Fuel, Not Foe? The Truth About Lactic Acid and Muscle Fatigue

Fuel, Not Foe? The Truth About Lactic Acid

It’s the telltale burning sensation that you feel in your legs when you’re 100 meters from the finish line, about to crest the top of a hill on your bike, or when you’re finishing that last set of squat jumps. It feels like every fiber of your muscles is on fire and begging you to stop.

Lactic acid. Most athletes see it as a villain, the cause of muscle fatigue, burn and delayed onset muscle soreness (aka DOMS). We’ve been led to believe it’s a waste product we need to “flush from our systems” with a massage or resting with our legs propped up on a wall.

But the latest research suggests that our bodies don’t, in fact, produce a substance called lactic acid when we exercise. Turns out, lactic acid — at least as we thought we knew it — is mostly just myth. Instead, our bodies actually create something called lactate — and we produce it not just when we exercise, but all the time. Furthermore, lactate is good for us, not bad! Has it been misunderstood all these years? Read on for the truth behind the burn.

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The Lactic Acid Myth

“It’s actually the accumulation of hydrogen ions that makes the surrounding environment acidic and causes our muscles to burn.”

For years, we’ve been told by coaches, trainers and science teachers that lactic acid causes our muscles to ache and tire when we exercise intensely. The theory is that when the body breaks down glucose for energy, it produces lactic acid as a by-product. Researchers concluded that the accumulation of lactic acid — and the increasing acidic environment in our muscles — is what causes muscle fatigue and failure.

“However, there is no experimental research to prove this. Just correlative data,” says Jeremy McCormick, PhD candidate in Exercise Science at the University of New Mexico. Ever since lactic acid was first linked to exercise metabolism in the 1920s, this theory has gone unquestioned for more than 80 years. A closer look at the biochemistry involved during exercise reveals a different story altogether.

When we exercise, our bodies require quite a lot of energy to fuel muscle contraction. We break down ATP (a high-energy compound), and a hydrogen ion is released in the process. During strenuous exercise where oxygen is limited, our metabolism can’t keep up with the ever-growing number of hydrogen ions in our body. And it’s actually the accumulation of hydrogen ions that makes the surrounding environment acidic and causes our muscles to burn.

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If too many hydrogen ions are the culprit, then why blame so-called lactic acid?

As exercise intensity increases, our bodies rely on glucose to keep up with the demand for energy. One of the end products of the break down of glucose is pyruvate, and this molecule begins to build up in our cells along with the hydrogen ions. Since our body doesn’t want these concentrations to rise unchecked, each molecule of pyruvate absorbs two of the hydrogen ions, forming lactate.

“Scientists have been confused because lactate and hydrogen ions are present together in the muscle when you exercise intensely, and they thought it was lactic acid,” says McCormick. “But it’s really lactate,” he says.

A Friend or Foe?

“Not only does lactate serve as a buffer, research also indicates that our bodies reuse lactate as a source of energy for our muscles, heart and brain.”

In order to keep our muscles functioning, our bodies try to reduce the acidic environment by neutralizing the growing number of hydrogen ions; lactate doesn’t cause the acidic environment, it tries to minimize it. It’s when this buffering process can’t keep up that our muscles start to burn.

“If we didn’t produce lactate, we would have an accumulation of hydrogen ions, and our muscles would get so acidic as the pH [a measure of acidity/alkalinity] keeps dropping to a point where muscles won’t function,” says McCormick. “Basically, you’d have mechanical failure.”

Our bodies constantly produce lactate, and it is usually cleared quickly from our system. When we exercise vigorously, “we hit a threshold where hydrogen ions accumulate, and you can’t clear it as fast as you’d like,” says McCormick.

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Not only does lactate serve as a buffer, research also indicates that our bodies reuse lactate as a source of energy for our muscles, heart and brain. During moderate and hard exercise, it can be shuttled back into the mitrochondria of muscle cells (the energy compartments in your cells) and converted into energy. “Some lactate also goes to the liver and forms more glucose. It’s a continuous cycle,” says McCormick.

While the difference between lactate versus lactic acid is mostly semantics, our new understanding opens avenues to think about fitness. Instead of avoiding lactate threshold training, we can exercise at or above this level to help our bodies become more efficient at clearing, disposing and using the lactate it produces. While we can’t avoid the burn in our muscles altogether, we can delay its onset. Maybe lactate isn’t so bad after all?

Originally published August 2014. Updated July 2016. 

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