Lactate production and clearance during exercise

The amount of lactate in the blood depends on how much lactate is produced and how quickly it is cleared. Both of these factors are influenced by intensity.

Lactate is a product of glucose use by muscle cells. The higher the flow of glucose into the cell, the higher the lactate production — regardless of oxygen availability.

The balance of production and clearance

During intense exercise, lactate production is many times higher than at rest, and the demand for its clearance rises accordingly. The better prepared your muscles and the other systems responsible for clearance are, the more and the longer you will be able to stay in balance between producing lactate and converting it back into energy.

You can often see a resting lactate level that is even slightly higher than when you begin your workout. This is precisely because during exercise you start to recruit more muscles and the body actively begins to use lactate. It’s important to understand that when using a lactate analyzer, we always measure the balance of production and clearance, not the absolute amount of lactate in the blood. This allows us to assess the level of the workload and the degree of adaptation of the systems.

The mechanism of lactate clearance

Clearing the lactate that has been produced (and, importantly, the metabolites associated with the onset of fatigue) involves transporting lactate out of the contracting muscle fibers to other locations, where it is either oxidized in the mitochondria or used in a process called gluconeogenesis — essentially the reverse conversion of lactate back into glucose/glycogen.

Lactate can be exported into the blood for clearance and energy use in virtually every organ of our body. However, this process takes time, whereas lactate is produced continuously during exercise.

The role of muscle conditioning

The higher your level of fitness, the more efficient your muscles are and the less lactate enters the blood, because the clearance (recycling) process happens right inside the muscles, which takes seconds or milliseconds. This is very advantageous, as it allows the contracting muscles to remove H⁺ more quickly, and it also results in faster "recycling" of lactate to obtain additional energy (ATP).

During exercise, lactate is mainly produced in the "fast" muscle fibers, which use a lot of glucose to generate energy. It is cleared mainly by the "slow" muscle fibers. This is a complex process involving various lactate-specific transporters and enzymes.

Energy from fats and carbohydrates

First of all, you should know that during exercise energy is produced almost entirely from either fats or carbohydrates. Generating energy from fats is slower than from carbohydrates. Thus, as the intensity of exercise increases, more of the energy is produced from carbohydrates via glycolysis, which leads to a higher rate of lactate production.

As more and more ATP (adenosine triphosphate — the body’s energy currency) is required to meet the demands of the 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 into lactate.

Oxygen and intensity

Lactate production and clearance also depend on the availability of oxygen relative to demand, which itself depends on the intensity of exercise. When training at a truly high intensity, the oxygen supply cannot meet demand, and most of the pyruvate has to be converted into lactate rather than oxidized directly. In addition, because of the limited availability of oxygen, the accumulating lactate cannot be oxidized and therefore cannot be cleared from the blood and muscles unless the exercise intensity is reduced.

In other words, at a higher exercise intensity the oxygen supply to the working muscles cannot meet demand, and the rate of lactate production begins to exceed the rate at which it can be cleared, leading to an exponential increase in lactate concentration.