Carb Loading: Topping Off for Long Races
Carb loading means eating enough carbohydrate in the last day or two before a long race to fill your muscles' store of carbohydrate, called glycogen, as far as it will go. It is a top-off for races longer than about 90 minutes, and a day of eating, not a pasta dinner. The amounts here are a starting point for a typical runner, and your own long runs revise them. This is general training guidance.
Which races it helps
Carb loading is for races that last longer than about 90 minutes: a marathon, an ultra, and a half marathon that takes you past that. A review of the studies from 1997 found that raising muscle glycogen above normal did nothing for a single effort under 5 minutes and nothing for running or cycling of 60 to 90 minutes, because plenty of glycogen is still in the muscle at the end. In efforts longer than 90 minutes it postponed fatigue by about 20%, and in events where a set distance is covered as fast as possible, high-carbohydrate diets were reported to improve performance by 2 to 3%1.
For a 5K, a 10K or a half marathon under 90 minutes, eat normally and skip the load. If your half marathon takes longer than 90 minutes, make the last day carbohydrate-forward, and you still do not need the giant pasta plate.
How much, and for how long
Coaches differ on how far to take it. Shorenstein, a sports dietitian for recreational and competitive endurance athletes, scales carbohydrate to about 10 to 12 grams per kilogram of body mass a day for a load, raised a slice of toast or a piece of fruit at a time with more fluid. David Roche, who coaches road, trail and ultra runners, does no grams-per-kilogram load in race week, because the taper fills the muscle's store. Part of why a larger load fills more: in 11 endurance-trained adults who spent the final 48 hours of three 5-day blocks on 6, 8 or 10 grams per kilogram a day, 10 produced clearly higher muscle glycogen than either of the others2.
Treat loading as a top-off and not a second fuel tank, and fuel during the race as usual. Let your own long runs revise the numbers.
What to eat
Shift the last day or two toward carbohydrate. Do not just eat more of everything, because the target is carbohydrate and not total food. A load is more carbohydrate than you eat on an ordinary day, so lean on easy sources such as rice, bread, pasta, potatoes, oats, bananas, juice and sports drink, and keep fiber and fat low. In the 2026 trial body mass did not change measurably, and the only stomach symptom that rose was fullness2, so expect to feel full.
Eat familiar foods, and try the two loading days once in training before a long run, because new or very high-fiber foods are a common source of trouble on race morning. The load lands in your taper, when you run less. See the guides on tapering, race-day fueling and carbohydrate during exercise for the race morning meal and the race itself.
Safety
If you have diabetes or another condition that affects how your body handles carbohydrate, check with your doctor before you change what you eat. This guide is not medical advice.
Common questions
Should I carb load for a half marathon?
Not if it takes you under 90 minutes: eat normally. If it takes longer, make the last day carbohydrate-forward and skip the giant pasta plate.
How many grams of carbohydrate should I eat?
Coaches differ. One sports dietitian scales it to about 10 to 12 grams per kilogram of body mass a day for the last day or two, and David Roche does no measured load, because the taper fills the muscle.
Do I need a big pasta dinner the night before?
No. The target is carbohydrate across the last day or two, from familiar foods low in fiber and fat, and not one large meal.
Will carb loading make me gain weight?
In the one 2026 trial that measured it, body mass did not change measurably across 6, 8 and 10 grams per kilogram a day, and fullness was the only stomach symptom that rose.
Sources
- Hawley JA, Schabort EJ, Noakes TD, Dennis SC. Carbohydrate-loading and exercise performance. An update. Sports Medicine 24(2):73-81 (1997). (Review of carbohydrate loading and exercise performance) ↩
- Jones R, Areta JL, Bennett S, Pugh J, Louis J. Dose-Response of Dietary Carbohydrate Intake on Skeletal Muscle Glycogen, Gastrointestinal Comfort and Body Composition in Endurance-Trained Individuals in Simulated Preparation for Competition. Scandinavian Journal of Medicine & Science in Sports 36(6):e70312 (2026). (11 endurance-trained adults (3 women, 8 men), three 5-day blocks with 6, 8 or 10 g of carbohydrate per kg a day in the final 48 hours) ↩
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