A System’s True Body
Humans needed to eat food to survive and function as living beings. The energy in their body was broken down from carbohydrates, fats, etc.
In the Darble World, there was the concept of Darlac, an energy source that was the condensation of one's stamina. It could be said that Darlac was the primary energy of the Darble World.
And in the apocalypse world, after the apocalypse, it was the Apocalypse Pearl.
In a cultivation world, it was Spirit Energy. And finally, in a magical world, it was Mana.
Basically, every living being relied upon some energy type. And, this energy type was something that they were naturally accustomed to dealing with.
Consider Darlac, for example. It was the condensation of stamina. Every flesh and blood creature had stamina. And, stamina was something that could be replenished through rest, eating food, and drinking water.