On the Full Metal Alchemist Wiki under the Category Alchemy. It says this...
The circle itself is a conduit which focuses and dictates the flow of power, tapping into the energies that already exist within the earth and matter. It represents the cyclical flow of the world's energies and phenomena and turns that power to manipulable ends.
But does this energy not come from a parallel world on the other side of the gate where alchemy took a different form of science? Did it not come from the lives of those who died in that world? Or is this different in FMA Brotherhood, because I am certain this is what Hohenhiem told Edward in the FMA Anime (2003).
Answers are appreciated. :)