The Health Graph allows individuals to track and correlate the various aspects of their personal health and fitness.
There are 9Million+ registered Health Graph users and more than 55 partners who have launched Health Graph integrated applications and devices, with hundreds more in development. Users choose which applications and devices they'd like to use to capture their fitness activities, strength training, sleep information, nutrition, and more. Users can then use reports and charts to analyze their data, comparing different factors side by side to see how changes in one area affect them in another.
You can learn more about the Health Graph and its OAuth and RESTful API here: http://blog.healthgraph.com/about