I artificially placed Health Boost on through the /effect command:
/effect playername 21 3000 125.
This increased my health to go off the screen.

I then filled my health with potions, regen, etc.

I then removed the effect with the command /effect playername 21 0 0.
The graphic went back to 10 hearts, but upon taking damage I lost no health. Only by repeatedly taking more and more damage did I eventually start to lose health.