Lactic acid fermentation is the most common fermentation process carried out by animals. In this process, the organic molecule acting as the electron acceptor is pyruvate. Its reduction to lactate, or lactic acid, is accompanied by the oxidation of NADH to NAD+ (see image below). This serves the purpose of recycling the NAD+, making it available for the glycolytic pathway.

Pyruvic acid + NADH → lactic acid + NAD+

A diagram outlining the steps in lactic acid fermentation. First, glucose undergoes glycolysis to produce 2 molecules of pyruvate and 2 molecules of ATP. Also during glycolysis, 2 molecules of NA Dplus are reduced to 2 molecules of NADH. Next, the 2 molecules of pyruvate undergo lactic acid fermentation to produce 2 molecules of lactic acid. During lactic acid fermentation, 2 molecules of NADH are oxidized to 2 molecules of NA Dplus. These NA Dplus molecules can then be used in the next round of glycolysis.

Lactic acid fermentation commonly occurs in muscle cells that have run out of oxygen.

If this metabolic pathway becomes the source of ATP for the cell because, for instance, oxygen levels remain low for a prolonged period, it can lead to a dangerous condition called lactic acidosis.