You may have curiously watched #june7 hashtags appended to many RunKeeper team members’ tweets in the last couple of weeks. That day is here, and along with it the biggest release in the company’s three year history — an API. Branded the Health Graph API, RunKeeper is launching with a set of well known partners including Foursquare, Withings, Polar, Wahoo, and Boston’s own Zeo.

The move is a big one in terms of strategy for RunKeeper, as APIs (Application Programming Interfaces) have been critical to many companies’ growth. While a lot of programming is involved to build an API (kudos to the RunKeeper developers), it is simply a layer that allows two different pieces of software to talk and integrate with one another. Facebook, for example, has built an API that allows developers to build things like games into the social network. Developers benefit because they gain access to Facebook’s pool of hundreds of millions of users; Facebook benefits because its users have more to engage with and thus more of a reason to frequent the social network.

In RunKeeper’s case, the company has built an API that allows any health monitoring device to integrate with its app. In doing so, a device – be it for sleep monitoring, calorie counting, glucose monitoring, heart monitoring or otherwise – gains access to RunKeeper’s 6+ million passionate and health conscious users who are logging their fitness activity. Beyond this, device manufacturers and developers can draw from all the fitness data RunKeeper has been tracking.

Enter the Health Graph.

Behind the Health Graph API is (go figure) the Health Graph, with the idea being that each person has his/her own unique graph. Making up this health graph is information in addition to the fitness activity that’s already tracked by RunKeeper — data like how many bars or gyms you hit up, cake or carrots you eat, and how many hours of sleep you knock down at night. RunKeeper’s engine powers this Health Graph, making correlations between separate health information over time, putting meaning behind it, and serving you the information in a way that allows you to easily make practical sense of it (read: actually become more healthy).

“Imagine a system that can identify correlations between a user’s eating habits, workout schedule, social interactions and more, to deliver an ecosystem of health and fitness apps, websites, and sensor devices that really work, based on a user’s own historical health and fitness data,” explained RunKeeper founder and CEO Jason Jacobs. “The Health Graph has the potential to completely alter the health and fitness landscape.”

Prior to today’s launch, RunKeeper has integrated in a private API with a wi-fi body scale, sleep monitoring devices, and heartrate transmitters. What they learned: As a user adds these devices to their profiles, their engagement with tracking and monitoring their health and fitness does as well.

“We learned a lot from integrating with a handful of health sensors through our private API in the last couple of years, and it became clear that the correlation engine we built could be utilized across a much broader range of sensors and health categories,” Jacobs told BostInno. “We are thrilled to be opening up a public API today so many other health sensors can tie into the underlying Health Graph that RunKeeper was built on and gain access to the large, passionate RunKeeper user community.”

To help spur device manufacturers and developers to integrate with the Health Graph API, RunKeeper has launched a corresponding affiliate program. These developers and device manufacturers also have the opportunity to get their brand within RunKeeper’s social features — its FitnessFeed and Twitter and Facebook integration — which have been critical to building and engaging its passionate fitness community to date.

And at the end of the day, the real beneficiaries in this release are RunKeeper users, who can now track, monitor and learn from various types of health data in addition to fitness activity – all in one place.

To learn more, like exactly what data is available for the Health Graph, read more on the RunKeeper blog.