Carb loading before a marathon: what actually works

Carb loading is probably the most misunderstood tactic in marathon nutrition. Most runners either skip it entirely or try to accomplish it in a single dinner the night before. Neither works. A proper carb loading protocol, executed over three to four days, increases muscle glycogen stores above baseline. In practical terms, that translates to delaying fatigue during a marathon.

This guide covers the science behind carb loading, the protocol to follow, and the mistakes that sabotage it. It links into the broader marathon nutrition plan that covers every phase from training block through post-race recovery.

Carb loading (glycogen supercompensation) is a dietary strategy that saturates muscle glycogen stores beyond their normal resting levels. Your muscles typically store 300 to 500 grams of glycogen. With proper loading, that increases. Since glycogen is the primary fuel source for sustained running above approximately 65% of VO2max, starting with more of it means you can maintain your target pace longer before your body shifts to the slower fat oxidation pathway.

Research has shown that runners who carb-loaded performed better in the final portion of endurance trials compared to those who did not. The difference was entirely in the back half of the race, which is exactly where most marathon runners lose time.

What is the best carb loading protocol before a marathon?

The modern protocol is simpler than the old depletion then supercompensation approach from the 1960s. No depletion phase is needed. Research confirmed that 36 to 48 hours of high carbohydrate intake combined with training taper achieves the same glycogen levels as the seven-day classical protocol.

A four-day protocol for a 70 kg runner:

Day	Carbs (g/kg)	Total carbs (70 kg)	Training	Focus
4 days out	8	560 g	30 min easy	Increase carbs. Start reducing fibre.
3 days out	10	700 g	20 min shakeout	Peak carb day. White rice, pasta, bread, sports drinks.
2 days out	10	700 g	Rest or 15 min walk	Continue loading. No new foods.
1 day out	10	700 g	Rest	Last solid meal by 7 PM. Familiar foods only.

700 grams of carbohydrate is roughly 2,800 calories from carbs alone. That is 4 to 5 cups of cooked white rice, or 7 to 8 large bagels, or some combination of rice, pasta, bread, potatoes, sports drinks, juice, and fruit. It is more food than most runners expect, and under-eating during the loading phase is the most common mistake.

What are the best foods for carb loading?

During the loading phase, you need high-glycemic, low-fibre, low-fat carbohydrate sources that are easy on the gut. This is not the time for whole grains, salads, or high-fibre cereals.

• Primary sources: white rice, white pasta, white bread, bagels, potatoes (boiled or baked, no skin), pretzels, rice cakes
• Supplementary sources: sports drinks, fruit juice, honey, jam, maple syrup, ripe bananas, applesauce
• Protein (keep at 1.2 to 1.4 g/kg): chicken breast, white fish, eggs, low-fat yogurt
• Avoid: beans, lentils, cruciferous vegetables, whole grains, high-fat sauces, fried foods, anything with more than 5 g fibre per serving

Runners who consumed high amounts of fibre during their loading phase were more likely to report GI distress on race day compared to those who reduced fibre.

What mistakes ruin a carb loading plan?

Five common errors that undo the protocol:

1. Starting too late. A single pasta dinner does not meaningfully increase glycogen stores. You need at least 36 to 48 hours at 10+ g/kg/day.
2. Not eating enough. Most runners underestimate 10 g/kg/day. Measure and count for the first time to calibrate.
3. Eating too much fibre. Fibre slows gastric emptying and increases GI risk. Switch to refined carbohydrates for the loading phase.
4. Adding fat. Cream sauces, cheese, and butter displace carbohydrate calories and slow digestion. Keep fat below 20% of total calories during loading.
5. Trying new foods. Every food you eat in the final four days should be something your gut already knows. Race week is not the time to discover you are sensitive to a specific grain or sauce.

Does carb loading work for half marathons too?

The evidence is weaker for the half marathon distance. Most runners have sufficient glycogen to complete 21 km at moderate pace without hitting the wall. However, a modified one to two day loading protocol can benefit half marathon runners targeting a personal best at high intensity, where carbohydrate oxidation rates peak and glycogen depletion happens faster.

For efforts under 90 minutes, standard carb loading is unnecessary. A normal carbohydrate-rich diet and a solid pre-race meal will suffice.

What about the cellular layer: does mitochondrial capacity matter during carb loading?

Carb loading fills the tank. But the efficiency of the engine that burns the fuel is equally important. Your mitochondria convert glycogen into ATP. If those mitochondria are running below capacity due to accumulated oxidative stress from weeks of training, even a perfectly loaded glycogen store will not deliver its full potential.

This is where the OLEUS Daily Shot fits into your race preparation. Taken daily in the weeks leading up to race day, the Daily Shot delivers oleuropein, magnesium, and vitamins B6 and C to support mitochondrial function and cellular energy production. On race morning, the Pre-Activity Shot provides targeted activation for the effort ahead.

Carb loading fills the fuel reserves. OLEUS supports the machinery that converts them into movement.

Build your race-day stack

Sources

1. Gherardi, G., et al. (2024). Mitochondrial calcium uptake declines during aging and is directly activated by oleuropein to boost energy metabolism and skeletal muscle performance. Cell Metabolism. DOI: 10.1016/j.cmet.2024.10.021

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