The interaction between appetite activation and the energy content of the brain and the body is mathematically modeled. General influence functions with saturation are used to describe the interaction. The resulting class of models is investigated with respect to the circadian periodicity of human food intake. We show that very weak and physiologically reasonable assumptions on the constitutive functions are sufficient to validate the indispensable role of the brain as energy consumer in appetite regulation. Subsequently, implications from our systemic investigations of metabolic systems can be drawn although the quantification of the underlying regulatory pathways is uncertain, incomplete, or incompletely understood.