Carb loading for a marathon means eating 10 to 12 grams of carbohydrates per kilogram of body weight per day for the 36 to 48 hours before race day. For a 70-kilogram (154-pound) runner, that works out to roughly 700 to 840 grams of carbohydrates daily, or about 2,800 to 3,360 calories from carbs alone. The goal is to pack your muscles with as much stored fuel as possible before you hit the start line.
Why Glycogen Matters at Mile 20
Your body stores carbohydrates as glycogen in your muscles and liver. These stores are surprisingly small: less than 3,000 calories total, even when fully topped off. Compare that to fat stores, which hold over 100,000 calories in a lean person. The problem is that running a marathon at a moderate to high intensity burns through glycogen far faster than fat, and when those stores run dry, your pace collapses. That’s the sensation runners call “hitting the wall,” typically somewhere around mile 18 to 22.
Carb loading triggers a process called supercompensation. After your taper reduces training volume, your muscles are primed to absorb and store more glycogen than they normally would. Flooding your system with carbohydrates during this window pushes glycogen levels above their baseline, giving you a larger fuel tank on race morning. Each gram of glycogen also pulls about 3 grams of water into storage with it, which means a fully loaded runner can expect to gain 2 to 4 pounds of water weight. That’s normal, temporary, and a sign the process is working.
The Loading Timeline
Start your carb load 36 to 48 hours before the race. If your marathon is on Sunday morning, that means shifting to a high-carb diet by Friday evening at the latest. Some runners stretch this to three days, but the research supports 36 to 48 hours as sufficient for full glycogen supercompensation.
For the two to three days before the race, focus on simple, easily digestible carbohydrates: plain pasta, white rice, white bread, potatoes, and similar low-fiber foods. This is not the time to eat whole grains, big salads, or anything heavy in fiber. Your goal is to get a high volume of carbs into your system without taxing your gut.
Your final large meal should be the night before the race, not race morning. Eating a huge plate of pasta minutes before the gun goes off is a recipe for stomach cramps and regret. On race morning, a smaller carb-rich meal two to three hours before the start (a bagel with jam, a banana, some oatmeal) tops off liver glycogen, which depletes overnight while you sleep.
What 10 to 12 Grams Per Kilogram Actually Looks Like
The numbers sound abstract until you translate them into food. For a 68-kilogram (150-pound) runner targeting 10 grams per kilogram, you need about 680 grams of carbohydrates across the day. Here’s a rough sense of scale:
- One cup of cooked white rice has about 45 grams of carbs
- One cup of cooked pasta has about 43 grams
- One large bagel has about 50 to 60 grams
- One medium banana has about 27 grams
- One large baked potato has about 60 grams
That means you’d need to eat the equivalent of 15 cups of rice in a single day to hit the target. Most runners find this surprisingly difficult. Spreading your intake across five or six smaller meals works better than trying to stuff it all into three. Sports drinks, juice, pretzels, crackers, pancakes with syrup, and even white bread with honey can help you reach the total without feeling painfully full. Cut back on fat and protein slightly during these days, not because they’re harmful, but because they take up stomach space you need for carbs.
How Much Faster Can It Make You
The performance payoff is real, though it varies by fitness level. In a classic study of a 30-kilometer cross-country race, runners improved their times by an average of 5.4% after carb loading. A more detailed treadmill study of a 25-kilometer time trial found that less experienced runners saw a 7.6% improvement, while experienced runners gained about 3.2%. Runners with higher aerobic fitness saw smaller (and sometimes statistically insignificant) gains, likely because their bodies are already more efficient at using fat as fuel. For a four-hour marathoner, even a 3% improvement translates to roughly seven minutes off the finish time.
Carb Loading for Female Runners
Early carb loading research was done almost exclusively on men, and for years there was a belief that the strategy didn’t work as well for women. The reality is more nuanced. Menstrual cycle phase does affect baseline glycogen storage: levels tend to be naturally higher in the luteal phase (the two weeks after ovulation) and lower in the follicular phase (the two weeks after your period starts).
However, research published in the International Journal of Sport Nutrition and Exercise Metabolism showed that women who consumed at least 8 grams of carbohydrate per kilogram of body weight per day could overcome that follicular-phase dip and achieve full glycogen supercompensation regardless of cycle timing. The catch is that many of the older studies used lower carb intakes (around 6 grams per kilogram), which simply weren’t enough. Women should aim for the same 10 to 12 grams per kilogram target. Because women tend to have lower body weight, the absolute number of grams is smaller, but the relative effort of eating that volume can still feel like a lot.
Foods to Avoid During Loading
The 48 hours before a marathon is not the time to experiment. Stick with foods you know your stomach tolerates well. Johns Hopkins Medicine specifically flags three categories to go easy on before a big race:
- High-fiber foods like beans, lentils, raw vegetables, and whole-grain bread, which can cause gas and cramping
- Spicy or high-fat foods that increase the risk of GI upset
- Caffeine in unfamiliar amounts, which can stimulate the GI tract and cause urgent bathroom stops mid-race
Your best bets are bland, white, refined carbohydrates. White bread, white pasta, white rice, potatoes without the skin, bananas, applesauce, pretzels, and low-fiber cereals. These foods are absorbed quickly and leave minimal residue in your digestive system. It sounds counterintuitive if you normally eat a high-fiber diet, but this is a short-term fueling strategy, not a long-term nutrition plan.
Common Mistakes
The most frequent carb loading error is simply not eating enough carbohydrates. Runners often think they’re loading when they’re really just eating a normal dinner with extra bread. If you’re not tracking your gram totals, at least roughly, you’re likely falling short. Use a food tracking app for those two days to make sure you’re in the 10 to 12 grams per kilogram range.
Another common mistake is adding carbs on top of your regular diet instead of replacing some fat and protein. Carb loading isn’t about eating more total calories than your body can handle. It’s about shifting the ratio so that 70 to 80% of your calories come from carbohydrates. If you simply pile extra food on top of your normal intake, you’ll feel bloated and sluggish on race morning.
Finally, don’t try carb loading for the first time before your goal race. Practice during a long training run or a tune-up race. Some runners find that certain foods cause bloating or discomfort at these volumes, and you want to discover that during training, not at mile 6 of a marathon you’ve spent months preparing for.