User blog comment:Water Dragon Slayer Shawn/Theory on why Gajeel and Natsu couldn't pass Freed's barrier./@comment-7418318-20140625112926/@comment-25100784-20140629053002

That's a very good replacement theory.

Maybe the offered blood or tissue is the reason for the clear change in the physical properties of their organs? Even if their bodies, and organs remain looking human like, their entire bodies would all have to have their physical properties changed to accommodate for accepting something like fire, or iron, or even air, as a form of sustenance and nutritional value. Gajeel has, after all, repeatedly proven he doesn't need regular human sustenance, seeing as all he ever eats is scrap metal.