"Health data can be stored in iCloud. When configured for iCloud storage, Health data is synced between devices and secured by encryption that protects the data both in transit and at rest. Health data is only included in encrypted iTunes Backups. It is not included in either unencrypted iTunes backups or iCloud Backup."

https://www.apple.com/business/docs/iOS_Security_Guide.pdf -- pg. 29 under the Health Data subsection.

Edit: @bobwaycott points out the significance of this quote in the below child comment.