Wearable health technologies and fitness apps are increasingly interconnected through APIs (application programming interface) through which data from sensors are transferred, translated and interpreted. This paper builds on a close critical reading of the technical documentation of some of the most popular fitness APIs (Apple HealthKit, Fitbit, HealthGraph and Withings among others) and aims at establishing an understanding of how human bodies and their activities are conceptualised and measured as data through APIs. Departing from these empirical readings, the paper conceptualises fitness APIs as “health data superstructures” that to various extents render certain self-tracking and body-monitoring practices more feasible than others. Such a conceptualisation is important since it allows for further explorations of the limits and possibilities of how wearable technologies are designed, developed and used in conjunction with other apps and technologies.