FIGURE 2.

Energy Balance is determined by the collective state of 3 interactive sets of control processes: Nutrient Acquisition, Nutrient Partitioning, and Energy Expenditure. It fluctuates between three states: Negative (A), in Balance (B), and Positive (C). ATP availability (pink box) is the principal regulated variable in Energy Homeostasis and acts at the pivot point of Energy Balance (bottom box in A, B, and C). The various control processes that regulate ATP availability are shown with blue text and arrows, the size of which indicates their relative activity in a particular state. They are as follows: Eating/Digestion (Nutrient Acquisition); the synthesis (anabolism) and breakdown (catabolism) of energy storage polymers from/to their constituent oxidizable monomers (Nutrient Partitioning); and the rate of Energy Expenditure. In Nutrient Partitioning these monomers are appropriately distributed between Oxidizable Fuels and Nutrient/Fuel Storage depending on the current state of Energy Balance. ATP generated by oxidizing fuel molecules drives 2 general processes, the sum of which determines overall Energy Expenditure: internal Cell Function (active transport, secretion, endergonic reactions, etc.); and Organism Function (movement, growth, reproduction, etc.). In an extended absence of eating (A), ATP availability is maintained by catabolizing fuel stores and reducing energy expenditure. The phosphorylated state of AMP kinase (the sensor of ATP availability; gray boxes) is favored under these circumstances. When energy stores deplete, Cell Functions are maintained by reducing the anabolism of macromolecules to an extent that can compromise growth, reproduction, and other Organism Functions. When Eating provides oxidizable fuels that are equal to the requirements to maintain ATP availability (B) then the rate of anabolic and catabolic processes is equal, i.e., a state of energy balance. If Eating provides an excess of fuels required to maintain ATP availability within its narrow range (C), then energy balance favors anabolism and the dephosphorylated state of AMP kinase then predominates.