In anaerobic organisms or in aerobic cells that are undergoing very high rates of glycolysis, the NADH generated in glycolysis (in the oxidation of glyceraldehyde-3-phosphate to 1,3 bisphosphoglycerate) cannot be reoxidized to NAD⁺ in the mitochondrion. When this situation occurs, the cell must find a way to convert NADH to NAD⁺, in order to maintain homeostasis.

Both in eukaryotic cells and in lactic acid bacteria, the mechanisms involves pyruvate being reduced to lactate. The enzyme catalyzing this reaction is lactate dehydrogenase. The equilibrium for this reaction lies far in favor of formation of lactate.

Figure 13.6, which depicts the energy profile of anaerobic glycolysis, shows that NADH produced in the oxidation of glyceraldehyde-3-phosphate is used to reduce pyruvate to lactate. Thus, during anaerobic glycolysis or lactic acid fermentation, an overall electron balance is maintained. (Note - alcohol fermentation is another type of anaerobic glycolysis).

Lactate is an important precursor of glucose in the Cori cycle (Figure 16.5)