Energy availability as an integrative framework: reinterpreting the energy balance and carbohydrate–insulin models in athletes

[Purpose] This review reinterprets the Energy Balance Model (EBM) and the Carbohydrate–Insulin Model (CIM) within athletic contexts and evaluates their explanatory relevance for low energy availability (LEA) and relative energy deficiency in sport (RED-S).[Methods] We conducted a structured conceptual review using a narrative synthesis approach. Literature searches were performed through January 2026 in PubMed, Web of Science, and SPORTDiscus, using terms related to EA, LEA, RED-S, EBM, CIM, and athlete metabolic, endocrine, and performance responses.[Results] Reductions in EA consistently induce endocrine, metabolic, and performance disturbances, independent of body mass changes. EBM primarily accounts for long-term regulatory adaptations associated with cumulative energetic strain, whereas CIM provides mechanistic insights into short-term alterations in substrate partitioning and hormonal signaling. Across sports disciplines, LEA has emerged as a coordinated multisystem physiological adaptation rather than an isolated consequence of reduced energy intake.[Conclusion] LEA and RED-S are most coherently interpreted within an EA-centered framework that integrates long-term energy regulation and short-term metabolic dynamics. This integrative perspective provides a unified physiological basis for understanding the metabolic adaptation, performance regulation, and health outcomes of athletes.