Lactic acid, also known as milk acid or 2-hydroxypropanoic acid, is an acid that both is formed by the body. It also exists in some foods. In the body, lactic acid develops generally in conjunction with exercise. As for foods, lactic acid exists in certain milk products, like yogurt, as well as some processed foods like some breads and beers.

In the body, lactic acid forms when there aren't enough oxygen molecules to completely breakdown the glucose in the body. A complex process known as glycolysis, breaks glycogen in the muscles into glucose and is one of the primary sources of the energy we need to exercise. During glycolysis, each glucose molecule splits into two pyruvic acid molecules. The resulting energy is released and forms into adenosine triphosphate (ATP). Normally, the pyruvic acid merges with mitochondria and undergoes the oxidation process, which produces more ATP. During strenuous exercise or activity, however, oxygen is lacking and the reaction cannot take place. The pyruvic acid is then transformed into lactic acid.

Lactic acid has always been thought of as only the result of metabolizing glucose for energy and is considered a waste product that can cause burning sensations in the muscles. It can, however, also be quite instrumental in avoiding hypoglycemia, which is a condition that causes a feeling of weakness and fatigue due to a lack of blood sugar. It also helps in the “fight or flight” survival instinct and serves as an important energy source.

Athletes rely on powerful muscle contractions that require energy or fuel from lactate during strenuous exercise. Lactate is dispensed from the muscle to the blood. When it reaches the liver, it converts into glucose and can then be applied as a fundamental energy source. When the threshold point is reached in the exercise session where the rate of lactic acid is building up faster then it is being dispensed, then build up occurs and fatigue follows.

A common belief among athletes is that specific muscle soreness following an intense workout is due only to a build up of lactic acid. It is still partially to blame for tired, sore or burning sensations of the muscles. Further examination has confirmed that the onset of muscle soreness and fatigue during high intensity sessions are from minuscule tears, injury, or inflammation. Proper training and exercise includes the warming up and cooling down of the muscles, adequate stretching, and a healthy, high carbohydrate diet. Doing all of these things can help prevent injury and aid in offsetting the negative effects of lactic acid buildup.

Very high levels of lactic acid can cause a serious, sometimes life-threatening condition called lactic acidosis.