Nothing ruins a marathon like hitting the wall at mile 20. Your legs turn to concrete, your pace collapses, and you watch months of training slip away in the final miles. The good news: this is preventable. Carb loading, when done correctly, fills your muscles with the fuel they need to carry you strong across the finish line.
This guide keeps things simple, practical, and easy to follow, even if you have zero nutrition background.
What Is Carb Loading
Carb loading is a strategy where you increase your carbohydrate intake in the days before a marathon so your muscles can store more glycogen. Glycogen is the fuel your body uses during long runs. Think of it like filling a gas tank before a long road trip. When your glycogen tank is full, you can run longer before fatigue hits.
A single plate of pasta the night before is not enough for a marathon. Effective carb loading requires eating extra carbs for a few days so your muscles reach maximum storage. Research shows that starting the race with full glycogen stores improves endurance and delays fatigue. One review found that proper carb loading can improve performance by 2-3% in events where you're racing a set distance as fast as possible (Hawley et al., Sports Medicine, 1997).
When To Start
Start carb loading 3 days before your marathon.
For a Sunday race:
- Start Thursday
- Continue Friday
- Finish Saturday
These are usually your taper days, so your body is training less and can store more energy. Spread your carbs across the full day. Do not wait for a huge pasta dinner the night before.
How Much To Eat (Simple Rule)
You do not need to weigh food or do math here. Keep it simple.
Aim for a lot more carbs than usual, across every meal and snack, for 3 full days.
If you want the science:
- Target about 8 to 12 grams of carbs per kilogram of body weight per day during your carb load
- Example: A 70 kg (~154 lbs) runner aims for roughly 560 to 840 grams of carbs per day
In practice, this looks like:
- Big carb-heavy breakfasts
- Carb-based lunches and dinners
- Several snacks
- Carbs in drinks (juice or sports drinks)
A useful mental shortcut: If your plate is not mostly carbs, it is not a carb loading meal.
What To Eat (Practical Examples)
Choose carbs that are easy to digest and familiar. Think simple, low fiber, low fat.
Great carb loading foods:
- Pasta, rice (not fried), potatoes, bread, bagels, tortillas, pancakes, waffles
- Fruit and fruit juice
- Low fiber cereals
- Pretzels, rice cakes, graham crackers
- Applesauce
- Jam, honey, maple syrup
- Low fat yogurt and milk
- Sports drinks, sweet tea, lemonade
- Energy bars, gels, gummies (as backup)
- Treats like cupcakes, muffins, or pastries (yes, really)
Don't forget liquid carbs. Juices, sports drinks, sweet tea, and even carb powders can add 100 to 200 grams to your day without filling you up. Many runners undereat during carb loading simply because they forget about drinks.
Examples of portion visuals:
- 1 large bagel: about 50 grams of carbs
- 2 cups of cooked pasta: about 80 to 90 grams of carbs
- 1 banana: about 25 to 30 grams of carbs
- 1 bottle of sports drink: about 30 to 40 grams of carbs
What To Avoid
During carb loading, you need to make room for a lot of carbohydrates. That means cutting back on foods that take up stomach space without actually helping you load. These are not bad foods, just the wrong foods for this specific window.
Fats that fill you up without adding carbs:
- Cream, butter, mayonnaise
- Fried foods (including fried rice)
- Heavy sauces and gravies
- Oily dressings
Protein beyond small portions:
- Large servings of meat or fish
- Eggs as a main dish
- Sausages, bacon
- Cheese (more than a light sprinkle)
High fiber foods:
- Salads and raw vegetables
- Beans, lentils, chickpeas
- Heavy whole grain bread
- Too much fruit (even though fruit has carbs, the fiber fills you up fast)
Other things to skip:
- Foods you have not eaten before
- Too much alcohol
- Overeating in one sitting
The goal is simple: fill your plate with carbs, not with foods that compete for stomach space. A small portion of chicken with your pasta is fine. A 12-ounce steak is not.
Sample 3-Day Carb Loading Plan
This sample is for a Sunday race and targets roughly 8-9g of carbs per kilogram of body weight, and the amounts shown are for a 70kg (154lb) runner. Scale portions up or down based on your size, appetite, and what you tolerate.
Thursday (3 Days Out): 550-650g carbs
Breakfast: Oatmeal with banana and honey, plus toast with jam
Snack: Pretzels and dried fruit
Lunch: Turkey sandwich on white bread, applesauce, sweetened iced tea
Snack: Yogurt with granola and a banana
Dinner: Pasta with marinara, roll, small serving of chicken, sports drink
Optional Dessert: Cereal with milk or frozen yogurt
Friday (2 Days Out): 550-650g carbs
Breakfast: Pancakes or waffles with syrup, berries, plus juice
Snack: Smoothie or sports drink with a granola bar
Lunch: Rice bowl with light protein and tortilla, soda or sports drink
Snack: Banana with yogurt and granola
Dinner: Personal size pizza with light cheese, side of pasta or potato
Late Snack: Rice cakes with jam
Saturday (1 Day Out): 400-500g carbs
Saturday is intentionally lighter because you are eating smaller, easier-to-digest meals to avoid GI issues while still topping off glycogen stores.
Breakfast: Two bagels with jam or honey, electrolyte drink
Snack: Applesauce and pretzels
Lunch: Chicken sandwich on a big bun, baked potato, sweet tea
Snack (optional): Gel or gummies
Dinner (early): Medium serving of pasta or rice with light sauce
Bedtime (optional): Half bagel or sports drink
Race Morning Protocol
Your carb load is already done. Race morning is about topping off the tank.
Eat Breakfast 2.5 to 3 Hours Before the Start
Choose foods you have used before. Examples:
- Bagel with jelly
- Oatmeal with banana
- Energy bar and juice
Aim for something like 200 to 400 calories of mostly carbs.
Drink With Breakfast
Water plus a sports drink is perfect. Coffee is fine if you normally drink it.
Optional Small Snack 30 to 45 Minutes Before the Start
Only if you feel hungry:
- One gel
- A few chews
- Half a banana
During the Race
Your carb load is done — now it's about maintaining energy and hydration as you go.
Fueling strategy:
- Take your first gel around 5K (or 25-30 minutes in) — don't wait until you feel tired
- Continue taking one gel every 5K through 35K
- After 35K, you likely don't need more fuel — your body is burning what's already in the system
Caffeine gels:
- Save caffeinated gels for the second half of the race (25K onwards) when you need a mental boost
- Warning: Never use a caffeinated gel on race day if you haven't tested it in training. Some runners experience jitters, GI distress, or heart pounding.
Hydration and electrolytes:
- Take water with your gels, not sports drink — doubling up on sugar can cause stomach issues
- If it's hot or you're a heavy sweater, consider electrolyte tabs or salt caps every 45-60 minutes
- Practice your hydration strategy in training — know how much you need before race day
The golden rule: Nothing new on race day. Your fueling plan should be tested and proven in your long runs.
Carb Loading Calculator
The Science Behind the Calculator
If you want to understand the logic that powers the calculator above.
Core formula:
Carb intake = 8 to 12 grams of carbs per kilogram of body weight per day
Variables the calculator uses:
- Body weight
- Carb targets (low, medium, high within the 8 to 12 g/kg range)
Examples:
- 60 kg runner at 8 g/kg = 480 grams per day
- 80 kg runner at 10 g/kg = 800 grams per day
Important science notes:
- No depletion phase needed
- Tapering is assumed
- Carb loading works for both men and women
- If total carbs per day exceed 800 to 900 grams, better to spread across 3 days instead of 2
Frequently Asked Questions
Do I need to deplete my glycogen first?
No. The old-school "depletion phase" (running hard while eating low carb) is outdated. Research shows you get the same glycogen storage benefits from 3 days of high-carb eating without the depletion phase, and you avoid the fatigue and immune suppression that comes with it.
Does carb loading work for women?
Yes. Earlier studies suggested women responded less to carb loading, but those studies used protocols that did not provide enough total calories. When women eat sufficient carbs (8-10g/kg), they see similar glycogen increases as men.
Can I carb load for a half marathon?
It is less critical but still helpful. For races under 90 minutes, your normal glycogen stores are usually enough. For half marathons taking 90+ minutes, a lighter 2-day carb load can provide a small performance boost.
What if I gain weight during carb loading?
This is normal and expected. For every gram of glycogen stored, your body holds about 3 grams of water. A 2-4 lb increase is a sign the loading is working, and that extra fuel will be burned during the race.
What's Next
Nail your carb loading, then make sure the rest of your race week is dialed in. Check out our Race Day Checklist so nothing catches you off guard on marathon morning.