Redefining stallion sperm bioenergetics: lessons from a decade of change in mammalian sperm metabolism

In brief: Sperm bioenergetics is now understood as an integrated system in which ATP production, redox homeostasis, substrate availability, and cellular heterogeneity jointly determine sperm function. This review uses the stallion as an oxidative-biased model to synthesize advances from 2016 to 2026 and proposes a metabolically plastic framework with direct implications for semen preservation and fertility assessment. Abstract: Over the last decade, mammalian sperm metabolism has moved from a binary view centered on glycolysis versus oxidative phosphorylation to an integrated model in which ATP production, redox homeostasis, metabolic plasticity, and cellular heterogeneity are functionally linked. This conceptual shift has relevance in the stallion, whose spermatozoa display strong mitochondrial engagement, marked redox sensitivity, and substantial responsiveness to media composition and preservation conditions. Here, we review advances from 2016 to 2026 that reshaped the field, with emphasis on extracellular flux analysis, multiparametric and label-free flow cytometry, proteomics, phosphoproteomics, metabolomics, and emerging stable-isotope approaches. We discuss evidence indicating that stallion spermatozoa operate within an oxidative framework in which mitochondrial respiration is supported by glycolytic input, whereas auxiliary pathways involving lactate-pyruvate cycling, and possibly neutral lipid mobilization and glycerol-glycerol-3-phosphate interconversion, may contribute primarily to redox stabilization and metabolic flexibility rather than to bulk ATP production. We further examine how these concepts inform diagnostics, semen preservation, cryopreservation, and intracytoplasmic sperm injection, emphasizing that single parameters are weak predictors of fertility and that integrated metabolic phenotypes are more informative. Finally, we identify major unresolved issues, including the need to standardize media composition and bioenergetic assays, distinguish pathway capacity from flux and coupling efficiency, and link metabolic phenotypes to fertility endpoints using rigorous experimental and statistical designs. A systems view of sperm metabolism offers a rational framework to improve equine fertility technologies and to explain interstallion variation in reproductive performance.