcome any significant weak areas.
It is hard to watch someone you care about neglect their health for years and years, especially when they insist that you get help for your issues (that's what happened with us). My older son has fatty liver and the doctors are all serious about it. Probably because of his age. It's true that carrying fat predominantly in the belly area is a bad sign.
She probably loves you and imagines what her life would be like if she got "that call" and had to go on without you. I thought about that a lot. Especially with special needs kids, if I went through that level of grief while they were still kids - and then, how would THEY handle the grief? Is the junk food really worth it? Won't you do this for me? That's where my head was at.
If she's not doing it lovingly, you can discuss that (nagging isn't nice). But it's important for those of us in our 50s with kids who still need us to at least attempt to stay alive, right?