This is not the central threat, but if you did want a mechanism, I recommend looking into the krebs cycle.

I think you've got something mixed up with the plot of "The matrix", where humans were used as "batteries", an idea that makes very little sense from a physics perspective and has been roundly mocked. 

The doomers say that misaligned AI will "use us for our atoms", but they're talking about a fanatical doombot that eats up all the matter on earth for it's purposes, human or not. The idea is also kinda dumb in my opinion, but people take it more seriously.