Marathon Fueling: How long before my marathon should I carb load?

October 20, 2025 •

4 min read

According to one survey, 62% of endurance athletes practice carb-loading before their events, but many question the optimal timing. Understanding how long before my marathon should I carb load? is a crucial part of race-week strategy to ensure you start with maximum energy stores.

Quick Summary

Successful carb-loading maximizes muscle glycogen by increasing carbohydrate intake and reducing training volume in the 2–3 days before a race. This approach helps prevent energy depletion and is more effective than older methods involving depletion phases.

Key Points

Timeline: Start carb-loading 2 to 3 days before your marathon, not just the night before, for optimal results.
Taper and Load: The increased carb intake works in synergy with the training taper to maximize muscle glycogen storage.
Focus on Carbs: Shift your diet to be 70–85% carbohydrates, reducing fat and protein slightly to avoid excess calories.
Low Fiber is Key: Choose easily digestible, lower-fiber carbs like white rice, pasta, and bagels to prevent stomach issues on race day.
Stay Hydrated: Drink extra fluids during the carb-loading period, as glycogen stores with water and aids hydration during the race.
Practice in Training: Experiment with your carb-loading strategy during your longest training runs to see what works best for your body.
Expect Water Weight: It is normal to gain 1–3 pounds during carb loading due to water retention, which benefits your race-day hydration.

The Science Behind Carb Loading

Carb loading is a nutritional strategy employed by endurance athletes to maximize the storage of glycogen in their muscles and liver. Glycogen is the body's primary fuel source for high-intensity exercise, and for an event as demanding as a marathon, having a full tank is critical to preventing fatigue, or "hitting the wall". The key is timing your increased carbohydrate intake with a reduction in training, known as tapering, which allows your body to store the extra energy instead of burning it off. This process can boost your glycogen stores by up to 30%.

The Optimal Timeline: How long before my marathon should I carb load?

The modern, science-backed approach to carb loading recommends a 2 to 3-day window immediately preceding the race. This short, focused period of high-carbohydrate intake, combined with your taper, is sufficient to supercompensate your glycogen levels without causing the negative side effects of older, more extreme methods. For a Sunday marathon, this means starting your carb load on Thursday evening or Friday morning. A single large meal the night before is not enough and can cause gastrointestinal distress. The strategy is to increase your carbohydrate percentage of total calories while slightly reducing your protein and fat intake to avoid over-eating.

Modern vs. Classic Carb-Loading Strategies


Feature	Modern Carb Loading (2-3 days)	Classic Carb Loading (6 days)
Timing	2 to 3 days before the event.	3 days of low carbs and heavy training, followed by 3 days of high carbs and light training.
Depletion Phase	No depletion phase is required.	Requires a phase of intense training and low-carb diet.
Stomach Distress	Lower risk of stomach issues from less dramatic diet shifts.	Higher risk of stomach upset and mood swings from restrictive phases.
Effectiveness	Equally effective for trained athletes due to metabolic adaptations.	Shows no significant performance advantage over the modern approach for trained athletes.
Simplicity	Simpler to execute and less disruptive to training and daily life.	More complex and mentally and physically taxing.

What to Eat During Carb Loading

When choosing foods for your carb load, the focus should be on easily digestible, low-fiber sources. This minimizes the risk of stomach upset during the race. Stick to familiar foods you've tested during training runs to avoid any surprises. Your diet should consist of 7–12 grams of carbohydrate per kilogram of body weight per day.

Breakfast: Oatmeal with banana and honey, pancakes with maple syrup, or bagels with jam.
Lunch: White rice with grilled chicken, pasta with marinara sauce, or a baked potato.
Dinner: Spaghetti with low-fat sauce, white rice with lean protein, or homemade pizza with simple toppings.
Snacks: Rice cakes, pretzels, bananas, fruit juice, or sports drinks to help meet your carb goals, especially if you find it hard to eat large volumes of food.

The Role of Hydration and Electrolytes

It's crucial to increase your fluid intake alongside your carbohydrates during the carb-loading phase. For every gram of glycogen your body stores, it also stores about 3-4 grams of water. This increased water retention is a normal and necessary part of the process, and it helps keep you well-hydrated throughout the race. Ensure you are drinking plenty of water and including some electrolytes to aid with fluid balance. Don't be alarmed if you notice a slight weight gain; it's a good sign that your glycogen stores are filling up.

Sample 2-Day Carb-Loading Meal Plan

Below is a sample plan for a runner weighing 70kg (154 lbs) aiming for 700g of carbohydrates per day, based on guidelines from Welsh Athletics.

Friday (2 Days Before Marathon)

Breakfast: Large bowl of cornflakes with 1% milk, 2 slices of raisin toast with jam, 200ml juice.
Snack: Large banana, mixed nuts and raisins.
Lunch: 10-inch panini with ham and tomato, 3 lemon and raisin scotch pancakes, 200ml juice.
Snack: 2 hot cross buns with jam, 200ml juice.
Dinner: Tuna pasta bake with 120g uncooked pasta, sweetcorn, and peas. 2 toasted waffles with syrup, 200ml juice.
Snack: Jam on 2 slices of white toast, hot chocolate with 1% milk.

Saturday (1 Day Before Marathon)

Breakfast: Granola with low-fat yoghurt, large banana, 200ml smoothie.
Snack: 2 raisin bagels with jam, 200ml juice.
Lunch: Sandwich with 3 slices of white bread, chicken, mango chutney, and lettuce. 2 chocolate brioche buns, 200ml juice.
Snack: 4 Jaffa cakes, 200ml juice.
Dinner: Chicken curry (low-fat sauce) with 120g white rice, pitta bread. Low-fat rice pudding with jam, 200ml juice.
Snack: 4 ginger nut biscuits, hot chocolate with 1% milk.

The Pre-Race Breakfast and Race Day Fueling

Your carb load culminates with a light, high-carbohydrate breakfast 2 to 3 hours before the race. This final meal tops off your liver's glycogen stores, which deplete overnight. During the marathon, continue to replenish your fuel with energy gels, chews, or sports drinks. A good target is 30–60 grams of carbohydrates per hour of running. Practicing your fueling strategy during long training runs is essential to figure out what works best for your body.

Conclusion

The optimal timeline to carb load for a marathon is the 2 to 3-day period directly before your race. This modern approach, combined with your training taper, effectively maximizes your glycogen stores and is more practical than outdated methods. By focusing on easily digestible, familiar, carbohydrate-rich foods and staying well-hydrated, you'll be properly fueled to avoid hitting the wall and run your best on race day. For personalized nutrition plans, it is always recommended to consult with a registered dietitian.

Frequently Asked Questions

You should start carb loading 2 to 3 days before your marathon to effectively maximize your glycogen stores for the race.

Yes, starting too early (e.g., a week in advance) is unnecessary and can be counterproductive, potentially leading to gastrointestinal issues without further increasing your glycogen stores.

Focus on easily digestible, lower-fiber carbohydrates like white rice, pasta, potatoes, bagels, and oats. Familiarity is key, so stick with what your stomach is used to from training.

Avoid high-fat foods, excessive protein, and high-fiber foods, especially in the 24-48 hours before the race. These can cause stomach upset and slow digestion.

A small amount of bloating or water weight gain is normal. For every gram of glycogen stored, your body stores extra water, which helps with hydration during the race.

Carb loading is most effective for events lasting over 90 minutes, like a marathon. While it can be beneficial for half marathons, a balanced diet and proper pre-race meal are often sufficient.

No, the classic method involving a glycogen-depleting phase is now considered unnecessary for trained athletes and can cause GI distress. The modern 2-3 day load with tapering is preferred.