During fermentation, lactate dehydrogenase converts pyruvate to L-lactic acid. Lactic acid is constantly produced during general metabolism and exercise, but its concentration generally does not rise. The concentration of lactic acid will increase only when the production process of lactic acid is accelerated and lactic acid cannot be transported away in time. The transport rate of lactate is affected by a series of factors, including the concentration and isomer form of monocarboxylic transporter, lactate dehydrogenase, and the oxidation capacity of tissue. Generally speaking, the concentration of lactic acid in blood is 1-2mmol/l when not exercising, and can rise to 20mmol/l when exercising strongly.

Generally speaking, the concentration of lactic acid will rise when the energy of the tissue cannot be satisfied through aerobic respiration, the tissue cannot obtain enough oxygen or cannot process oxygen quickly enough. In this case, pyruvate dehydrogenase could not convert pyruvate to acetyl coenzyme A in time, and pyruvate began to accumulate. In this case, if the lactate dehydrogenase does not reduce pyruvate to lactic acid, the glycolysis process and the synthesis of adenosine triphosphate will be controlled.

The process of producing lactic acid is: pyruvic acid +nadh+h+ → lactic acid +nad++2h

The significance of this process lies in the reconstruction of nicotinamide adenine dinucleotide (nad+) required for glycolysis to maintain the synthesis of adenosine triphosphate. Lactic acid can be oxidized to pyruvate in oxygen rich muscle cells, and then directly used as the fuel of the tricarboxylic acid cycle. It can also be converted into glucose by the gluconeogenic cycle in the liver. Lactobacillus bacteria can also carry out lactic acid fermentation. These bacteria can live in the mouth, and the lactic acid they produce is the cause of dental caries. In medicine, lactic acid is often used in Ringer's lactate solution. This is a solution of sodium chloride, potassium chloride and lactic acid isotonic with human blood in distilled water. Ringer's lactate solution is often used to supplement blood loss after injury, operation or burn.