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What is lactic acid?

Lactic acid: Image shows sprinters racing round track
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What is lactic acid? If you’ve ever tried to sprint as fast as you can at the end of a race or bang out a set of heavy squats in the gym, you’ve probably experienced an uncomfortable burning sensation and overwhelming fatigue in your legs. 

For many years, exercise scientists attributed this muscle burning to lactic acid, which was thought to be a by-product of the metabolic reactions carried out in your muscles in order to generate the energy they need to power your hard workout. 

But is lactic acid to blame? To learn more about lactic acid and help separate the myths and misconceptions from the facts, we spoke to Bianca Grover (opens in new tab) an exercise physiologist, medical exercise specialist and personal trainer.

What is lactic acid?

Lactic acid is an organic acid produced by the body when glucose (sugar) is broken down to generate ATP (cellular energy) in the absence of oxygen.

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When you exercise, your muscles need energy to work and enable your movement. To do this, your muscles produce cellular energy (adenosine triphosphate (ATP)) through different metabolic pathways. 

A metabolic pathway is basically a chain of chemical reactions. One of our most important metabolic pathways, known as glycolysis, breaks down  glucose molecules (simple sugars from the foods we eat) into pyruvate. This chemical is then used as a source of energy for the body – but it can only be harvested as an energy source in the presence of oxygen.

When you're exercising at a high intensity — and your body needs a lot of energy quickly — your fast-twitch muscle fibers will kick in and start producing energy anaerobically (without oxygen.) The fibers will still be relying on the glycolysis process to produce this energy, but as the pyruvate chemical can't be harvested for this purpose it gets turned into a waste produce instead: lactic acid. 

Does lactic acid accumulate in the body?

Although exercise physiologists used to believe lactic acid could accumulate in the muscles and bloodstream during hard exercise, research in the journal Physiology (opens in new tab) has elucidated the fact that lactic acid as a molecule cannot exist in its intact form in the body because the pH of human blood is too high. In other words, the pH of our blood is too alkaline, or not acidic enough to sustain the bond between the hydrogen ion and the lactate molecule.

As a result, lactic acid in the body freely dissociates into the freestanding lactate molecule and lone hydrogen ions. Therefore, there is no build-up of lactic acid in your legs during intense exercise, and lactic acid is clearly not the cause of muscle burning and fatigue during intense exercise.

Although blood lactate concentration does increase during intense exercise, the lactic acid molecule itself dissociates and the lactate is recycled and used to create more ATP.

"Your body naturally metabolizes the lactic acid, clearing it out. The liver can take up some of the lactic acid molecules and convert them back to glucose for fuel," says Grover. "This conversion also reduces the acidity in the blood, thus removing some of the burning sensation. This is a natural process that occurs in the body. Things such as stretching, rolling, or walking will have little to no impact."

The burning sensation you feel in your legs during a heavy workout probably isn't caused by lactic acid, but instead by tissue damage and inflammation. 

It’s also important to remember that lactate itself isn’t 'bad'. In fact, research in Bioscience Horizons (opens in new tab) suggests that lactate is beneficial to the body during and after exercise in numerous ways. For example, lactate can be used directly by the brain and heart for energy or converted into glucose in the liver or kidneys, which can then be used by nearly any cell in the body for energy.

Are there other sources of lactic acid?

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Muscle cells aren't the only sources of lactic acid. Red blood cells also produce lactic acid as they roam the body, according to the online text Anatomy and Physiology (opens in new tab) published by Oregon State University. Red blood cells don't have mitochondria — the part of the cell responsible for aerobic respiration — so they only respire anaerobically.

Many species of bacteria also respire anaerobically and produce lactic acid as a waste product. In fact, these species make up between 0.01-1.8% of the human gut, according to a review published in the Journal of Applied Microbiology (opens in new tab). The more sugar these little guys eat, the more lactic acid they produce. 

Slightly more insidious are the lactic acid bacteria that live in our mouths. Because of the acidifying effect they have on saliva, these bacteria are bad news for tooth enamel, according to a study published in Microbiology (opens in new tab).

Finally, lactic acid is commonly found in fermented dairy products, like buttermilk, yogurt and kefir. Bacteria in these foods use anaerobic respiration to break lactose — milk sugar — into lactic acid. That doesn't mean that lactic acid itself is a dairy product, however — it's 100% vegan. It happens to get its name from dairy simply because Carl Wilhelm, the first scientist to isolate lactic acid, did so from some spoiled milk, according to a study published in the American Journal of Physiology (opens in new tab).

Is lactic acid responsible for muscle pain?

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Grover says that lactic acid does not induce muscle soreness. "The burning sensation is due to the increased acidity in the blood because of the low amount of oxygen available," she says, referring to the hydrogen ions that dissociate from the lactic acid molecules produced during anaerobic glycolysis.

Essentially, during intense exercise, the muscles produce energy through a metabolic pathway that produces usable energy, lactate, and hydrogen ions. 

The lactate can be processed in the liver and utilized for energy elsewhere in the body while the hydrogen ions are metabolic by-products that lower the pH in the muscles and blood, causing an acidic environment that yields a burning sensation and intense fatigue in your muscles. Soreness after exercise is more likely due to tissue damage or inflammation. 

So, next time you overhear someone saying their legs are sore from lactic acid, you can think to yourself, "It’s not lactic acid per se…"

Additional resources: 

Amber Sayer is a fitness, nutrition, and wellness writer and editor, and contributes to several fitness, health, and running websites and publications. She holds two masters degrees—one in exercise science and one in prosthetics and orthotics. As a certified personal trainer and running coach for 12 years, Amber enjoys staying active and helping others do so as well. In her free time, Amber likes running, cycling, cooking, spending time outside, and tackling any type of puzzle.

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