I like this idea. It makes a lot of sense.  I think perhaps an additional requirement would be mandatory attendance to nutritional courses and exercise classes. If you don't make those or don't stick to a prescribed diet (which the courses teach you how to make), then you're removed from health care eligibility. These classes could even be open to anyone who wants to - but only fats will be tracked - so it could still be a positive learning experience used in the system without catering to fatasses.