Originally Posted by mrsD

Magnesium stearate is not absorbed. It is a filler added to powders so they will flow easily thru the tablet compression machine. It is sort of a powder lubricant. Your body cannot absorb it, and it passes out of your digestive tract in the stool.

If you dissolve a product with magnesium stearate in it, in a glass of water, you'll find that the stearate remains as a residue.
In capsules it even floats on the surface of the water.