Lactate — The Key Marker in Endurance Training

What Is Lactate

Our body is designed so that everything is interconnected — when one system suffers, the entire training system is disrupted. We won't dive into complex processes and biology basics here. Instead, let's describe in simple terms what lactate is and why it's the key marker in endurance sports.

To fully understand what lactate is, we need to look at our energy systems. Energy systems need fuel to produce energy. One such fuel is carbohydrates.

The energy system uses glucose (carbohydrates) to produce energy. The end product of this process is lactate! In other words: lactate is the end product of glycolysis.

The less lactate during exercise, the higher the percentage of fat you're using as a fuel source — peak fat oxidation occurs in the range of 1.3 to 2.5 mmol/L. The higher the lactate, the more carbohydrates you're using.

Lactate and Fatigue?

One of the main stereotypes is that lactate is our enemy and causes fatigue, but this is not the case. When exercise intensity increases, athletes begin to use more carbohydrates as fuel. As a result, more lactate is produced.

This is one of the reasons people have come to think that lactate is a byproduct that causes fatigue: during high-intensity exercise, lactate concentration is high, and athletes get tired. However, there is no causal relationship between lactate and fatigue.

Lactate does NOT cause fatigue — it's not even a "waste product." It's FUEL!

We've seen that the anaerobic energy system produces lactate. The aerobic energy system uses this lactate as fuel to produce even more energy.

Lactate Threshold

The lactate threshold is the highest intensity at which lactate in muscles and blood can reach a steady concentration, which is why it's commonly called the maximal lactate steady state.

You may also see the term lactate threshold used interchangeably with anaerobic threshold (AT), LT2, or VT2. However, all these terms have slightly different meanings due to different measurement methods.

The lactate threshold is typically around 4 mmol on average. On average, you can sustain this intensity for approximately 40–130 minutes depending on the amount of available carbohydrate energy.

The lactate threshold is the point at which the rate of lactate production exactly equals the rate of clearance, where any decrease in intensity will lead to a drop in lactate levels, and any increase in intensity will lead to non-linear lactate accumulation.

Lactate Production

Lactate is produced by the body all the time, even at rest, since both aerobic and anaerobic energy production (mitochondrial respiration, glycolytic system, or creatine phosphate) are used at any given moment.

As more and more ATP (adenosine triphosphate — the body's energy currency) is required to meet the demands of increasing workload, the contribution of the glycolytic anaerobic system also increases.

The end product of the glycolytic anaerobic system is pyruvate, which is either oxidized in the mitochondria or converted to lactate.

Lactate Clearance

Removal of produced lactate (and, importantly, associated metabolites linked to the onset of fatigue) involves transport of lactate from contracting muscle fibers to other locations where it is either oxidized in mitochondria or used in a process called gluconeogenesis — essentially the reverse conversion of lactate back to glucose/glycogen.

Some adaptations that lead to improved lactate transport include training volume, training in the steady lactate zone, at threshold level and slightly above — the key is proper dosing.

Another key factor affecting the ability to clear lactate is VO2max, as it influences the rate of lactate oxidation. Lactate oxidation contributes most to lactate clearance during moderate and high-intensity exercise.

Why Measure Lactate

There are many reasons why athletes and coaches are interested in measuring lactate levels. Let's look at some examples.

Lactate: A Marker of Glycolytic Activity

We learned that lactate is always the end product of glycolysis (also called the anaerobic energy system). Thus, the rate of lactate production tells you something about how active the anaerobic energy system is.

Lactate: A Marker of Fatigue

We learned that lactate does not cause fatigue. However, blood lactate concentration is a good marker of fatigue. For example, when an athlete shows a constant increase in lactate concentration, you can be sure they will fatigue.

Since fatigue markers like heart rate are not always accurate, especially with increasing load, lactate directly reflects how you're currently loading yourself, which energy system you're using, and consequently what training objectives you're addressing.

Lactate: A Marker of Carbohydrate Use

Finally, since all carbohydrates "pass through" the anaerobic energy system, knowing the rate of lactate production also tells you something about carbohydrate utilization. It's therefore not surprising that the lactate production curve resembles the carbohydrate burning curve.

Exercise Intensity and Blood Lactate Levels

Blood lactate concentration changes depending on exercise intensity. At higher exercise intensities, you will ultimately get higher blood lactate values.

Exercise Duration and Blood Lactate Levels

Blood lactate concentration can also change over time, even when exercise intensity remains the same. Imagine, for example, an athlete running 10 km. The running pace stays approximately the same. However, the lactate concentration at the finish will be higher than in the middle of the race.

In other words: lactate concentration increases even when pace remains constant. There's a term for this — the rate of lactate appearance. The lower this value is for you, the better.

Lactate concentration depends on both time and intensity. Each person will respond differently to these two factors. Therefore, if you want to know how you respond to load, it may make sense to use different durations and different exercise intensities in your lactate testing protocol.

What to Consider in Lactate Analysis

Now that we have blood lactate test results, what do we do with them? First, we need to fully understand what we're looking at. As briefly mentioned earlier, we should remember that measuring blood lactate levels is a practical alternative to what we really want to measure: lactate production in the muscles.

Also: blood lactate concentration is the result of lactate production and its "burning." We learned that the glycolytic (anaerobic) energy system produces lactate. We also learned that the aerobic energy system uses this lactate as an energy source.

There are average values for determining lactate thresholds, but they may differ slightly for each individual. In our training approach, we use 6 training zones built entirely on the energy supply model. In the following articles, we'll discuss zones and training models related to lactate.