Lactic acid, also known as 2-hydroxypropanoic or milk acid, is a compound formed when glucose is broken down under certain conditions in a living creature or by some types of bacteria. In a person, for example, it is an important part of producing energy for strenuous exercise and helps with certain liver functions. During extremely intense exercise, it can buildup to excess and cause short-term burning sensations in muscles. This acid can also be found in certain dairy products, such as yogurt, as well as sourdough breads and some beers and wines as a result of fermentation.

Production and Use in the Body

Skeletal muscles and other tissues commonly produce lactic acid, even while at rest. The body creates this acid as it breaks down carbohydrates to produce energy. The acid only becomes an issue when there are unusually large amounts of it.

This can happen when there is not enough oxygen in the body to completely break down glucose during physical activity. Energy in a person's body is typically created with the assistance of oxygen during aerobic exercise. When oxygen levels reach their limit, but more energy is needed, then the activity becomes anaerobic, which means that energy needs to be produced through other methods. Through a complex process known as glycolysis, glycogen in the muscles breaks down into glucose and then into pyruvate or pyruvic acid.

Ad

During aerobic exercise, the pyruvate undergoes an oxidation process that helps remove it. When someone participates in strenuous, anaerobic exercise, however, his or her body does not have the oxygen available to do this. Under these conditions, the excess pyruvate produces lactic acid, which helps generate short-term bursts of energy. A "fight-or-flight" response, for example, often relies on this acid for the energy a person needs to quickly run at high speeds.

Use By The Liver

One common use for lactic acid in a human body is the formation of glucose. Moderate amounts of this acid can move through someone's blood stream and reach the liver, where it undergoes a process called gluconeogenesis to become glucose. This is then either used to maintain healthy glycogen levels within the liver or passed back into the body for use as blood-sugar.

Extreme Activity

During prolonged strenuous activity, the high amounts of lactic acid may produce hydrogen ions that cause burning sensations in muscles. This is typically quite painful, and many serious athletes and bodybuilders experience this discomfort during intense exercise or weightlifting. The pain is fairly brief, however, and helps prevent serious injury since it usually makes the person stop using a certain muscle group.

Common Myths

A widespread belief among some people is that ongoing muscle soreness following an intense workout is due to a buildup of lactic acid. In truth, most research indicates that only immediate soreness or burning sensations result from too much of this compound. Minuscule tears and inflammation in the muscles usually cause soreness and fatigue that last for several days. Proper training and exercise, including warming up and cooling down muscles, adequate stretching, and a healthy, high-carbohydrate diet, can help prevent injury.

Potential Health Concerns

Very high levels of lactic acid can cause a serious, sometimes life-threatening condition called lactic acidosis. Symptoms of this condition include rapid breathing, sweating, and nausea and vomiting. Healthcare professionals typically draw a blood sample to check acid levels when they suspect a person might have this condition. While extreme over-exercising and overheating can result in lactic acidosis, it can also be caused by alcohol poisoning, liver disease, and a lack of oxygen, due to something like carbon monoxide poisoning.

Use In Food

A number of foods also include lactic acid to alter the pH balance or change the flavor. Certain types of bacteria added to milk, for example, produce the acid, which helps create the texture and tartness found in yogurt. Sourdough bread often relies on both yeast and bacteria in the air around the dough's starter for its tart flavor, usually caused by the formation of acid within the bread. Beer and wine sometimes include bacteria that produces this acid, which can help eliminate other, somewhat unpleasant flavors produced during fermentation.

Lactic acid is an organic compound with the formula CH₃CH(OH)COOH. In its solid state, it is white and water-soluble. In its liquid state, it is colorless. It is produced both naturally and synthetically. With a hydroxyl group adjacent to the carboxyl group, lactic acid is classified as an alpha-hydroxy acid (AHA). In the form of its conjugate base called lactate, it plays a role in several biochemical processes.

In solution, it can ionize a proton from the carboxyl group, producing the lactate ion CH
₃CH(OH)CO⁻
₂. Compared to acetic acid, its pK_a is 1 unit less, meaning lactic acid deprotonates ten times more easily than acetic acid does. This higher acidity is the consequence of the intramolecular hydrogen bonding between the α-hydroxyl and the carboxylate group. Lactic acid is chiral, consisting of two optical isomers. One is known as L-(+)-lactic acid or (S)-lactic acid and the other, its mirror image, is D-(−)-lactic acid or (R)-lactic acid. A mixture of the two in equal amounts is called DL-lactic acid, or racemic lactic acid. Lactic acid is hygroscopic. DL-lactic acid is miscible with water and with ethanol above its melting point which is around 17 or 18 °C. D-lactic acid and L-lactic acid have a higher melting point.

In animals, L-lactate is constantly produced from pyruvate via the enzyme lactate dehydrogenase (LDH) in a process of fermentation during normal metabolism and exercise. It does not increase in concentration until the rate of lactate production exceeds the rate of lactate removal, which is governed by a number of factors, including monocarboxylate transporters, concentration and isoform of LDH, and oxidative capacity of tissues. The concentration of blood lactate is usually 1–2 mM at rest, but can rise to over 20 mM during intense exertion and as high as 25 mM afterward.^[4][5] In addition to other biological roles, L-lactic acid is the primary endogenous agonist of hydroxycarboxylic acid receptor 1 (HCA₁), which is a G_i/o-coupled G protein-coupled receptor (GPCR).^[6][7]

In industry, lactic acid fermentation is performed by lactic acid bacteria, which convert simple carbohydrates such as glucose, sucrose, or galactose to lactic acid. These bacteria can also grow in the mouth; the acid they produce is responsible for the tooth decay known as caries.^{[8][9][10][11]} In medicine, lactate is one of the main components of lactated Ringer's solution and Hartmann's solution. These intravenous fluids consist of sodium and potassium cations along with lactate and chloride anions in solution with distilled water, generally in concentrations isotonic with human blood. It is most commonly used for fluid resuscitation after blood loss due to trauma, surgery, or burns.