Nitrogen Homeostasis Reveals an Internal Inconsistency in the Energy Balance Model’s Predicted Coincidence of Energy Balance and Body Weight Stability

The Energy Balance Model predicts that energy balance coincides with body weight stability. The present analysis shows that this relationship emerges from the canonical equations of the model rather than from an assumption introduced by the study. Further analytical examination demonstrates that this predicted state corresponds to fat mass and fat-free mass stability and consequently requires nitrogen homeostasis. These conditions lead to a coupled mass- and energy-accounting system. When solved without enforcing nitrogen homeostasis, the equilibrium solution requires persistent carbohydrate accumulation, protein depletion, and negative nitrogen balance, a combination physiologically incompatible with a prolonged steady body weight. Conversely, enforcing the already-derived requirement of zero nitrogen balance eliminates these macronutrient imbalances but requires obligatory non-urinary nitrogen losses to vanish, which is physiologically impossible. Thus, the contradiction arises from the model’s equilibrium structure. By formal analytical reasoning, prolonged body weight stability must therefore coexist with energy imbalance, a conclusion consistent with observations of stable body weight despite apparent energy imbalance and randomized metabolic-ward data in which energy imbalance failed to correctly predict both the direction and magnitude of body weight change. Energy imbalance alone cannot sufficiently describe body weight dynamics because energetic quantities do not uniquely specify material fluxes.