For the two to three days before a marathon, most runners should aim for 10 to 12 grams of carbohydrates per kilogram of body weight per day. For a 70-kilogram (154-pound) runner, that works out to roughly 700 to 840 grams of carbs daily. On race morning, the target drops to 1 to 4 grams per kilogram in the one to four hours before the starting gun.
Those numbers sound enormous, and they are compared to a normal diet. But carb loading isn’t about one giant pasta dinner. It’s a multi-day strategy that fills your muscles with stored fuel, and doing it right can shave meaningful minutes off your finish time.
Why Carb Loading Works
Your muscles store carbohydrates as glycogen, which is the primary fuel source when you’re running at marathon pace. A typical runner’s glycogen stores can power roughly 90 minutes of sustained effort. Since most people take well over 90 minutes to finish a marathon, topping off those stores beforehand delays the point where your body runs low and performance starts to crater.
Research on carb loading and endurance performance shows consistent benefits. In one well-known study, runners improved their 30-kilometer race times by an average of 5.4% after carb loading. Less experienced runners saw even bigger gains (7.6%), while highly trained runners still improved by about 3.2%. A separate trial found a 4.2% improvement in runners with moderate fitness levels. For a four-hour marathoner, a 4% improvement translates to roughly 10 minutes faster.
The Loading Phase: 2 to 3 Days Out
The loading window runs from about 72 hours before the race through the evening before race day. During this stretch, you’re replacing most of your normal diet with carbohydrates, aiming for that 10 to 12 grams per kilogram target. This doesn’t mean eating double your usual calories. You’re shifting the ratio so that carbs make up roughly 85 to 95% of what’s on your plate, while reducing fat and protein.
Here’s what that looks like in practice for different body weights:
- 55 kg (121 lbs): 550 to 660 grams of carbs per day
- 70 kg (154 lbs): 700 to 840 grams of carbs per day
- 85 kg (187 lbs): 850 to 1,020 grams of carbs per day
Spreading this across five or six meals and snacks makes it more manageable than trying to cram it into three sittings. White rice, white bread, pasta, bagels, pretzels, pancakes, fruit juice, and sports drinks are all reliable options. The key is choosing foods that are easily digestible and low in fiber. This is the one time you actively want refined carbs over whole grains. Brown rice, whole wheat bread, and high-fiber cereals are harder to digest in large quantities and increase the risk of bloating or stomach trouble on race day.
Tapering Makes It Work
Carb loading pairs with your training taper. In the final days before the race, you’re running less (or not at all), which means your muscles aren’t burning through glycogen the way they normally would. That reduced demand, combined with high carb intake, allows your body to pack glycogen stores well above their normal capacity. This “supercompensation” effect is what gives you the extra fuel on race morning. Without the taper, simply eating more carbs won’t have the same impact because your training sessions would keep drawing down those stores.
The Pre-Race Meal
Your final meal before the marathon serves as a top-off, not a replacement for the loading phase. Eat it one to four hours before the start, targeting 1 to 4 grams of carbs per kilogram of body weight. The closer you eat to the gun, the smaller this meal should be. Three hours out, a 70-kilogram runner might eat 200 to 280 grams of carbs. One hour out, something lighter like a banana and a sports drink is more realistic.
Stick with foods you’ve tested during training. A plain bagel with jam, oatmeal with honey, or a bowl of white rice are common choices. Avoid anything high in fat, fiber, or protein, which slow digestion and can cause cramping mid-race.
Carb Loading Differs for Women
Most carb-loading research was originally conducted on men, and it turns out the standard recommendations don’t translate directly to female runners. The hormone estradiol, which fluctuates across the menstrual cycle, appears to shift the body toward burning fat instead of glycogen during endurance exercise. This has a real impact on how women respond to carb loading.
In one study where both men and women increased their carb intake (men to about 8 grams per kilogram, women to about 6.4 grams per kilogram) while keeping total calories the same, men boosted their glycogen stores by 41% and improved performance significantly. Women, however, showed no increase in glycogen storage and only a modest 5% performance gain. The carbs simply weren’t being stored the same way.
A follow-up study changed the approach. When both men and women consumed 12 grams of carbs per kilogram of fat-free body mass and women increased their total calorie intake by about 34% during the loading phase, both sexes achieved similar levels of glycogen supercompensation and similar performance improvements. The takeaway for female runners is that you likely need to increase not just the proportion of carbs but also your overall calorie intake during those final days. Aiming for 10 to 12 grams per kilogram while boosting total calories by roughly a third appears necessary to get the same loading benefit.
Common Mistakes to Avoid
The most frequent carb-loading error is waiting until the night before the race to have one oversized pasta dinner. A single large meal can’t accomplish what two to three days of sustained high-carb eating can. You’ll just end up bloated and uncomfortable at the start line with glycogen stores that are only partially filled.
Another common mistake is loading up on high-fiber or high-fat carb sources. Whole grain bread, beans, and broccoli are great on a normal training day, but during the loading phase they increase the odds of gastrointestinal distress. Opt for white, refined, easily digestible options instead.
Some runners also make the mistake of dramatically increasing total calories rather than shifting the ratio. Unless you’re a female athlete who needs that caloric bump for effective loading, the goal is to replace protein and fat calories with carbohydrate calories, not to pile food on top of your existing diet. A modest caloric increase is fine, but doubling your intake just leads to discomfort and sluggishness.
Finally, don’t try carb loading for the first time on race week. Practice it before a long training run so you know which foods sit well, how your stomach handles the volume, and how your energy feels on run day.