We are trying to decide on our gamification and points scoring strategy for our health game, we are modeling after "candy crush" with it's near immediate gratification, with extra pointfor photo uploads of fruit and veggie and water photos, and extra point for sharing with friends, does it make sense to set up this simple system with Game Center at apple for the initial sprint, then add in points for weight loss with the next release?
Not directly related to your question, but I would be careful about passing user health data to the Game Center. I don't have enough details about your product, but here in the States there are strict privacy requirements dictated by HIPAA when it comes to protected health information (PHI). You may not want to tie a user account (in game center) to health activity depending on exactly how it's going to be used and whether it would fall under PHI as defined by HIPAA.
Also, as a diabetic myself, the idea sounds pretty cool. If someone could gamify my regular blood sugar testing I'm sure I'd be more compliant with my testing schedule :)