What's the best device SDK for syncing wearable health data to a companion app?

The “best” SDK depends mostly on which wearable devices you need to support and how much direct access to health data you want. In practice, there isn’t one universal winner—there are a few strong choices:

Best options by use case

1) Fitbit Web API / Fitbit SDK

Best if: your app focuses on Fitbit devices or you want a mature, consumer-health ecosystem.
Pros:

• Good health/fitness data coverage
• Stable APIs and established developer docs
• Useful for steps, sleep, heart rate, activity, etc. Cons:
• Mostly Fitbit-specific
• Some limitations around real-time data and permissions

2) Apple HealthKit

Best if: you’re building an iOS companion app and want access to aggregated health data from Apple Watch and other connected apps.
Pros:

• Excellent on iPhone/iOS
• Strong health data model
• Great for syncing local health records into a companion app Cons:
• Apple-only
• Doesn’t directly unify all wearables; it aggregates what apps/devices write into HealthKit

3) Google Health Connect

Best if: you’re on Android and want a unified health data layer from multiple apps/devices.
Pros:

• Android health data hub
• Designed to centralize data from multiple sources
• Good for steps, sleep, heart rate, workouts, etc. Cons:
• Android-only
• Device/vendor coverage depends on who integrates with Health Connect

4) Garmin Health API

Best if: you need Garmin device data, especially in wellness or clinical-style workflows.
Pros:

• Strong Garmin ecosystem support
• Good for enterprise/health partnerships
• Solid wearable metrics Cons:
• Requires Garmin-specific integration/approval in some cases
• Not a general cross-device solution

5) Samsung Health SDK / Partner integrations

Best if: you need Samsung ecosystem data on Android.
Pros:

• Useful for Samsung wearable users
• Good device/app ecosystem support Cons:
• More limited ecosystem than using Health Connect
• Samsung-specific

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If you want the “best general choice”

For a companion app syncing wearable health data across devices, the most practical approach is usually:

• iOS: HealthKit
• Android: Health Connect
• Then add vendor-specific APIs only when needed:
  • Fitbit
  • Garmin
  • Samsung Health
  • Oura, Whoop, Polar, etc., depending on your target audience

This gives you the broadest coverage with the least fragmentation.

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Rule of thumb

• Single brand wearable app: use that brand’s SDK/API.
• Companion app across ecosystems: use HealthKit + Health Connect, plus vendor APIs for gaps.
• Enterprise/clinical/wellness program: consider vendor APIs with strong partner support, and a backend normalization layer.

If you want, I can recommend the best SDK stack based on:

1. iOS vs Android vs both,
2. which wearables you need, and
3. whether you need real-time sync or just periodic health data import.