Lactate vs. Lactic Acid

Aug 11, 2022

Photo Credit: Jenny Hill

By: Danielle Abel

Metabolic Physiology

Let's say that you just ate a donut. That donut would be digested and broken down into glucose. Glucose consists of 6 carbon molecules linked together.

  • C-C-C-C-C-C

Glucose can be broken down for energy into 2) 3 carbon chain pyruvate molecules 

  • C-C-C
  • C-C-C

Glycolysis

When glucose is broken down into 2 pyruvate molecules, glycolysis has occurred. The word glycolysis simply means the splitting of glucose - gluco (glucose) & lysis (splitting or being broken down). 

From here, the pyruvate can go down 2 different pathways based on whether or not oxygen is present. 

  • With oxygen
    • Pyruvate is broken down into acetyl CoA and enters the Krebs cycle
    • This process occurs in the sarcoplasm of the muscle cell
    • This is an aerobic energy process
  • Without oxygen:  
    • Pyruvate accepts a hydrogen ion and becomes lactate
    • This process occurs in the sarcoplasm of the muscle cell
    • This is an anaerobic energy process

The hydrogen ions come from ATP (adenosine triphosphate) hydrolysis when ATP (the energy currency of the body) is broken down into ADP. 

When hydrogen ions build up, muscle acidosis occurs because the hydrogen ions cause the pH level to drop in the blood. Low pH is more acidic, high pH is more basic. Remember, when acidosis occurs, it's because the intensity of the work is so high that the body can't supply adequate oxygen fast enough to cover the exercise workload demands. This could be things like intervals or high-intensity training.

Lactic Acid and Lactate Buffering

The lactate molecules (the 3 carbon chain pyruvates with the hydrogen ion attached) are actually buffering the acidic environment of the blood. So it's not that lactate makes the blood acidic; it's the anaerobic metabolism of ATP being broken down without oxygen available that the acidic environment forms. At the same time, lactate forms when glucose is broken down into pyruvate, and pyruvate picks up a hydrogen ion. As hydrogen ions accumulate in the blood, lactic acid accumulation occurs. 

So the processes are occurring separately but at the same time. 

Now that the lactate has a hydrogen ion, you may be thinking, "is that now lactic acid?" The answer to this is no; it's actually helping the pH come back up to neutral because it's buffering the acidic hydrogen ions out of the bloodstream. 

Lactate is actually a base. 

Concurrent Anaerobic & Aerobic Metabolism

Even though lactate is formed for the most part during anaerobic metabolism, it's important to keep in mind that we have some percentage of anaerobic metabolism going on at all times. Since anaerobic metabolism is faster, it's the quickest energy source for the body; it's just a matter of how hard the effort is that it becomes noticeable. 

When performing anaerobic exercise, a greater percentage of pyruvate is turned into lactate in comparison to the percentage at rest. 

Lactate Threshold

The lactate threshold test is a finger prick blood test that tests the point at which formation exceeds clearance and lactate levels rise sharply. At higher heart rates, lactate cannot keep up with clearing the hydrogen ions from the blood, and lactic acid accumulation occurs. 

An athlete who knows where this level occurs recognizes where their highest rate of force production and max force production occurs, knowing that after this level, they shift into their aerobic energy systems, which are more fatigue resistant but don't produce as much speed or force. 

If an athlete can stay just below their lactate threshold, their perceived level of fatigue may be reduced, which will allow them to maintain longer, increasing their work capacity. Knowing where these zones lie can also be helpful in determining where long runs should occur in addition to where interval training should occur. 

Lactic Acid Clearance 

Lactic acid clearance happens very quickly when movement stops; it can be cleared in a number of ways. Oxidative Type I muscle fibers can clear lactate by cleaving the hydrogen molecule off the lactate and combining it with oxygen (H20). When the hydrogen is cleared, pyruvate is left, which can be used again for energy. 

Cardiac muscle has the capability to use lactate as an energy source, whereas skeletal muscle cannot, so cardiac muscle can also clear lactate from the blood. 

The liver can also clear lactate from the blood by going through the Cori cycle, a metabolic process in which lactate is converted into glucose to be reused for energy. 

The brain can also clear lactate from the bloodstream as well. 

Summary

In summary, lactate is actually a base that forms from the combination of pyruvate and hydrogen ions. The hydrogen ion build-up itself in the bloodstream from ATP hydrolysis (the phosphagen energy system) is the cause of acidosis (lactic acid). 

Lastly, muscle soreness is not from lactate or lactic acid accumulation either. It's likely that muscle soreness is simply from the microdamage that occurs in the muscle during resistance training. 


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