Have you purchased life insurance?  You get a physical, and they take down your weight.  The premium depends on whether you are overweight, smoke, have high-blood pressure, etc.  That is fair, and I think the same should be done for health insurance, especially if it gets nationalized. 
  
There is a difference between suffering from an ailment due to "bad luck" (including genetics) and purposely engaging in an unhealthy, destructive lifestyle.