How to Carb Load Before a Marathon: The Science-Backed 2025 Guide

If you've ever hit the wall in a marathon, you already know the reason: you ran out of fuel.

Carb loading — the strategic increase of carbohydrate intake before race day — is the proven way to maximize glycogen stores and delay fatigue.

Here's how to carb load the right way before your marathon — backed by sports science and made easy with MAVR, the AI-powered fueling app built for endurance athletes.

Start carb loading 2–3 days out, target 8–10 g of carbs per kilogram daily, keep meals low in fiber and fat, hydrate steadily with electrolytes, and top off with a familiar, high-carb breakfast on race morning.

• Carb load with familiar, low-fiber staples and taper training stress.
• Hydrate consistently and include sodium so glycogen stores lock in water.
• Use MAVR to calculate carb targets, meal timing, and race-day breakfast.

Carb loading, or carbohydrate supercompensation, is the process of maximizing muscle glycogen stores before a long endurance event.

Your body can store roughly 400–600 grams of glycogen in muscle and another 100 grams in the liver — around 2,000–2,800 calories of ready-to-burn fuel.

After 90–120 minutes at marathon pace those stores deplete, leading to bonking unless you fuel mid-race. Proper carb loading ensures you start with a full tank.

Carb loading isn't a single meal; it is a 2–3 day strategy timed with your taper so glycogen saturation occurs without unnecessary weight gain.

Begin two to three days before the marathon when training volume drops, and aim for 8–10 grams of carbohydrate per kilogram of body weight per day (3.6–4.5 g per pound).

• Example: A 150 lb (68 kg) runner needs roughly 540–680 g of carbs per day during the final 2–3 days.
• Distribute intake across breakfast, snacks, lunch, dinner, and an evening top-off to keep digestion comfortable.
• Use MAVR to update daily carb goals automatically as your taper shifts.

You don't need exotic foods — prioritize easy-to-digest, high-carb meals that you've already tested in training.

• White rice, pasta, bread, and bagels.
• Oatmeal, pancakes, waffles, and low-fiber cereals.
• Potatoes, sweet potatoes, and rice cakes.
• Bananas, applesauce, dried fruit, and fruit juice.
• Sports drinks or carbohydrate drink mixes for liquid calories.

• Too much fiber that can cause bloating or digestive distress.
• High-fat meals that slow gastric emptying and crowd out carbs.
• Excess protein that fills you up before carb needs are met.
• Unfamiliar foods that could upset your stomach on race morning.

Every gram of stored glycogen binds about three grams of water, so expect to gain 2–4 pounds — that is a sign your muscles are fueled and hydrated.

• Drink steadily throughout the day instead of chugging before bed.
• Add electrolytes to at least one bottle daily to support glycogen storage.
• Avoid constant clear urine; balance fluids with sodium to prevent overhydration.

Keep meals high in carbs, low in fat and fiber, and finish your final substantial meal by early evening so digestion wraps before sleep.

• Eat breakfast, lunch, and dinner on schedule with carb-rich staples.
• Skip heavy sauces, fried foods, and large protein portions.
• Avoid alcohol, which can impair glycogen storage and hydration.
• Prep your race-day breakfast and hydration plan ahead of time.

Replace overnight liver glycogen with a familiar meal 3–4 hours before the start, targeting 1–4 g of carbs per kilogram of body weight.

Example for a 150 lb (68 kg) runner: 70–270 g of carbs from options like a bagel with banana and a sports drink or oatmeal with honey and toast.

• Keep fiber and fat minimal to protect your stomach.
• Sip 400–600 ml of fluid alongside breakfast and continue small sips until the start.
• Stick with foods you have tested in long runs — race day is not the time to experiment.

• Starting the carb load the night before the race instead of 2–3 days out.
• Relying on high-fiber foods that trigger digestive distress.
• Overdoing protein or fat and crowding out essential carbohydrates.
• Ignoring electrolytes and diluting sodium with plain water.

MAVR removes the guesswork from carb loading so you arrive at the start line fully fueled, not overstuffed.

• Enter your race date and body weight to generate precise carb targets.
• Receive daily meal timing and hydration cues that adapt as your taper evolves.
• Sync your training calendar so long runs, travel days, and rest days all factor into your fueling plan.