Upgraded to iOS 9.0.1 and lost all health weight data
Solution 1:
Backups don't backup health data unless they're encrypted backups.
Thus, if you restore one of these unencrypted backups, you won't restore the health data. This data is gone—it's not stored in iCloud for privacy reasons.
Solution 2:
Same happened to me this morning. Switching phone off and back on seems to have done the trick.