How do I tell my parents that I am concerned about thier weight without really offending them?

My parents just came to visit me for three months since I live in Florida and the winters in Missouri are aweful. My mother had major back surgery in Aug 2009 and I was there for that. At the same time my mom was in surgery my dad had a massive gall bladder attack and had to have his gall bladder removed. He is a heart patient and my mom besides the back surgery has high blood pressure. In the 4 mos. that I have seen them.. she has put on at least 30 lbs and my dad has put on about as much.. I know this is not healthy for either of them and added weight to my mom is not going to help out her back. How do I approach the subject or even should I?