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What is Lactic Acid?

Lactic acid is a substance that is produced when the body converts carbohydrates to energy. It is a combination of lactate and hydrogen ions and is also called lactate. The body produces the maximum amount of lactic acid in muscle cells, red blood cells, and skin, brain, and gastrointestinal tract cells. However, all tissues in the body can make lactic acid. 

How is lactic acid produced?

When oxygen is available, the body uses oxygen to convert glucose into energy. However, when there is insufficient oxygen (these are called anaerobic conditions), the body uses a process called fermentation to produce energy and fuel cells. Lactic acid is a waste product of this fermentation process. In other words, the lactic acid fermentation process generates ATP (the cell’s energy currency) in the absence of oxygen. 

The Cori Cycle

The Cori cycle (also called the lactic acid cycle) is named after the two scientists who discovered it. It is the metabolic pathway in which anaerobic glycolysis (breakdown of glucose in the absence of oxygen) leads to the production of lactic acid, as described above. 

The most common cause is production of lactic acid in muscle cells during a strenuous workout. The lactic acid is removed from the bloodstream by the liver and oxidized to pyruvate, which is ultimately converted to glucose through the Cori Cycle. This process of glucose formation is called gluconeogenesis. The newly formed glucose is subsequently released to provide energy. 

As noted, intense exercise is one of the most common conditions in which the body makes lactic acid. But other conditions that make the body use more oxygen than usual can also cause cells to make lactic acid.

What does the lactic acid do in the body?

Lactic acid has three main functions in the human body:

• Energy production when cells are working harder than usual and oxygen demand cannot keep up with supply.

• Gluconeogenesis (conversion of lactic acid to new glucose by the liver after the kidneys and liver filter lactic acid out of blood). 

• Molecular signaling to attract immune system cells to areas of cell damage.

The Importance of Lactic Acid in Intense Exercise

Lactic Acid in Aerobic vs Anaerobic Exercise

During aerobic exercise, the body uses oxygen to convert carbohydrates to energy. During intense exercise, the supply of oxygen cannot keep up with the demand. This is called anaerobic exercise. In this situation, the body breaks down glucose and other carbohydrates without oxygen, and lactic acid is a byproduct of the process. The liver and kidneys filter the lactic acid from blood and break it down to glucose. This newly made glucose is used to provide energy to cells. 

Any intense exercise, such as a sprint at the end of a long run, lifting weights in the gym, or doing yard work can cause a short-term increase in lactic acid in the body. This excess lactic acid is temporary, harmless, and does not cause any symptoms. 

Lactic Acid and Muscle Fatigue

It is a common myth that exercise-induced muscle soreness occurs due to lactic acid buildup in muscle cells. Research has shown this is not true. The soreness in muscles that develops during and after a new or tough workout occurs due to microscopic trauma (tiny tears in the muscle fibers). The muscle damage triggers inflammation, which causes muscle soreness and kickstarts the body’s natural healing process. 

Does Lactic Acid Affect Athletic Performance?

Lactic acid may improve muscle performance during high-intensity exercise. As mentioned above, during a vigorous workout, oxygen demands cannot keep up with supply. This causes the body to produce lactic acid by breaking down carbohydrates using a process that doesn’t use oxygen. The lactic acid thus produced is filtered from the blood and used to make glucose in the liver. The glucose provides energy to cells, thereby potentially enhancing athletic performance.

Lactic Acid Levels: What Do They Mean?

As described above, the body makes lactic acid when it breaks down carbohydrates for energy when oxygen levels are low. This happens during vigorous workouts, and the increase in lactic acid is temporary in this case. The body can get rid of lactic acid buildup through the liver and kidneys.

However, you can develop lactic acidosis (dangerously high lactic acid concentrations) if you push yourself beyond your usual limits during sports, workouts, or any type of intense physical activity. The lactate threshold is the point during intense exercise at which lactic acid buildup occurs at a rate that is faster than the rate at which the body can get rid of lactic acid. That’s why doctors do not recommend “playing through your pain” or forcing yourself to keep going when you’re exhausted. 

Too much lactic acid can also occur in health conditions that cause reduced oxygen delivery to the body’s tissues. A lactic acid test (blood test) can be done to measure the lactic acid level in the body. A dangerously high lactic acid level is called lactic acidosis, and it can be fatal.

How Much Lactic Acid is Normal?

Normal lactic acid levels in the body range between 0.5 and 2.2 mmol/L (millimoles per liter) or 4.5 and 19.8 mg/dL (milligrams per deciliter). These values may be slightly different depending on the laboratory. 

Note: You should not exercise for several hours before a lactic acid test. Also, do not clench your fist or have an elastic band in place before the blood draw. This can cause a false elevation in lactic acid levels. 

What Does a Lactic Acid Level of 3 Mean?

A lactic acid level of 3 mmol/L means you have too much lactic acid in your body (normal levels are between 0.5 and 2.2 mmol/L). This may be a sign that your tissues are not getting enough oxygen. Your healthcare team can use this information to diagnose an underlying medical condition. 

What is a Critical Level of Lactic Acid?

Critically ill patients who have a blood lactate concentration of more than 5 mmol/L and a pH of less than 7.35 have a very poor prognosis with a mortality rate of 75%.

What Causes High Lactic Acid?

Some of the medical conditions that can increase lactic acid levels include:

• Diabetic ketoacidosis

• Heart failure

• Kidney failure

• Liver failure

• Cirrhosis

• Lung disease

• Severe infection 

• Sepsis

• Cancer

• Hypoxia (low oxygen levels in the heart, lungs, and other tissues)

• Exposure to certain toxins

• Conditions in which there is a dramatic but transient increase in metabolic demand, for example, post-seizure lactic acidosis

• Congenital lactic acidosis due to inborn errors of metabolism, such as abnormalities in the cellular respiratory chain, pyruvate dehydrogenase, gluconeogenesis, or the tricarboxylic acid (TCA) cycle

• Certain medications, such as metformin, nucleoside reverse transcriptase inhibitors (HIV medicines), and beta-agonists like albuterol and salmeterol

How is Lactic Acidosis Treated?

You can treat mild post-exercise elevation in lactic acid at home with rest and hydration. For high levels of lactic acid caused by a medical condition (lactic acidosis), doctors use IV fluids, oxygen therapy, and other resuscitation measures immediately, followed by treatment of the underlying health conditions. 

Types of Lactic Acidosis

Type A lactic acidosis is the more dangerous form. It occurs due to hypoperfusion (reduced blood flow and oxygen deficiency in tissues). For example, elevated lactic acid caused by sepsis, shock, lung disease, heart failure, or cardiac arrest.

Type B lactic acidosis is less dangerous. It occurs due to localized tissue hypoxia (reduced oxygen supply). For example, elevated lactate levels caused by vigorous muscle use, seizures, cancer, and ingestion of certain toxins or medications. 

Most lactic acid in the human body is L-Lactate. D-Lactic acid is produced in small quantities by bacteria in the colon. In people with certain gastrointestinal conditions, D-lactic acid builds up to dangerously high levels.

Symptoms of Lactic Acidosis

Common symptoms of lactic acidosis include nausea, vomiting, abdominal pain, and muscle weakness.

Note: Dominant symptoms in people with lactic acidosis are usually symptoms of the underlying cause (health condition) that has resulted in elevated lactic acid levels.