If you are wearing armor with an attribute modifier to give extra base health, upon saving and reloading the world the extra health is lost and has to be regenerated.
MC-81113
attribute modifier health lost on save
Resolved
Incomplete
0
Is this still an issue in the current Minecraft Snapshot 15w47c or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.