Lactate Metabolism for Runners: Fuel, Not a Waste Product

You’ve probably heard that lactic acid doesn’t cause fatigue. But if that’s true, why does your body slow down just as blood lactate spikes during an all-out effort? It’s a fair question — and the answer changes how you think about training.

The bottom line: lactate is a fuel source, not a fatigue toxin. What actually slows you down is the hydrogen ion released alongside lactate — these ions acidify your body fluids and impair metabolic function.

This article explains how lactate is produced during running and how your body recycles it back into usable energy.

By understanding lactate metabolism, you’ll gain a clearer picture of why different training sessions target different physiological adaptations — and how to structure your training more effectively.

How Lactate Is Produced and Metabolized

Here’s a step-by-step look at how lactate is generated and how your body processes it.

Carbohydrate Metabolism: The Glycolytic Pathway

To understand how lactate is produced, you first need to understand how your body converts carbohydrate into energy.

The pathway that converts carbohydrate — either glucose in the blood or glycogen stored in muscles and organs — into energy is called the glycolytic system (glycolysis).

Through a series of reactions, glycolysis converts carbohydrate into pyruvate. This process generates 2 ATP (adenosine triphosphate) per glucose molecule (3 ATP when starting from glycogen).

When pyruvate enters the mitochondria, it undergoes complete oxidation via the Krebs cycle, generating an additional 30 ATP.

How Lactate Is Produced

As exercise intensity increases — as you run faster — the glycolytic system ramps up, and carbohydrate becomes the dominant fuel source.

When glycolysis accelerates, more pyruvate is produced than the mitochondria can immediately process. Mitochondrial pyruvate uptake has an upper limit, so pyruvate begins to accumulate.

To prevent pyruvate from reaching harmful concentrations, a dehydrogenation reaction converts the excess pyruvate into lactate. This is how lactate is produced.

How Lactate Is Metabolized

When pyruvate is converted to lactate, hydrogen ions are simultaneously released into the bloodstream. As blood lactate rises, so does hydrogen ion concentration — causing blood pH to fall and making the internal environment more acidic.

This acidification impairs muscle contraction and metabolic function. In response, the body works to reduce circulating lactate — that is, it ramps up lactate metabolism as quickly as possible.

Lactate is converted back into energy through the following pathways:

Brooks (1986) ※1 reported that more than 75% of lactate produced during steady-state exercise is oxidized during that same exercise bout. Much of this oxidation occurs through the lactate shuttle described below.

Lactate that is not immediately oxidized in the mitochondria is released into the bloodstream. From there, it is either taken up by other cells for mitochondrial oxidation, or transported to the liver.

When lactate reaches the liver, the Cori cycle converts it back into glucose through gluconeogenesis — making it available again as an energy source.

Lactate Metabolism and Marathon Training

Here’s how this plays out in the context of marathon training.

At easy conversational pace — what Daniels calls E pace — mitochondria can fully oxidize all pyruvate as fast as it is produced. Lactate is cleared within the muscle fiber before it can accumulate. This is why E pace is defined as an intensity at which blood lactate does not rise.

At higher intensities such as interval training, lactate is produced faster than it can be cleared. The excess spills into the bloodstream, raising blood lactate.

The intensity at which blood lactate begins to rise is called the lactate threshold (LT). The accompanying rise in hydrogen ions acidifies body fluids, impairing performance.

To sustain a fast marathon pace for a long time, you need to train your body to clear lactate faster. That is the underlying goal of most marathon training.

Lactate Production and Clearance by Muscle Fiber Type

As energy demand rises sharply, glycolysis accelerates and lactate production increases. Here’s a closer look at how different muscle fiber types contribute to that process.

Fast-Twitch Fibers Generate Lactate; Slow-Twitch Fibers Burn It

As exercise intensity increases, the proportion of fast-twitch muscle fibers being recruited also increases.

Fast-twitch fibers contain few mitochondria, making it difficult for them to fully oxidize the pyruvate they produce — and they cannot clear their own lactate. As a result, lactate generated in fast-twitch fibers is readily released into the bloodstream.

In contrast, slow-twitch muscle fibers are mitochondria-rich. They consume pyruvate and clear lactate quickly, leaving them with the capacity to absorb lactate released by fast-twitch fibers.

Across the whole body, the net result is: lactate generated in fast-twitch fibers is released into the blood, taken up by slow-twitch fibers, and fully oxidized in their mitochondria.

This phenomenon is called the cell-to-cell lactate shuttle (Brooks 1986) ※1.

The slow-twitch fibers acting as lactate consumers are not limited to the leg muscles. The cardiac muscle and respiratory muscles — both rich in slow-twitch fibers — also oxidize lactate via their mitochondria during exercise.

Type IIa Muscle Fibers: The Self-Clearing Hybrid

Type IIa muscle fibers (also called FOG fibers — fast oxidative glycolytic) are a hybrid fiber type. Like fast-twitch fibers, they can generate substantial force. But they also contain a high density of mitochondria, sharing characteristics with slow-twitch fibers.

Type IIa fibers are believed to metabolize their own lactate internally — in effect, running the cell-to-cell lactate shuttle at high speed within a single fiber.

This means the lag time between lactate production and energy reuse is shorter in type IIa fibers — that is, lactate clearance is faster.

What Determines Lactate Clearance Rate?

Faster lactate clearance can improve performance in high-intensity events such as the 5,000m, where sustaining speed is critical.

Since lactate produced in slow-twitch fibers is cleared locally within those fibers, the limiting factor is lactate generated in fast-twitch fibers.

Three factors govern how quickly that lactate is cleared:

① Lactate Diffusion and Blood Transport (MCT4)

The transporter responsible for releasing lactate from fast-twitch fibers into the bloodstream is MCT4 (monocarboxylate transporter 4). MCT4 is found predominantly in fast-twitch muscle fibers.

MCT4 increases consistently with high-intensity training. Pilegaard et al. (1999) ※2 found a 32% increase in MCT4 after 8 weeks of high-intensity training. Juel et al. (2004) ※3 also reported a statistically significant increase in MCT4 following high-intensity interval training.

By contrast, low-intensity endurance training tends not to increase MCT4.

To upregulate MCT4, training at intensities that heavily recruit fast-twitch fibers — at interval pace or faster — appears to be necessary.

② Lactate Uptake Into Cells and Mitochondria (MCT1)

For lactate to move from the bloodstream into the cytoplasm and mitochondria of consuming cells, the transporter MCT1 (monocarboxylate transporter 1) is required. MCT1 is found predominantly in slow-twitch muscle fibers.

Unlike MCT4, MCT1 increases with both endurance and high-intensity training. Dubouchaud et al. (2000) ※4 showed a statistically significant increase in MCT1 expression following moderate-intensity endurance training.

Pilegaard et al. (1999) ※2 also reported a 76% increase in MCT1 with high-intensity training — confirming that MCT1 responds across a wide range of training intensities.

③ Mitochondrial Oxidation Rate

To increase the rate at which mitochondria oxidize lactate, training that both increases mitochondrial quantity and improves mitochondrial function is required.

Evidence indicates that mitochondrial quantity depends on total training load, while mitochondrial function depends on training intensity.

For a detailed look at how to increase mitochondria and improve their function, see the article below.

Training to Improve Lactate Clearance

Both low-intensity and high-intensity training improve lactate clearance — through different mechanisms.

MCT1, which imports lactate into slow-twitch fibers for oxidation, increases across all training intensities. MCT4, which exports lactate from fast-twitch fibers into the blood, increases only with high-intensity training.

Mitochondrial quantity grows with total training load; mitochondrial function is strongly tied to training intensity.

This is why a balanced mix of easy and hard training progressively raises your lactate threshold — each intensity targets a different component of the clearance system.

Among the training methods available for improving LT, sweet spot training (SST) is widely considered the most cost-effective option — producing maximum adaptation with minimum fatigue.

SST is performed slightly below LT intensity. For a detailed breakdown of how to implement it, see the article below.

References

※1 Brooks GA (1986) “The lactate shuttle during exercise and recovery” Med Sci Sports Exerc

※2 Pilegaard H et al. (1999) “Effect of high-intensity exercise training on lactate/H+ transport capacity in human skeletal muscle” American Journal of Physiology

※3 Juel C et al. (2004) “Effect of high-intensity intermittent training on lactate and H+ release from human skeletal muscle” American Journal of Physiology – Endocrinology and Metabolism

※4 Dubouchaud H et al. (2000) “Endurance training, expression, and physiology of LDH, MCT1, and MCT4 in human skeletal muscle” Am J Physiol Endocrinol Metab