[Popular myths vs. physiological realities: The rehabilitation of lactate]

Lactate, long relegated to the status of a mere metabolic waste product, is now recognized as a central metabolite in energy metabolism, a recyclable substrate, and a cellular signaling mediator. Its production, primarily by glycolytic muscle fibers, allows for the regeneration of cytosolic NAD⁺ necessary for the continuation of glycolysis during high-intensity exercises. Its export via MCT4 lactate/proton transporters and its uptake by other tissues, especially oxidative muscle fibers via MCT1 transporters, underscore its role as a metabolic intermediate rather than a waste product. Its co-transport with protons across the sarcolemmal membrane contributes to intracellular pH regulation. Furthermore, lactate plays a role in regulating oxidative energy metabolism adaptations, by activating the mitochondrial biogenesis cofactor PGC-1α and modulating the expression of mitochondrial markers. Additionally, by binding to the G protein-coupled receptor GPR81, lactate may modulate key signaling pathways such as ERK1/2 and Akt-mTOR, which are involved in muscle hypertrophy, insulin sensitivity, and lipid regulation. In doing so, it links its metabolic functions to major adaptive processes triggered by physical exercise. However, this functional role still requires further elucidation.