for a literal fountain of youth, as in you consume something and you get literally younger, perhaps the mass just dosen't disappear it's ether shed/perspired or forms lumps that sluff off the person, perhaps while making noises or you get to uneat every meal between the current you and the new young you.
in a less literal fountain of immortality, perhaps the process turns you into a type of cancer.
or perhaps the fountain was created by a jerk ass genie, it "resurrects" you in the form of a child using your current body as reaction mass, maybe you don't even know it and only get the Chest Burster feeling.
Perhaps it requires you to do something horrible (and keep doing it) to maintain your "youth". And if you don't do it, you die in some disgusting fashion. I know that's been done before, but properly pulled off it could be cool.
If I were to write some of the strange things that come under my eyes they would not be believed. ~Cora M. Strayer~An idea I had is that the Fountain causes the user to grow a new, younger body that is physically grafted on the old one, which will grow older and die. Essentially, whoever uses it will have to carry around his own corpse as extra body weight.
https://soundcloud.com/rich-justice-hinmen Too white for the black kids, too white for the white kids.

into a much darker instance, ala Body Horror. How should I communicate to people this? Any way to do so to provoke revulsion and a slow realization of what is happening is not good.
I am a big fan of Body Horror and want to use this trope in a much squickier way. For example, what kind of pain would the transformee go through? What kind of degeneration would the mind go through as it travels backward thru time.