Lactate Production & Clearance During Exercise
The amount of lactate in the blood depends on how much lactate is produced and how quickly it is utilized. Both of these factors are influenced by exercise intensity.
Lactate is a product of glucose utilization by muscle cells. The higher the glucose flux into the cell, the higher the lactate production — regardless of oxygen availability.
Balance of Production and Utilization
During intense exercise, lactate production is many times higher than at rest, and accordingly the need for its utilization also increases. The better prepared your muscles and other systems responsible for utilization are, the more and longer you can maintain a balance between lactate production and its recycling back into energy.
You can often see resting lactate levels even slightly higher than when you begin exercising. This is precisely because during exercise you start activating more muscles and the body actively begins using lactate. It is important to understand that when using a lactate analyzer, we always measure the balance between production and clearance, not the absolute amount of lactate in the blood. This allows us to assess the level of workload and the degree of system adaptation.
Lactate Clearance Mechanism
The removal of produced lactate (and, importantly, the metabolites associated with the onset of fatigue) involves the transport of lactate from 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 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, while lactate is continuously produced during exercise.
The Role of Muscle Fitness
The higher your fitness level, the more efficient your muscles become and the less lactate enters the bloodstream, as the utilization (recycling) process occurs directly within the muscles in a matter of seconds or milliseconds. This is highly advantageous as it allows the contracting muscles to remove H⁺ more quickly, and also enables faster 'recycling' of lactate for additional energy (ATP).
During exercise, lactate is mainly produced in 'fast-twitch' muscle fibers, which use a lot of glucose for energy. It is primarily cleared by 'slow-twitch' muscle fibers. This is a complex process involving various lactate-specific transporters and enzymes.
Energy from Fats and Carbohydrates
First and foremost, you should know that energy is almost entirely generated from either fats or carbohydrates during exercise. Energy generation from fats is slower than from carbohydrates. Therefore, as exercise intensity increases, a greater proportion of energy is produced from carbohydrates through glycolysis, leading 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 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 exercise intensity. When training at very high intensity, oxygen delivery cannot meet demand, and most of the pyruvate must be converted to lactate rather than being directly oxidized. Furthermore, due to limited oxygen availability, the accumulating lactate cannot be oxidized and therefore cannot be cleared from the blood and muscles unless exercise intensity is reduced.
In other words, at higher exercise intensities, the oxygen supply to 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.
