How Carbs Fuel Your Running: Carbohydrate Metabolism Explained

Many runners cut carbs to lose weight — but could that approach actually be working against you?

Carbohydrates are essential for running fast. If you’re training hard while restricting carbs, you may be undermining your own performance without realizing it.

I went through a period of low-carb dieting myself. But I quickly found that restricting carbohydrates made it impossible to train with quality — and my performance suffered as a result.

In this article, I’ll break down how carbohydrate metabolism works and explain the right approach to fueling your training.

As exercise intensity increases, your body relies more heavily on carbohydrates for fuel. And when you train with adequate carbohydrate stores, you become better at using them efficiently.

If your training targets VO2 max or lactate threshold (LT) improvements, adequate carbohydrate intake is non-negotiable.

On the other hand, training in a low-carbohydrate state can be a useful strategy when the specific goal is to enhance fat adaptation.

Energy Systems and Their Importance Across Race Distances

Let’s start by looking at the different energy systems and how their importance varies across race distances.

The body has five different pathways for producing ATP (adenosine triphosphate), the energy source that powers muscle contraction.

The diagram below shows the proportion of energy supplied by anaerobic and aerobic systems across different events.

This graph shows how the energy supply ratio shifts with the duration of maximal effort.

Because carbohydrates fuel both anaerobic (glycolytic) and aerobic (oxidative) pathways, they are an exceptionally important nutrient for runners. Understanding how carbohydrate metabolism works — and how to fuel and train accordingly — is key to improving performance.

Why Carbohydrate Metabolism Is Essential for Running Fast

Carbohydrate metabolism capacity is essential for running fast.

In a 5000m race, the vast majority of energy comes from carbohydrates. A high lactate clearance capacity matters for fast 5K performance, but without a strong ability to use carbohydrates in the first place, lactate doesn’t even get produced.

In marathon running, fat utilization tends to receive the most attention — but carbohydrates are just as critical. By using carbohydrates to sustain a fast pace and continuously processing the lactate produced, you can maintain a high pace for a longer time.

Building your carbohydrate utilization capacity first, and then adding LT training to improve lactate clearance, is the most efficient path to raising your lactate threshold.

How Your Body Processes Carbohydrates

Here is a breakdown of how carbohydrate metabolism works in the body.

From Food to Muscle Glycogen: How Carbs Are Stored

We take in carbohydrates through foods like rice, bread, and sugar. Rice and bread contain polysaccharides — long chains of sugar molecules — while table sugar contains disaccharides, formed from two linked sugar molecules.

By the time carbohydrates reach the small intestine, they have been broken down into glucose (also called blood sugar). Glucose is a monosaccharide — the simplest unit of sugar.

Glucose travels from the small intestine through the bloodstream to the liver, where it is converted into glycogen — a branched chain of many linked glucose units. Glycogen stored in the liver is called liver glycogen.

At the same time, glycogen synthesis also occurs in muscle tissue. Glycogen stored in the muscles is called muscle glycogen.

How Your Body Uses Carbohydrates for Energy

When your body uses carbohydrates for energy, it draws from blood glucose and glycogen stored in the muscles and liver.

Glucose and glycogen are first converted to glucose-6-phosphate, then further broken down into pyruvate.

Once pyruvate is produced, it is taken up by the mitochondria, converted to acetyl-CoA, and fully oxidized through the Krebs cycle (TCA cycle) to produce energy.

The pathway from glycogen (or glucose) to pyruvate, which generates 3 ATP, is called the glycolytic system. This pathway requires no oxygen and is characterized by an extremely fast rate of energy production.

ATP (adenosine triphosphate) is the energy currency that powers muscle contraction. Once pyruvate enters the mitochondria, it undergoes aerobic metabolism.

Aerobic metabolism in the mitochondria produces far more energy than the glycolytic system alone.

The Relationship Between Carbohydrates and Lactate

As exercise intensity rises, fast-twitch muscle fibers become increasingly active, driving up carbohydrate utilization.

Greater carbohydrate use leads to more pyruvate production — but the rate at which mitochondria can process pyruvate and convert it to energy has a ceiling.

Excess pyruvate that cannot be processed is converted into lactate.

Most of the lactate produced is taken up by the heart and slow-twitch muscle fibers and oxidized as fuel — a process known as the lactate shuttle ※1. Research shows that more than 75% of the lactate generated during steady-state exercise is used directly as an energy source during that same session ※1.

The remaining lactate returns to the liver, where it is converted back to glucose through gluconeogenesis. This recycling pathway is called the Cori cycle. For a detailed explanation of lactate metabolism, see the related article below.

Why Low Carbs Also Impair Fat Metabolism

Carbohydrates don’t just provide energy directly — they also play a critical role in enabling fat metabolism. When carbohydrate availability is low, the efficiency of fat burning drops as well.

When carbohydrates run low, the production of oxaloacetate — a key intermediate required to keep the Krebs cycle running — decreases ※2. Without sufficient oxaloacetate, the cycle slows and can no longer fully process the acetyl-CoA supplied by fat oxidation, effectively limiting fat burning capacity ※2.

This has been confirmed in direct measurements of human skeletal muscle: glycogen depletion reduces TCA cycle intermediates and impairs aerobic energy production ※2.

In short, burning fat requires carbohydrates. For a detailed explanation, see the related article below.

The Trade-Offs of a Low-Carb Approach for Runners

Many runners pursue weight loss as a strategy to improve their marathon time.

If you’re using a low-carb diet to lose weight, you may be inadvertently reducing the training stimulus on your glycolytic system.

Glycolytic training derives its effectiveness from using muscle glycogen and blood glucose as the primary fuel source. The intensity and the act of depleting carbohydrate stores is what drives the adaptation.

But when you’re restricting carbohydrates, your muscle glycogen stores are already depleted before you even begin training — which means carbohydrate availability is limited from the start.

Because glycolytic training relies on carbohydrates for its training effect, combining it with a low-carb weight-loss approach is inherently counterproductive.

It’s advisable to ensure adequate carbohydrate intake before any session focused on glycolytic training.

In a randomized controlled trial (RCT), competitive runners who consumed a low-carb diet (under 1.5 g/kg/day) the day after a glycogen-depleting workout showed approximately a 2% decline in 1500m performance compared to a high-carb group (over 5 g/kg/day) ※3. This demonstrates that carbohydrate deficiency directly limits performance even in high-intensity events.

A meta-analysis pooling results from multiple studies found no statistically significant difference in endurance performance between periodized low-carb training and conventional high-carb training ※4.

On the positive side, training in a carbohydrate-depleted state may amplify mitochondrial adaptation signals. The trade-off, however, is a reduction in peak intensity during high-intensity sessions ※4.

The practical takeaway: ensure adequate carbohydrate availability for high-intensity interval sessions and threshold runs — sessions that rely on the glycolytic system — and reserve low-carb training for low-intensity recovery runs.

According to ACSM guidelines, athletes training at high intensity for 1–3 hours per day should consume 6–10 g of carbohydrate per kilogram of body weight per day ※5. Runners who train hard daily and restrict carbs too aggressively risk falling well below this benchmark.

How to Train Your Carbohydrate Metabolism

Based on everything covered so far, training your carbohydrate metabolism comes down to two goals: ① increasing the rate of glycolytic energy production, and ② improving lactate clearance capacity — the ability to process the lactate produced during aerobic carbohydrate metabolism.

These two goals are difficult to develop simultaneously. To increase glycolytic rate (①), repeated high-intensity training above VO2 max is most effective.

A study of seven weeks of sprint interval training (repeated 30-second all-out sprints) found statistically significant increases in key glycolytic enzymes — phosphofructokinase (PFK) and lactate dehydrogenase (LDH) ※6. High-intensity intervals of this kind train both the glycolytic and aerobic systems simultaneously.

For lactate clearance (②), accumulating as much training volume as possible just below the intensity where blood lactate begins to rise is the key strategy.

For ①, rep workout-style training is most effective; for ②, LT-pace training is the recommended approach.

References

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

※2 Sahlin K, Katz A, Broberg S (1990) “Tricarboxylic acid cycle intermediates in human muscle during prolonged exercise” Am J Physiol

※3 Venckunas T et al. (2024) “Effect of Low vs. High Carbohydrate Intake after Glycogen-Depleting Workout on Subsequent 1500 m Run Performance in High-Level Runners” Nutrients

※4 Gejl KD, Nybo L (2021) “Performance effects of periodized carbohydrate restriction in endurance trained athletes – a systematic review and meta-analysis” J Int Soc Sports Nutr

※5 Thomas DT, Erdman KA, Burke LM (2016) “American College of Sports Medicine Joint Position Statement. Nutrition and Athletic Performance” Med Sci Sports Exerc

※6 MacDougall JD et al. (1998) “Muscle performance and enzymatic adaptations to sprint interval training” J Appl Physiol