So Ponies consume products of the life in the world, creating nonliving matter. Dragons consume the inedible matter and purge it of pollutants and chemicals, leaving it revitalized for life.
I like that sort of idea.
That argument doesn’t work so well when you consider the fact that humans have a massive resistance to things that easily kill many other animals, such as theobromine.
Well, most myths about dragons say even non-fire-breathing ones have poisonous breath, and dragon blood supposedly has all kinds of freaky magical properties. So yeah, Spike’s probably immune to most things that can kill a pony, and probably considers them essential nutrients.