system
Given the following post, respond as if you're commenting on Reddit:
human
[Avatar] Does bending require chemical energy from the bender? Do they burn calories when bending like someone would exercising?

Earth benders can levitate massive rocks out of the ground and throw them with incredible speed. To physically do that would require a huge expenditure of energy accelerating that rock.
But are earth benders expending all the energy necessary to move that rock? Is chemical energy from the bender being converted into potential/kinetic energy of the object?  

If so, how does a bender like toph manage such energy output without eating 10 meals a day? Is there an alternative source of energy from the spirit world?

If benders are not using their own energy, what is preventing a fire bender from creating a heat engine that uses little to no fuel?  They could attain unlimited power generation from such technologies by breaking the laws of thermodynamics.
gpt
The Mind, Body, and Spirit are all tied to together in ways that aren't always understandable. Bending usually involves all three working in harmony (the exceptions are those such as Sparky-Sparky Boom Man who could bend with just his mind). However, the energy behind a thrown rock or a burst of flame does not come from the individual bender, but from the cosmic energy that permeates reality. It requires physical and mental exertion to shape that energy; we see benders sweat and grow tired with continued action. Thus, it's actually more efficient to burn fuel with a firebender there to regulate the flame than have one work a steam engine 24-7. In Republic City, we see that Mako has a job lightning-bending into a generator at the power-plant, but the size and machinery involved seems to indicate that he was either enhancing or regulating the conventional machinery. 

TL,DR:  Blah-blah, spiritual mumbo jumbo, blah-blah, something about space.