Lactate Explained From a Physiological Perspective

Before going down the road of explaining what lactate testing is about, let’s spend a few moments on defining two terms that are often used interchangeably but are, in fact, chemically distinct. 

Lactic acid is a product of metabolism that occurs in varying amounts relative to the availability of oxygen in carbohydrate metabolism. It is called an acid because it has the ability to share protons in the form of hydrogen atoms in the presence of water. The compound that remains is called lactate.

For the chemistry aficionados, lactate is called the conjugate base of lactic acid and is a naturally occurring metabolite in the metabolic conversion of glucose into energy. As a performance metric to assess the physiological adaptations conducive to the design of training programs, lactate is what is being tested. Hence, that is the term used in this blog post. 

How Lactate Is Tested and Produced 

When lactate testing is performed, what is actually measured is lactate in the blood and not in the muscle where it is formed—though, on a practical basis, the error introduced is negligible. 

During a performance test, a very small sample of blood is usually removed from one’s fingertip, or sometimes an earlobe. It is placed on a specialized strip and then analyzed in an instrument, producing results in as little as 13 seconds. 

Lactate production is proportional to the intensity of exercise. When energy demands increase, a corresponding rise in lactate production follows. As long as levels do not appreciably accumulate, then lactate does not become a performance limiter.  

3 Energy-Generating Pathways In Your Metabolic Transmission

Many endurance athletes and coaches are familiar with training zones. Loosely defined, these zones refer to varying levels of intensity which are assessed via heart rate or workload (e.g., speed, pace or power). 

Similar to your car’s transmission, which has different gears to accommodate an engine’s intensity, so too does the human body. Your metabolic transmission contains its own gears that are expressed as metabolic pathways, represented as different levels within the training zone model—typically five zones. 

Our metabolic gearbox contains three energy-generating pathways: 

• ATP-CP
• Glycolysis 
• Oxidative

Glycolysis and the oxidative metabolic pathways can be further subdivided based on whether sufficient oxygen levels are present to support oxygen-assisted glycolysis (oxidative glycolysis) or the high energy yield from the metabolic decomposition of lipids (fats as triglycerides). However, unlike an automobile transmission—where you use only one gear at a time—your metabolic transmission uses a blend of metabolic pathways depending on the energy demands from the intensity of the exercise. 

As previously mentioned, lactate is a product of glycolysis (carbohydrate metabolism). However, since all metabolic pathways occur in varying percentages simultaneously, lactate is produced all the time. 

In low to moderate intensities, when energy demands are relatively low and oxygen availability is high, very little lactate is generated. It also does not accumulate due to mechanisms that remove or clear it from the muscle environment. These mechanisms include being oxidized, moved to neighboring cells for energy production, transferred to the liver to be stored as glycogen or converted to glucose and recirculated to exercising muscles as a source of immediate fuel.

How Lactate Supports Energy Development

Lactate is often erroneously blamed for causing fatigue and sore muscles. But as previously stated, lactate actually provides positive contributions to cellular energy. 

Lactate is a gluconeogenic substance, which means it can form glucose via its association with the liver. The liver receives the lactate—a metabolic byproduct of nonoxidative (without oxygen) glycolysis, formed primarily during high-intensity exercise—and converts it into glucose. The newly formed glucose enters the bloodstream to be received by the active muscles to support their metabolic requirements from the demands of exercise intensity. 

Moreover, even the heart and brain, via cerebral spinal fluid, benefit from lactate. 

The Negative Side of Lactate 

What happens when exercise intensity raises energy demands, oxygen supply can no longer meet the requirements of oxidative glycolysis and nonoxidative glycolysis assumes a greater role in producing energy? Now we have a problem. 

Associated with lactate production is an accumulation of hydrogen ions (electrically charged atoms), which is the culprit in fatigue and muscle soreness. Hydrogen ions, originally associated with the lactic acid, increase in number and create an acidic environment (a drop in pH). A drop in pH interferes with metabolic enzyme formation and muscle fiber contractile processes. It may also excite pain receptors. All of which contribute to a drop in performance. 

Final Thoughts

For many endurance athletes, lactate is regarded as a nemesis to performance. Though lactate is associated with high-intensity exercise, it is more a marker of anaerobic (without oxygen) metabolism, which eventually leads to the accelerated onset of fatigue and muscle soreness and a decline in performance. Hopefully, this blog brought some clarification to this often misunderstood metabolite. 

About the Author

Michael D. Weiss, M.S., ACSM: EP-C, EIM, is a former university professor of exercise science. He is an exercise physiologist, endurance sports consultant, running coach and an avid endurance athlete engaged in trail running, cycling and cross-country skiing. Michael’s professional interests include the human responses and adaptations to exercise in environmental extremes, with emphasis on heat and altitude conditioning. Additionally, via his own business, he combines his background in cardiometabolic conditioning with his practical skills in performance testing to develop high-performance training programs for endurance athletes.

Please visit Alta Health and Performance Solutions to learn more about Michael Weiss and performance testing.