In HEK293 cells, adding 5 mM pyruvate for 20 minutes at 37°C pushed extracellular lactate about 3× even at 21% O2. Is this primarily LDH-A mass action shifting the cytosolic NADH/NAD+ balance, or are we seeing a malate–aspartate shuttle bottleneck that forces glycolytic NAD+ regeneration via lactate despite intact mitochondrial flux?