Where does health data actually come from?

For most metrics you have three routes: the on-device platform stores (Apple HealthKit on iOS, Android Health Connect), a specific wearable's cloud API (Fitbit, Garmin, Oura, WHOOP, Strava), or a health-data aggregator that normalizes many of them behind one integration. Which route is best depends on the metric — some are nearly universal (steps, heart rate), others need specific hardware (body composition needs a smart scale).

Is the data measured or estimated?

It varies by metric, and it matters. Heart rate and HRV are measured; steps are counted (algorithmically); calorie burn, VO2 max, and sleep stages are modeled estimates, not clinical measurements. Each page here is explicit about which, because presenting an estimate as a measurement is both misleading and, for health features, a compliance risk. Treat consumer-device data as a wellness signal, not a diagnosis.

Do I need a wearable for every metric?

No. Steps, basic activity, and GPS can come from the phone itself. But recovery and physiology metrics (HRV, VO2 max, blood oxygen, sleep stages) generally require a wearable, and body composition needs a smart scale or manual entry. Each page notes what hardware, if any, the user must have.

Health Data

Health Data by Metric: Which API for Each

Need a specific health metric in your app? Each page below takes one — heart rate, steps, sleep, calories, HRV, VO2 max, and more — and answers the practical questions: which sources expose it via an API, how to access it, whether it’s measured or a modeled estimate, and the best pick for your build. Organized by the data you need, not by provider.

Want the provider view instead? Compare by category in the fitness API landscape, or read up on the concepts behind the numbers in the explainers. When you know the source, the integration guides take it from there.

Cardio & recovery

The signals from the heart — measured and estimated.

Activity & movement

What the body does — steps, workouts, routes, energy.

Sleep & body

Overnight staging and body measurements.

Frequently asked questions

Where does health data actually come from?: For most metrics you have three routes: the on-device platform stores (Apple HealthKit on iOS, Android Health Connect), a specific wearable's cloud API (Fitbit, Garmin, Oura, WHOOP, Strava), or a health-data aggregator that normalizes many of them behind one integration. Which route is best depends on the metric — some are nearly universal (steps, heart rate), others need specific hardware (body composition needs a smart scale).
Is the data measured or estimated?: It varies by metric, and it matters. Heart rate and HRV are measured; steps are counted (algorithmically); calorie burn, VO2 max, and sleep stages are modeled estimates, not clinical measurements. Each page here is explicit about which, because presenting an estimate as a measurement is both misleading and, for health features, a compliance risk. Treat consumer-device data as a wellness signal, not a diagnosis.
Do I need a wearable for every metric?: No. Steps, basic activity, and GPS can come from the phone itself. But recovery and physiology metrics (HRV, VO2 max, blood oxygen, sleep stages) generally require a wearable, and body composition needs a smart scale or manual entry. Each page notes what hardware, if any, the user must have.

Independent comparison, last reviewed July 24, 2026. Pricing, rate limits, and feature availability change often — confirm current details in each provider’s official documentation before you commit. Product and company names are trademarks of their respective owners; AIFitnessAPI is not affiliated with, endorsed by, or sponsored by any product listed here.