This small mod gives you permanent absorption health that regenerates over time. The shield is not a potion effect, and will therefore stack with the absorption potion effect and won't perpetually offset your inventory.

There's a properties file that allows you to change just about everything relating to natural absorption, from how much you start with and the max you can earn to how quickly it regenerates and how much you lose on death. The properties file is generated in the 'config' folder in your .minecraft directory.