Human cells have two strategies for burning glucose, or blood sugar, to liberate energy. The more commonly used of the two strategies -- aerobic respiration -- requires abundant oxygen. Less commonly used, because it yields approximately 15 times less energy per glucose molecule, is an anaerobic strategy, called fermentation, which does not depend upon oxygen. When human cells ferment glucose, one of the products is lactic acid. As such, the process is often called lactic acid fermentation, which has several products.
One product of lactic acid fermentation is lactic acid itself. Humans, animals and some bacteria engage in lactic acid fermentation as an anaerobic metabolic strategy, in contrast to yeast and other bacteria that use ethanolic fermentation instead. As noted by Drs. Reginald Garrett and Charles Grisham in their book, "Biochemistry," lactic acid differs from ethanol by one carbon atom; lactic acid has three carbons, while ethanol has two. As such, one glucose, with six carbon atoms, splits neatly into two molecules of lactic acid, meaning that unlike ethanolic fermenters, lactic acid fermenters don't produce carbon dioxide as a byproduct.
The process of fermentation doesn't actually yield energy. In fact, without oxygen, glucose is split into two pyruvate molecules through the metabolic process of glycolysis, which generates a small amount of energy. Pyruvate is converted to lactic acid through lactic acid fermentation, but the purpose of the conversion is not to yield additional energy. Instead, glycolysis requires the involvement of a substance called NAD+. Fermentation serves the purpose of regenerating NAD+, explain Drs. Mary Campbell and Shawn Farrell in their book, "Biochemistry." NAD+ is an important product of lactic acid fermentation because it allows the energy-yielding process of glycolysis to continue.
Lactic acid itself isn't a particularly useful product -- instead, it's produced as a byproduct in the process of making NAD+. Once produced, it's essentially metabolic waste. Still, lactic acid does represent a valuable source of reduced carbon, which has the potential to yield energy. The liver can recycle lactic acid by converting it back into pyruvate, which can be burnt under oxygen-rich conditions for further energy, note Drs. Garrett and Grisham. In essence, by converting lactic acid to pyruvate, the body maintains a valuable source of carbon and avoids wasting potentially energy-yielding molecules.