Side note: I talked to an Apple developer at WWDC and was told that HealthKit data never leaves the device (isn't synced with iCloud). Hence the lack of the iPad app. Given that, if the data is lost, its probably gone forever.
Apple backs up your Health data in iCloud, if you enable it.
"Your data in the Health app is encrypted with keys protected by your passcode, and never leaves your device unless you choose to back it up or grant access to a third-party app. When you do choose to back up your Health data through iCloud, it is encrypted both in transit and on our servers."
"...only in encrypted iTunes backups...", I think certain encrypted data is also stored in iCloud backups as you're required to authenticate the restore before it continues. An iCloud restore recently brought back all my email password settings etc, but an iTunes non-encrypted restore lost them.
"Your data in the Health app is encrypted with keys protected by your passcode, and never leaves your device unless you choose to back it up or grant access to a third-party app. When you do choose to back up your Health data through iCloud, it is encrypted both in transit and on our servers."
https://www.apple.com/privacy/privacy-built-in/