During intense physical activity, your body produces a molecule called lactate. For a long time, this substance was incorrectly labeled a metabolic waste product and blamed for muscle soreness. Current scientific understanding, however, shows that lactate is not a harmful byproduct. It is a useful molecule that plays a part in how your body generates energy when you push its limits.
The Production of Lactate During Exercise
Your body’s primary method for breaking down glucose to create energy is a process known as glycolysis. When you exercise at a low to moderate intensity, there is enough oxygen available for aerobic glycolysis. In this state, pyruvate is produced from glucose and then shuttled into the mitochondria to generate large amounts of energy with oxygen.
When you increase the intensity of your exercise, your muscles’ demand for energy can outpace the oxygen supply. This forces your body to rely more on anaerobic glycolysis, a faster but less efficient energy pathway that does not use oxygen. In this high-demand situation, pyruvate begins to build up in the muscle cells.
To allow energy production to continue, the body must manage this excess pyruvate. The enzyme lactate dehydrogenase converts pyruvate into lactate. This conversion is a temporary solution that helps reduce the accumulation of other metabolic byproducts, allowing high-intensity effort to continue for a little longer.
The production of lactate is not a sign of failure but an adaptation. It is a continuous process that occurs even at rest, but it accelerates dramatically during anaerobic efforts like sprinting or heavy weightlifting. This creation of lactate ensures that muscles can continue to generate the energy needed for powerful, short-duration movements.
Lactate as a Fuel Source
Once produced, lactate does not stay trapped in the muscle where it was created. It enters the bloodstream and is transported throughout the body to be used as a fuel source. This process, known as the lactate shuttle, is a factor in energy distribution during exercise. It moves from tissues that produce it in large quantities, like fast-twitch muscle fibers, to tissues that can use it for energy.
The heart and brain are two primary consumers of lactate. The heart is well-equipped to take up lactate from the blood and use it to fuel its continuous contractions during strenuous activity. Similarly, slow-twitch muscle fibers, which are rich in mitochondria, can absorb lactate from the circulation and oxidize it for energy. This shuttling mechanism allows the body to recycle this resource.
Another destination for lactate is the liver, where it participates in the Cori cycle. In this metabolic pathway, the liver takes up lactate from the bloodstream and converts it back into glucose. This newly formed glucose can then be released back into the blood to fuel the working muscles or be stored as glycogen for future use. This cycle is a way for the body to sustain energy levels during prolonged exertion.
The body’s ability to use lactate as a fuel source is a sophisticated adaptation. It transforms what was once considered a waste product into a versatile energy substrate that supports various organs and tissues. This reframes lactate as part of metabolic regulation, helping to sustain performance.
Understanding the Lactate Threshold
As you increase your exercise intensity, you reach a point where lactate is produced faster than your body can clear it from the bloodstream. This point is known as the lactate threshold (LT). It marks the transition from a state where lactate production and clearance are balanced to one where lactate begins to accumulate. This threshold is a marker in exercise physiology.
The accumulation of lactate is associated with the onset of fatigue and the burning sensation felt in muscles during intense workouts. However, lactate itself is not the direct cause of this sensation. The “burn” is caused by the buildup of hydrogen ions, which are released during the rapid breakdown of glucose in anaerobic glycolysis and increase acidity within the muscle cells.
A higher lactate threshold is a strong predictor of endurance performance. Athletes with a higher LT can maintain a faster pace or a greater power output for a longer period before fatigue sets in. For example, elite marathon runners often compete at an intensity that is very close to their lactate threshold.
This threshold is not a fixed value; it is dynamic and can change with training, nutrition, and even daily factors like hydration. It is measured as a specific heart rate, pace, or power output. Understanding your lactate threshold can provide insight for structuring your training, as it helps define the boundary between sustainable and unsustainable effort.
Training to Improve Lactate Dynamics
You can train your body to become more efficient at clearing and using lactate, which effectively raises your lactate threshold. Specific training methods stimulate physiological adaptations that improve how your body handles lactate during intense exercise. These adaptations allow you to sustain higher intensities for longer durations.
One effective method is tempo or threshold training. This involves exercising at an intensity at or just below your current lactate threshold for a sustained period, typically 20 to 60 minutes. Training at this “comfortably hard” pace encourages your body to improve its ability to clear lactate from the blood and use it as fuel.
High-intensity interval training (HIIT) is another tool for improving lactate dynamics. HIIT workouts involve short, all-out bursts of exercise performed well above the lactate threshold, followed by brief recovery periods. This training forces the muscles to produce large amounts of lactate, which stimulates the body to increase the number of lactate transport proteins that shuttle lactate out of the muscle cells.
Both tempo and HIIT workouts contribute to an increase in mitochondrial density. Mitochondria are the parts of the cell responsible for aerobic energy production and lactate oxidation. Having more mitochondria means your body has a greater capacity to use oxygen and burn lactate as fuel, ultimately enhancing your endurance performance.