It’s always a good idea to periodically check and see what apps have access to your data, health or otherwise. Via Gruber:
HealthKit was designed from the ground up to be cryptographically secure in this way. You can check which apps have access to what Health data in Settings → Health → Data Access & Devices.
I do trust Apple with my data. Perhaps naïvely? While they’re not immune from potentially becoming a bad actor in this space, they’ve shown time and again their unwillingness to use customer data against them, or hand over customer data or keys to customer data to law enforcement.