Abstract
<jats:p>Two leading paradigms dominate discussions of obesity: the Energy Balance Model (EBM), which attributes weight gain to a surplus of caloric intake over expenditure, and the Carbohydrate-Insulin Model (CIM), which emphasizes carbohydrate-driven hormonal shifts that promote fat storage. Here we introduce the Protein Partitioning Model (PPM), a muscle-centric framework proposing that dietary protein and muscular activity interact to influence energy partitioning and thereby shape body composition even under constant caloric intake. By integrating PPM with the Rational Addiction framework from economics, we model how food choices and body composition co-evolve over time through their effects on lean and fat tissue. The model predicts that, when paired with resistance exercise, protein intake supports functional lean mass, which in turn enhances insulin sensitivity and increases the marginal utility of protein consumption. This creates a virtuous feedback loop characterized by bounded escalation, converging on stable states with relatively low adiposity. In contrast, carbohydrate-consumption feedback can produce relatively volatile, binge-prone trajectories with much higher steady-state adiposity. By treating muscle as an active regulator of nutrient partitioning rather than a passive consequence of energy balance, PPM unifies key features of both existing paradigms while suggesting a proactive, muscle-centric approach to obesity prevention and long-term metabolic health.</jats:p>