be weakened .weaken

The only coherent sentence would be "The absorption of Vitamin D will be weakened."

If you write: "The absorption of Vitamin D will weaken," you have an incoherent sentence because it does not tell what will be weakened by that absorption.

ja ...thank u . i get it

Well, S., yes and no. Written that way we'd assume it somehow weakens the body not the absorption

We? You might, and i would not be surprised at that.