What is lactic acid? If you’ve ever tried to run as fast as possible at the end of a race or do a series of heavy squats at the gym, you’ve probably experienced an uncomfortable burning sensation and crushing fatigue in your legs.
For many years, exercise scientists have attributed this muscle burn to lactic acid, which was thought to be a byproduct of the metabolic reactions carried out in the muscles to generate the energy they need to fuel your hard workout.
But is it the fault of lactic acid? To learn more about lactic acid and help separate myths and misconceptions from facts, we spoke to Bianca Grover (opens in a 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.
When you exercise, your muscles need energy to work and allow for your movements. To do this, your muscles produce cellular energy (adenosine triphosphate (ATP)) through various 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 in the foods we eat) into pyruvate. This chemical is then used as an energy source for the body, but can only be collected as an energy source in the presence of oxygen.
When you train at high intensity – and your body needs a lot of energy quickly – your fast-twitch muscle fibers fire up and start producing energy anaerobically (without oxygen). The fibers will continue to rely on the glycolysis process to produce this energy, but because the chemical in pyruvate cannot be harvested for this purpose, it is instead transformed into a waste product: lactic acid.
Does lactic acid build up in the body?
Although exercise physiologists believed that lactic acid could build up in muscles and bloodstream during hard exercise, research in the journal Physiology (opens in a new tab) made it clear 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 support the bond between the hydrogen ion and the lactate molecule.
As a result, lactic acid in the body freely dissociates into the independent lactate molecule and lone hydrogen ions. Therefore, there is no accumulation of lactic acid in the legs during strenuous exercise, and lactic acid is clearly not the cause of muscle burning and fatigue during strenuous exercise.
Although the concentration of lactate in the blood increases during strenuous exercise, the lactic acid molecule itself dissociates and the lactate is recycled and used to create more ATP.
“Your body naturally metabolizes lactic acid, eliminating it. The liver can absorb some of the lactic acid molecules and convert them back into glucose for fuel, “says Grover.” This conversion also reduces acidity in the blood, thereby eliminating some of the burning sensation. This is a natural process that occurs in the body. . Things like stretching, rolling or walking will have little or no impact. “
The burning sensation in the legs during a heavy workout is probably not caused by lactic acid, but by tissue damage and inflammation.
It is also important to remember that lactate itself is not “bad”. Indeed, research in Bioscience Horizons (opens in a 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 almost any cell in the body for energy.
Are there other sources of lactic acid?
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 a new tab) published by Oregon State University. Red blood cells have no mitochondria (opens in a new tab) – the part of the cell responsible for aerobic respiration – so they only breathe anaerobically.
Many species of bacteria also breathe 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 a new tab). The more sugar these kids eat, the more lactic acid they produce.
Slightly more insidious are the lactic bacteria that live in our mouths. Due to the acidifying effect they have on saliva, these bacteria are bad news for tooth enamel, according to a study published in Microbiology. (opens in a new tab).
Finally, lactic acid is commonly found in fermented dairy products, such as buttermilk, yogurt, and kefir. The bacteria in these foods use anaerobic respiration to break down lactose, the sugar in milk, into lactic acid. That doesn’t mean that lactic acid itself is a dairy product, however: it’s 100% vegan. It gets its name from dairy products simply because Carl Wilhelm, the first scientist to isolate lactic acid, made it from spoiled milk, according to a study published in the American Journal of Physiology (opens in a new tab).
Is lactic acid responsible for muscle pain?
Grover says lactic acid doesn’t induce muscle soreness. “The burning sensation is due to the increased acidity in the blood due to the low amount of oxygen available,” he says, referring to the hydrogen ions that dissociate from the lactic acid molecules produced during anaerobic glycolysis.
Essentially, during intense exercise, muscles produce energy through a metabolic pathway that produces usable energy, lactate and hydrogen ions.
Lactate can be processed in the liver and used for energy in other parts of the body, while hydrogen ions are metabolic byproducts that lower the pH in muscles and blood, causing an acidic environment that produces a burning sensation and intense fatigue in the muscles. . Pain after exercise is more likely due to tissue damage or inflammation.
So the next time you hear someone say they have sore legs from lactic acid, you may think, “It’s not lactic acid per se …”