A metabolic framework for reward: Redefining dopamine and opioids as physiological agents

Energy efficiency is a foundational principle of life and evolution. In living organisms, any reduction in metabolic burden frees resources that can be allocated to other vital functions. Optimizing the balance between metabolic expenditure and gain is thus a core computation of the brain, enabling animals to survive and thrive. Yet despite its centrality, this principle has not been systematically integrated into accounts of reward and motivation. Instead, reward is typically modeled as a subjective signal of pleasure or value, leaving unclear its biological function. From a metabolic perspective, however, reward and motivated behavior primarily serve a fundamental biological role: optimizing the body's metabolic economy. Drawing on interdisciplinary evidence, we demonstrate that dopamine and opioids, traditionally regarded as agents of reward and motivation, should be redefined as physiological regulators. Dopamine upregulates, whereas opioids downregulate, physiological processes through cellular and network-level modulation. This countervailing regulation enables rapid coordination across systems and scales, thereby enhancing the metabolic efficiency of homeostasis. The physiological roles of dopamine and opioids reveal a metabolic mechanism underlying behavior and learning: motivation and reinforcement are not directly encoded by neuromodulator release but emerge from its metabolic consequences, with behavior initiated by physiological needs and reinforced through their resolution. Ultimately, the metabolic framework reframes reward not as a subjective experience but as a biologically grounded and quantifiable function, where behavior and learning operate most basically in the service of metabolic optimization.