Energy Flux, Lactate Shuttling, Mitochondrial Dynamics, and Hypoxia.

Our understanding of what happens in working muscle and at the whole-body level at sea level and at high altitude is different from that a few years ago. If dietary CHO and nutrition are adequate, at sea level metabolism shifts from a mix of lipid and CHO-derived fuels toward carbohydrate (glycogen, glucose, and lactate) oxidation at moderate and greater exercise intensities. As given by the Crossover Concept, a percentage to total energy expenditure, lipid oxidation is greatest at exercise power outputs eliciting 45-50 % of VO2max with greater intensities requiring relatively more CHO and lesser lipid oxidation. At altitude, a given exercise power output is achieved at a greater relative intensity expressed as % VO2max. Hence, exercise under conditions of hypoxia requires greater glycolytic flux, and lactate production than does the same effort at sea level, normoxic conditions. Glycolytic flux is further augmented at altitude by the effect of hypoxemia on sympathetic nervous system activity. Hence, augmented lactate production during exercise is adaptive. Over the short term, accelerated lactate flux provides ATP supporting muscle contraction and balances cytosolic redox. As well, lactate provides and energy substrate and gluconeogenic precursor. Over a longer term, via redox and ROS-generating mechanisms, lactate may affect adaptations in mitochondrial biogenesis and solute (glucose and lactate) transport. While important, the energy substrate, gluconeogenic, and signaling qualities of lactate production and disposal at altitude need to be considered within the context of overall dietary energy intake and expenditure during exercise at sea level and high altitude.