Marathon Carb Loading: How Much, When to Start, What to Avoid

Glycogen stored in your muscles and liver is the fuel that runs out first in a marathon. Carb loading exists to start the race with that tank full. The current prescription is short: 10 to 12 g of carbohydrate per kilogram of body weight per 24 hours, for the 36 to 48 hours before the start. That is the guidance of the joint position statement of the American College of Sports Medicine, the Academy of Nutrition and Dietetics and Dietitians of Canada (Thomas, Erdman and Burke, 2016).

Few runners get there. Among 50 endurance athletes racing events of 2.5 hours or longer, only 10% reached the 10 to 12 g/kg range the day before, and even those who could recite the guideline averaged 6.1 g/kg (Sampson, Morton and Areta, 2024). The tables below turn the guideline into grams, plates and clock times for runners of 132, 154 and 176 lb (60, 70 and 80 kg).

Key takeaways

• Carb loading pays off for events longer than 90 minutes: the marathon, and the half marathon if you finish over 1:30.
• Target: 10 to 12 g of carbohydrate per kg of body weight per day for 36 to 48 hours. For 154 lb (70 kg), that is 700 to 840 g a day. There is no depletion phase.
• Cut fiber, fat and protein on those days so the carbohydrate fits and your gut stays quiet. The scale goes up because of glycogen and water, not fat.
• Race-morning breakfast: 1 to 4 g/kg, 1 to 4 hours before the gun. During the marathon: 60 to 90 g per hour from glucose plus fructose, all rehearsed on long runs.
• Diabetes, bowel disease or a restricted diet calls for a plan built with a registered dietitian or physician.

What carb loading is and when it matters

Carb loading is the strategy of pushing muscle glycogen above its everyday level in the days before a race. Table 1 of the 2016 position statement separates two cases: events shorter than 90 minutes call for 7 to 12 g/kg in the preceding 24 hours, the same as a normal hard-training day. Events longer than 90 minutes of sustained or intermittent exercise call for 10 to 12 g/kg per 24 hours for 36 to 48 hours (Thomas et al., 2016). The German Nutrition Society's sports nutrition working group lands on the same numbers and the same 90-minute cutoff (König et al., 2020). The marathon qualifies. So does the half marathon for anyone finishing north of 1:30. A 5K or 10K does not.

The old protocol, three days of depletion followed by three days of loading, has been retired. Eight trained athletes who ate 10 g/kg of high-glycemic carbohydrate while resting reached maximal muscle glycogen within 24 hours, from roughly 95 to 180 mmol per kg of muscle, with no depletion phase and no further gain over the next two days (Bussau et al., 2002). A review of marathon nutrition confirms that starting with high glycogen does not require the classic supercompensation protocol (Jeukendrup, 2011). A 36 to 48 hour window on top of the taper's reduced training is enough.

How much to eat: 10 to 12 g/kg per day for 36 to 48 hours

The math is per kilogram of body weight, per day. For a Sunday-morning start, the window covers all of Saturday plus Friday from lunch onward.

A runner who eats 500 g of carbohydrate in a day feels like they overdid it; at 154 lb (70 kg), that is still only 7 g/kg. The 7 g/kg column comes from an observational study of 257 runners at the 2009 London Marathon: those who ate more than 7 g/kg the day before ran faster and held their pace better than those who ate less (Atkinson et al., 2011). It is correlation, not proof of cause, but it points the same way as the guideline.

Outside the window, the same Table 1 sets the rest of taper week by training load: 5 to 7 g/kg per day for about an hour of daily exercise, 6 to 10 g/kg for one to three hours (Thomas et al., 2016). Since volume drops in marathon week, a full week at 10 g/kg adds energy without adding glycogen.

A sample day with 700 g of carbohydrate (154 lb / 70 kg runner)

Seven hundred grams becomes a mountain of food if it all comes from rice and bread. Juice, honey and simple sweets earn their place because they deliver carbohydrate with little bulk and little fiber, the kind of choice the position statement suggests for hitting the target while keeping the gut comfortable (Thomas et al., 2016).

Values come from USDA FoodData Central (SR Legacy), in grams of carbohydrate per 100 g: plain bagel 52.4; honey 82.4; raw orange juice 10.4; banana 22.8; hard pretzels 80.4; cooked long-grain white rice 28.2; baked potato with skin 21.2; white bread 49.4; dry pasta 74.7.

For 132 lb (60 kg, 600 g), drop the evening snack, the mid-morning juice and one slice of bread at lunch, about 100 g. For 176 lb (80 kg, 800 g), add 1 cup of rice at lunch (45 g), 2 oz more dry spaghetti at dinner (43 g) and a tablespoon of honey (17 g), about 105 g. Notice what the day leaves out: beans, whole grains, big raw salads, nuts, peanut butter. Protein shows up in small portions, fat is incidental, and water goes with every meal.

Fiber, fat and protein: what to adjust

On the pre-race meal, the position statement says choices high in fat, protein or fiber "may need to be avoided to reduce risk of gastrointestinal issues during the event" (Thomas et al., 2016). During loading, sources low in fiber and residue help athletes hit the target and meet goals for gut comfort or a lighter "racing weight." Restricting fat, the same text says, only makes sense in those acute scenarios, never as a chronic habit.

A 2025 review puts that advice in perspective: a low-residue diet for 48 to 72 hours is common practice in loading protocols and reduces the bulk of gut contents. The evidence, though, is level 7, built on clinical experience and athlete testimonials, with no randomized trials (Mancin, Burke and Rollo, 2025). If your gut has never complained, trimming fiber the day before is usually enough; with a history of cramping, urgency or diarrhea in races, start two days out.

The swaps: brown rice becomes white, whole-wheat bread becomes white, beans and lentils come off the plate or shrink to a spoonful, raw salad becomes a small portion of cooked vegetables, fruit with skin becomes banana and strained juice. Meat, eggs and cheese stay, in portions that do not crowd out the carbohydrate. The traps are fried food, cream sauces and cheese-heavy pizza: they look like "pasta night," but much of the energy is fat.

The scale will go up, and that is a good sign

Each gram of glycogen is stored in muscle with at least 3 g of water. In trained cyclists who replaced carbohydrate without replacing all the fluid they had lost, the measured ratio was 1 g of glycogen to 3 g of water, and it climbed higher with full fluid replacement (Fernández-Elías et al., 2015). The extra pounds on race morning are that package of fuel and water, not fat, and they leave as the glycogen is burned. That is why the 2025 review lists offsetting the added body mass among the reasons for a low-residue diet (Mancin, Burke and Rollo, 2025). Do not try to shed that weight by cutting fluid or food: it is what you want to carry to the start line.

The race-morning breakfast: 1 to 4 g/kg, 1 to 4 hours before

For exercise longer than 60 minutes, the guideline is 1 to 4 g of carbohydrate per kg, eaten 1 to 4 hours before the start, with amount, type and timing adjusted to the event and your own experience (Thomas et al., 2016). The more time before the gun, the bigger the meal; the closer to the start, the smaller and more liquid.

A 2 g/kg example for 154 lb (70 kg), three hours before: 1 plain bagel (52 g of carbohydrate) with 2 tbsp honey (35 g), 1 medium banana (27 g) and 8 fl oz orange juice (25 g), 139 g in total. Low in fiber, low in fat and with almost no protein. Runners who cannot take anything in during the race may prefer lower-glycemic choices, which the position statement describes as a more sustained fuel source in that situation. Whatever you pick needs to have been tested before long runs.

Carbohydrate during the race: 30 to 90 g per hour

The guideline scales with duration: for 1 to 2.5 hours, 30 to 60 g per hour; beyond 2.5 to 3 hours, up to 90 g per hour, using products that combine multiple transportable carbohydrates, meaning glucose (or maltodextrin) plus fructose (Thomas et al., 2016). A 2011 review had already set 30 to 60 g per hour as the appropriate target for longer events (Burke et al., 2011). Absorption explains the ceiling: a single carbohydrate source is oxidized at up to roughly 60 g per hour, and only the combination with fructose gets near 90 g per hour. That is also why the advice is in grams per hour rather than per kg: the limit sits in the gut and does not track body weight (Jeukendrup, 2014).

For a half marathon between 1:30 and 2:30, aim for 30 to 60 g per hour. For the marathon, 60 to 90 g per hour: in a four-hour race, 60 g per hour adds up to 240 g. Gels, sports drink, chews and bananas all work; the amount per serving varies by product, so read the label and add up grams instead of counting packets. At low intensity, carbohydrate oxidation is lower too, and the bottom half of the range is usually enough (Jeukendrup, 2014).

Train your gut on long runs

Between 30% and 50% of athletes report gastrointestinal problems regularly (Jeukendrup, 2017). The gut responds to training: the capacity to absorb carbohydrate rose in cyclists who spent 28 days at 8.5 g/kg per day versus 5.3 g/kg in the control group, and stomach comfort improved in runners who repeated sessions with high fluid intake. The review's practical conclusion: practice the race strategy in training, include high-carbohydrate days and take in carbohydrate during exercise regularly. Table 1 of the position statement says the same thing: the athlete should practice until they find a refueling plan that meets their goals, including hydration and gut comfort.

In the last four weeks, use your long runs as the lab: the same brand of gel, the same grams per hour, the same breakfast at the same hour. One long run deserves the full dress rehearsal, with 36 hours of loading beforehand. Runners on low-carb or ketogenic diets need extra care: those diets may reduce the capacity to absorb carbohydrate, and the review suggests building in high-carbohydrate days before competing (Jeukendrup, 2017).

Common mistakes

Eating "more of everything" instead of more carbohydrate. Lasagna, burgers and pizza add calories, but fat and protein take up the room the carbohydrate needs. The target is grams of carbohydrate, not full plates.

Loading only at the night-before pasta dinner. The pasta party covers one meal; the guideline covers 36 to 48 hours. A single dinner does not deliver 700 g.

Staying under target for fear of the scale. Among the 50 athletes assessed in 2024, 90% stayed below the recommended range (Sampson, Morton and Areta, 2024); in London, runners who ate less than 7 g/kg lost more pace (Atkinson et al., 2011). The extra weight is glycogen and water.

Keeping fiber high and debuting new food. Whole-grain toast, granola and a big salad the day before, plus an unfamiliar gel at mile 18, are the classic ingredients of a porta-potty stop.

Skipping breakfast because of nerves. A closed stomach is normal; the fix is to eat earlier and go more liquid, not to fast. Juice, a bagel with honey and a banana three hours out already cover 2 g/kg.

When to see a professional

Type 1 or type 2 diabetes, inflammatory bowel disease, irritable bowel syndrome, celiac disease, a history of disordered eating, or a strict vegetarian, low-carb or medically restricted diet changes the math: these amounts alter blood glucose, medication doses and symptoms, and need individual adjustment. The position statement itself recommends referring athletes to a registered dietitian for a personalized plan (Thomas et al., 2016). The same goes for anyone who has had a serious episode of GI distress in a race: a sports dietitian or sports medicine physician can build the load around your history.

Frequently asked questions

Do I need to carb load for a half marathon?

It depends on your finish time. The guideline's cutoff is 90 minutes of exercise: above it, 10 to 12 g/kg for 36 to 48 hours; below it, 7 to 12 g/kg in the preceding 24 hours is enough (Thomas et al., 2016). A 1:45 or 2:00 half marathoner is in the first group.

Can I carb load on food alone, without gels or maltodextrin?

For the load itself, yes: rice, bread, pasta, potatoes, bananas, honey and juice deliver the grams with no supplements, as the sample day shows. During the race, the limit is practical: carrying 60 to 90 g per hour as bananas and bread is hard, and the glucose-plus-fructose blends in gels and sports drinks exist to get around the absorption ceiling of a single carbohydrate source (Jeukendrup, 2014).

Wrapping up

Carb loading is arithmetic: 10 to 12 g per kg per day for the 36 to 48 hours before the race, with less fiber and fat. Then 1 to 4 g/kg in the 1 to 4 hours before the start, and 60 to 90 g per hour during the marathon, all rehearsed on long runs. Done that way, it brings to the start line the stores your marathon plan built over months. Afterward, the work continues in recovery, and everyday nutrition for running takes over again.

References

• Thomas DT, Erdman KA, Burke LM. Nutrition and Athletic Performance. Joint Position Statement of the Academy of Nutrition and Dietetics, Dietitians of Canada and the American College of Sports Medicine. Medicine & Science in Sports & Exercise. 2016;48(3):543–568. doi:10.1249/MSS.0000000000000852. Full text PDF (Dietitians of Canada).
• Burke LM, Hawley JA, Wong SH, Jeukendrup AE. Carbohydrates for training and competition. Journal of Sports Sciences. 2011;29(Suppl 1):S17–S27. doi:10.1080/02640414.2011.585473.
• Jeukendrup AE. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine. 2014;44(Suppl 1):25–33. doi:10.1007/s40279-014-0148-z.
• Jeukendrup AE. Training the gut for athletes. Sports Medicine. 2017;47(Suppl 1):101–110. doi:10.1007/s40279-017-0690-6.
• Jeukendrup AE. Nutrition for endurance sports: marathon, triathlon, and road cycling. Journal of Sports Sciences. 2011;29(Suppl 1):S91–S99. doi:10.1080/02640414.2011.610348.
• Bussau VA, Fairchild TJ, Rao A, Steele P, Fournier PA. Carbohydrate loading in human muscle: an improved 1 day protocol. European Journal of Applied Physiology. 2002;87(3):290–295. doi:10.1007/s00421-002-0621-5.
• Atkinson G, Taylor CE, Morgan N, Ormond LR, Wallis GA. Pre-race dietary carbohydrate intake can independently influence sub-elite marathon running performance. International Journal of Sports Medicine. 2011;32(8):611–617. doi:10.1055/s-0031-1273739.
• Fernández-Elías VE, Ortega JF, Nelson RK, Mora-Rodriguez R. Relationship between muscle water and glycogen recovery after prolonged exercise in the heat in humans. European Journal of Applied Physiology. 2015;115(9):1919–1926. doi:10.1007/s00421-015-3175-z.
• Sampson G, Morton JP, Areta JL. A broken link: knowledge of carbohydrate requirements do not predict carbohydrate intake around competition in endurance athletes. European Journal of Sport Science. 2024;24(10):1395–1404. doi:10.1002/ejsc.12183.
• Mancin L, Burke LM, Rollo I. Fibre: the forgotten carbohydrate in sports nutrition recommendations. Sports Medicine. 2025;55(5):1067–1083. doi:10.1007/s40279-024-02167-1.
• König D et al. Position of the working group sports nutrition of the German Nutrition Society (DGE): carbohydrates in sports nutrition. Deutsche Zeitschrift für Sportmedizin. 2020;71(7-8-9):185–191. doi:10.5960/dzsm.2020.456.
• U.S. Department of Agriculture, Agricultural Research Service. FoodData Central, SR Legacy. fdc.nal.usda.gov.