Is the bar "filled"? Ie. do you have a second sprite of an empty bar with an internal color fill? Have you tried simply setting the bar width to 100 or another set value and reducing its width the amount of damage taken? If its doesn't drain and show an empty sprite you could also put the bar on a lower layer than the end cap and put a background colored sprite behind the end cap to edge of screen and then shift the health bar to the left "behind" them with each increment of damage.
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.
newt wrote:@emoaeden's example is a great formula. Just remember it's current health, and initial size at a 100 percent. Health*(256/100)
@imothep85 : The reason it's 256 is because it's 256 pixels wide, so if you have bars that are longer or shorter, just make sure you adjust accordingly. Probably an obvious statement, but just in case you're super new, thought I'd point it out