A workshop in Copenhagen brought together obesity experts to discuss causal models of obesity pathogenesis. The energy balance model (EBM) and carbohydrate-insulin model (CIM) are two competing theories explaining obesity. The EBM suggests obesity results from excess energy intake, while the CIM posits that altered fuel partitioning leads to fat storage. Both models agree on the physiological basis of energy metabolism but differ in their causal pathways. The EBM emphasizes energy intake, while the CIM focuses on metabolic changes in fuel use. The workshop aimed to clarify terms, compare models, and identify key scientific questions. It highlighted the need for experimental research to test these models. The EBM suggests that increased food intake drives obesity, while the CIM argues that changes in fuel partitioning lead to fat storage. Both models recognize the role of biological, behavioral, and environmental factors. The debate has implications for obesity management and public health. The workshop emphasized the importance of understanding the complex interplay of factors contributing to obesity. Further research is needed to resolve the debate and develop effective strategies for prevention and treatment. The discussion underscores the complexity of obesity and the need for a multifaceted approach to address it.A workshop in Copenhagen brought together obesity experts to discuss causal models of obesity pathogenesis. The energy balance model (EBM) and carbohydrate-insulin model (CIM) are two competing theories explaining obesity. The EBM suggests obesity results from excess energy intake, while the CIM posits that altered fuel partitioning leads to fat storage. Both models agree on the physiological basis of energy metabolism but differ in their causal pathways. The EBM emphasizes energy intake, while the CIM focuses on metabolic changes in fuel use. The workshop aimed to clarify terms, compare models, and identify key scientific questions. It highlighted the need for experimental research to test these models. The EBM suggests that increased food intake drives obesity, while the CIM argues that changes in fuel partitioning lead to fat storage. Both models recognize the role of biological, behavioral, and environmental factors. The debate has implications for obesity management and public health. The workshop emphasized the importance of understanding the complex interplay of factors contributing to obesity. Further research is needed to resolve the debate and develop effective strategies for prevention and treatment. The discussion underscores the complexity of obesity and the need for a multifaceted approach to address it.