Abstract for: The Tonic/Dynamic Energy Balance System: Results from system dynamics modelling of five conditions of weight change

Understanding energy balance and body-weight dynamics requires capturing complex feedback mechanisms between energy intake, expenditure, and body composition. However, existing approaches often lack a unified dynamic representation of these interactions. This study develops a quantitative system dynamics (SD) model to represent the relationships among energy intake, energy expenditure, fat-free mass (FFM), and fat mass (FM). An SD model of energy balance was developed from existing conceptual frameworks and experimental studies to capture key feedback mechanisms. The Tonic Subsystem represents the influence of body size (primarily FFM) on energy intake. The Dynamic Subsystem, activated by changes in FFM, contains two balancing loops: a ‘Metabolic Dial’ that adjusts energy expenditure and an ‘Appetite Dial’ that regulates energy intake. The model was validated against five body-weight reference modes: weight maintenance, passive calorie loss, diet-induced weight loss and regain, short-term overfeeding, and gradual weight gain. Feedback mechanisms were critical for reproducing these patterns, with Tonic and Dynamic effects on intake sometimes opposing each other. FFM had a greater impact on energy balance than FM, and the Dynamic Subsystem showed asymmetry, exerting a stronger effect during weight loss than gain. The Tonic/Dynamic Model of Energy Balance helps explain weight-change dynamics, including the asymmetric ‘ratchet effect’ that more strongly protects against weight loss than weight gain.