You could use some voodoo magic (*cough*Math*cough*) to calculate the current percentage of your max health, then scale the upper layer (content) of the bar to said percentage of its max width. Or, as chrisinfinger said put a second background at the end of your bar, and increase that in size the more health someone loses.
Note that you'll want two values for this to work, a MaxHealth value and a (Current)Health value so you can Mathematician your way to the desired result. And two sprites, of course. A separate background and content sprite for each bar.