What Are Lactic Acid and Lactic Acid Fermentation? Why Do They Occur?

Although “lactic acid” is frequently used in everyday language in medical contexts, the substance measured in the blood is mostly lactate. Lactate is a part of normal energy metabolism and is naturally produced in the body under certain conditions. Therefore, the presence of lactate alone is not considered abnormal or dangerous. What matters is under which conditions lactate is elevated, how long it persists, and with which symptoms it is accompanied in the person.

How Is Lactate Produced in the Body?

Cells break down glucose to produce energy. The breakdown of glucose starts with a process called glycolysis. At the end of glycolysis, pyruvate is formed, and in this way the cell rapidly obtains energy. Pyruvate can be used in different pathways depending on the cell’s energy requirements and metabolic conditions. When glycolysis accelerates, a portion of pyruvate is converted to lactate, contributing to the continuation of energy metabolism.

In this conversion, the enzyme lactate dehydrogenase, or LDH, plays a role. During the reaction, NADH is converted back to its NAD+ form. In other words, lactic acid fermentation is a functional adaptation pathway that allows the cell to maintain short-term energy production.

The important point here is that the formation of lactate does not automatically mean disease. Lactate can also appear during intense muscle activity and can be used as a fuel by some tissues. The heart, muscles, and other tissues can utilize circulating lactate for energy. In this respect, lactate is not a passive but an active part of metabolism.

How Does Lactic Acid Fermentation Occur?

Lactic acid fermentation is the process by which pyruvate formed through glycolysis is converted to lactate. Thanks to this conversion, NAD+ is regenerated and glycolysis can continue for a certain period of time. This mechanism becomes more important in situations where energy demand increases rapidly and oxygen utilization capacity is temporarily insufficient. As a result, the cell is able to maintain energy production for a short while longer.

In the human body, this process is most commonly explained through the example of intense exercise. During short-term high effort, the energy requirement of the muscles rises very rapidly. During this period, glycolysis accelerates, pyruvate production increases, and part of it is converted to lactate. However, lactic acid fermentation is not a process unique only to muscle cells. Different tissues may also produce lactate or utilize the lactate produced, depending on metabolic conditions.

The liver has a distinct role in this cycle. A portion of the lactate that enters the circulation can be converted back into glucose in the liver. This recovery pathway is known as the Cori cycle.

Lactic acid fermentation is not observed only in human physiology. Some bacteria also produce lactic acid during food fermentation. The process in yogurt and similar fermented products is an example of this. However, fermentation in foods and elevated lactate levels in the blood are not the same thing. One is a fermentation process carried out by microorganisms, while the other is a laboratory finding related to human metabolism.

Why Does Lactate Increase During Exercise, and How Does It Return to Normal?

During intense exercise, the energy demand of the muscles rises markedly within a short time. During this period, the person may experience a burning sensation, faster breathing, and a temporary decline in performance. This picture is often the body’s natural response to increased energy needs.

When exercise stops or the pace slows down, metabolic balance changes again. Circulating lactate can be used by different tissues; part of it participates in energy production, and another part can be converted into glucose in the liver via the Cori cycle. As breathing and circulation stabilize, oxygen delivery recovers and lactate levels tend to decrease.

Post-exercise pain is not solely caused by lactate. Especially muscle pain that becomes more noticeable hours after exercise or the next day is more related to exercise-induced changes in muscle tissue and inflammatory processes than to lactate accumulation. Therefore, presenting lactic acid as the sole explanation for all exercise-related complaints is not scientifically correct.

Are Lactic Acid and Lactic Acidosis the Same Thing?

These two concepts do not mean the same thing. Elevated lactate may in some cases be only a laboratory finding. Lactic acidosis, however, refers to a serious clinical condition in which metabolic acidosis accompanies elevated lactate. Therefore, not every elevation in lactate is lactic acidosis.

Physicians sometimes use the term hyperlactatemia. Hyperlactatemia indicates that lactate in the blood is elevated, but does not by itself show that acidosis is present. The meaning of the result is evaluated together with the patient’s general condition, physical examination findings, circulation, respiration, blood gas, and coexisting diseases. A diagnosis of lactic acidosis is not made based on a single symptom or a single test result.

While a short-term increase in lactate after intense exercise may be physiological, lactate that rises during a severe infection, serious circulatory disorder, or shock is assessed differently. It should be kept in mind that the clinical context is more decisive than the value itself.

What Is a Lactate Test and Why Is It Ordered?

A lactate test is a laboratory examination performed to assess the lactate level in the blood. This test may be ordered particularly in the evaluation of conditions such as severe infection, suspected sepsis, serious circulatory disorder, shock, marked respiratory distress, or an unexplained poor general condition. Lactate measurement is also performed in some sports physiology applications; however, this use has a different purpose from emergency disease evaluation.

The test result alone does not establish a definitive diagnosis. The conditions under which the measurement was performed, the person’s current complaints, accompanying laboratory examinations, and clinical examination are evaluated together. The necessary investigations are not the same for every individual; they are planned based on complaints and the clinical picture.

In Which Situations Should Elevated Lactate Be Taken Seriously?

Transient exercise-induced elevation in lactate should be distinguished from the increase seen during illness. If there is rapid or labored breathing, severe fatigue, worsening accompanied by nausea and vomiting, confusion, low blood pressure, or a situation suggestive of shock, urgent evaluation is required. Worsening that occurs in the setting of severe infection, sepsis, or serious heart-lung disease is also an important warning sign.

Such clinical findings may also be an indication of different underlying health problems. In cases such as shortness of breath that does not improve with rest, deterioration in general condition, or changes in consciousness, the nearest emergency department should be visited without delay. For unexplained, recurrent complaints that affect overall health status, consultation with an Internal Medicine specialist is recommended for a detailed evaluation.

In summary, lactic acid and lactate are natural components of energy metabolism. Lactic acid fermentation helps glycolysis to continue by enabling the regeneration of NAD+ through the conversion of pyruvate to lactate. While transient elevation of lactate during exercise is often physiological, lactic acidosis is a more serious condition and must be evaluated together with symptoms. Correctly distinguishing these concepts both reduces unnecessary anxiety and enables truly important conditions to be recognized in a timely manner.