A by-product of anaerobic respiration formed when glucose is metabolized for energy production when oxygen levels are low. A buildup of lactic acid results in lactic acidosis which causes muscle fatigue and discomfort. The accumulation of lactic acid in muscles can produce sufficient discomfort or pain to cause the horse to slow down or stop the exercise and enable the muscle to revert to the less intense aerobic metabolism utilizing oxygen. This will limit lactic acid production and enable clearance from the muscle tissue. As such, it acts as a safety mechanism to protect the muscle from over exertion. The lactic acid produced is converted to lactate which in the presence of adequate oxygen can be used as a fuel for energy production.
Lactic acid is also produced by bacterial fermentation of simple carbohydrates like sugars in the gut. When these carbohydrates reach the hind gut the Lactobacillus bacteria utilize them and produce lactic acid which if present in sufficient quantity lowers the pH of the gut contents and can cause local damage to the gut lining. See also Dysbiosis.