Elemental puppets and bionic bodies are very similar in many respects.
Inside Naioksate's bionic body, there is an alchemical stomach combination that can breakdown "food" and convert it to the energy needed by various organs during operation.
The "food" here is not things like soda, fine wine, fruits, and white bread, nor is it secondary energy sources such as kerosene and combustible animal fats that can burn to heat up a boiler boiling water.
It's the primary energy alchemical product produced by the Energy Research Institute: highly concentrated energy fluid, containing pure elemental energy. It can't be absorbed by a normal human body, and is only usable for alchemical energy devices such as alchemical stomach.
In situations where elemental energy can't be used, Naioksate can only maintain his life with concentrated energy fluid temporarily, keeping his alchemical organs operating normally.