The scientific study of obesity has been dominated throughout the twentieth century by the concept of energy balance. This conceptual approach, based on fundamental thermodynamic principles, states that energy cannot be destroyed, and can only be gained, lost or stored by an organism. Its application in obesity research has emphasised excessive appetite (gluttony), or insufficient physical activity (sloth), as the primary determinants of excess weight gain, reflected in current guidelines for obesity prevention and treatment. This model cannot explain why weight accumulates persistently rather than reaching a plateau, and underplays the effect of variability in dietary constituents on energy and intermediary metabolism. An alternative model emphasises the capacity of fructose and fructose-derived sweeteners (sucrose, high-fructose corn syrup) to perturb cellular metabolism via modification of the adenosine monophosphate (AMP)/adenosine triphosphate (ATP) ratio, activation of AMP kinase and compensatory mechanisms, which favour adipose tissue accretion and increased appetite while depressing physical activity. This conceptual model implicates chronic hyperinsulinaemia in the presence of a paradoxical state of 'cellular starvation' as a key driver of the metabolic modifications inducing chronic weight gain. We combine evidence from in vitro and in vivo experiments to formulate a perspective on obesity aetiology that emphasises metabolic flexibility and dietary composition rather than energy balance. Using this model, we question the direction of causation of reported associations between obesity and sleep duration or childhood growth. Our perspective generates new hypotheses, which can be tested to improve our understanding of the current obesity epidemic, and to identify novel strategies for prevention or treatment.