Why We Need Carbs During a Marathon

Most runners think hitting the wall happens when you “run out of glycogen.” But that’s not how the physiology actually works. and understanding the real mechanism is the difference between a controlled, confident marathon and a slow, painful fade after 30 km.

This article gives you the deeper reasoning behind why hourly carbohydrate intake is one of the most powerful performance tools you have.

🧠 The Real Carbohydrate Cost of a Marathon

A well‑trained 70 kg athlete running a ~3‑hour marathon typically burns around 600–700 g of carbohydrate. This aligns with metabolic data from endurance studies such as Romijn et al., 1993, which demonstrated high carbohydrate oxidation rates at marathon‑relevant intensities.

With a proper carb‑load, most athletes start the race with:

• 600–900 g of muscle glycogen

• 80–120 g of liver glycogen

Total: ~700–1,000 g stored.

So on paper, you might think: “I’ve got enough stored to get through the race — why fuel at all?”

Because the body doesn’t operate on total glycogen. It operates on local availability.

🔍 Why You Can Hit the Wall Even With Glycogen Left

“Hitting the wall” isn’t global depletion, it’s critical depletion in two specific systems.

1️⃣ Liver glycogen

Liver glycogen maintains blood glucose. When it drops too low, blood glucose falls, your brain panics, and perceived effort increases to protect you.

Studies like Coyle et al., 1986 showed that fatigue during prolonged exercise strongly correlates with falling blood glucose and liver glycogen depletion.

2️⃣ Muscle glycogen inside the fibres doing the work

You don’t use all muscle fibres equally. If the fibres responsible for your marathon pace hit a low threshold of glycogen, force production collapses — even if other fibres still have plenty (for example if fibres in your quadriceps are low, but have excess in your calves).

This “local depletion” concept is supported by Casey et al., 1995 and Vøllestad et al., 1984, showing that fatigue is fibre‑specific rather than whole‑muscle.

This is one of the reasons why athletes can finish a marathon with hundreds of grams of glycogen still in the body, yet still hit the wall.

🔒 The “Glycogen Spare” Concept

To keep both liver and muscle systems stable, you want a buffer; a safety margin of glycogen that prevents either system from crashing.

A practical target is to aim for ~250 g of glycogen spare by the end of the race to ensure the safety net isn't breached.

Let’s run the maths:

• Start with ~850 g

• Burn ~660 g

• Finish with ~190 g spare

That’s below the safe zone.

This is why fueling matters — not to “add energy,” but to protect your liver and muscle glycogen from dropping into the danger zone.

🍌 Why 60 g for the whole race is useless, but 60 g per hour is powerful

Many runners think:

“I’ll take a gel or two — that’s about 60 g total. Should be fine.”

But total intake is irrelevant. What matters is rate of delivery.

❌ 60 g for the whole race

Your liver and muscles still drain faster than you can support them and you're right on the limit of the glycogen depletion threshold.

✔ 60 g per hour

Keeps you well away from the glycogen depletion threshold.

This is strongly supported by Jeukendrup’s 2014 review, which demonstrated that exogenous (external) carbohydrate intake of 60–90 g/h (and up to 120 g/h with glucose–fructose blends) maximises carbohydrate oxidation and spares internal glycogen.

It’s the difference between dribbling fuel into the system versus having a steady stream of fuel.

🎯 So what should you aim for?

For most marathoners:

• Minimum: 60 g/hour

• Optimal for many: 70–90 g/hour

• Well‑trained athletes: up to 120 g/hour with glucose–fructose blends (Jeukendrup & Moseley, 2010)

If our example athlete wants to finish with ~400 g spare (a safer buffer), they’d need:

• ~210 g extra carbs during the race

• Over 3 hours → ~70 g/hour

This aligns perfectly with modern sports nutrition guidelines and the consensus position of the International Olympic Committee (IOC) on fueling for endurance performance.

Reference List

Casey, A., Constantin-Teodosiu, D., Howell, S., Hultman, E., & Greenhaff, P. L. (1995). Metabolic response of type I and type II muscle fibers during repeated bouts of maximal exercise in humans. American Journal of Physiology, 268(3), E400–E407.

Coyle, E. F., Coggan, A. R., Hemmert, M. K., & Ivy, J. L. (1986). Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate. Journal of Applied Physiology, 61(1), 165–172.

Hultman, E., & Bergström, J. (1967). Muscle glycogen synthesis in relation to diet studied in normal subjects. Acta Medica Scandinavica, 182(1), 109–117.

Jeukendrup, A. E. (2014). A step towards personalized sports nutrition: Carbohydrate intake during exercise. Sports Medicine, 44(Suppl 1), S25–S33.

Jeukendrup, A. E., & Moseley, L. (2010). Multiple transportable carbohydrates enhance gastric emptying and fluid delivery. Scandinavian Journal of Medicine & Science in Sports, 20(1), 112–121.

Maughan, R. J., Burke, L. M., Dvorak, J., Larson-Meyer, D. E., Peeling, P., Phillips, S. M., ... & Engebretsen, L. (2018). IOC consensus statement: Dietary supplements and the high-performance athlete. International Journal of Sport Nutrition and Exercise Metabolism, 28(2), 104–125.

Romijn, J. A., Coyle, E. F., Sidossis, L. S., Gastaldelli, A., Horowitz, J. F., Endert, E., & Wolfe, R. R. (1993). Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. American Journal of Physiology, 265(3), E380–E391.

Sherman, W. M., Costill, D. L., Fink, W. J., & Miller, J. M. (1981). Effect of exercise-diet manipulation on muscle glycogen and its subsequent utilization during performance. International Journal of Sports Medicine, 2(2), 114–118.