Carb Loading: How to Do It Right Before a Race
2026-06-06 · 8 min read
Rapid answer
To carb load, eat 10 to 12 grams of carbohydrate per kilogram of body weight per day for the 1 to 3 days before a race lasting longer than 90 minutes, while tapering your training. You do not need the old depletion phase. Expect to gain 1 to 2 kg of water weight; that is stored fuel, not fat, and it is the point.
Carb loading is one of the few race-day tactics with decades of solid science behind it, and also one of the most commonly botched. Done right, it tops off your muscle and liver glycogen so you reach the wall later or not at all. Done wrong, it leaves you heavy, bloated, and no better fueled than if you had eaten normally. This guide covers what carb loading actually does, the modern protocol that has replaced the miserable old one, exactly how much to eat, and when to start.
What is carb loading?
Carb loading is the practice of eating a very high carbohydrate diet for one to three days before an endurance event to maximize the glycogen stored in your muscles and liver. A trained athlete normally stores around 400 to 500 grams of glycogen; loading can push muscle glycogen to roughly 150 to 200 percent of its normal level, extending the time before you deplete and hit the wall. It works because glycogen, not fat, is the fuel that limits hard endurance efforts, and your tank for it is finite.
The technique was discovered in the late 1960s by Scandinavian physiologists using muscle biopsies, and it remains a cornerstone of endurance race prep today. The science has not changed; the protocol has.
Why do athletes carb load?
Athletes carb load because muscle glycogen is the rate-limiting fuel for sustained efforts above about 65 percent of VO2 max, and running out of it is what causes the marathon "wall" around mile 20. A normal glycogen store lasts roughly 90 to 120 minutes of hard exercise. Loading buys you extra minutes at race pace before you are forced to slow down. For anyone racing longer than about 90 minutes, that margin is the difference between holding pace and surviving.
This is also why carb loading is pointless for short races. A 5K or 10K finishes long before you would deplete a normal glycogen store, so loading just adds dead weight.
The modern carb loading protocol (no depletion needed)
The current evidence-based protocol is simple: eat 10 to 12 grams of carbohydrate per kilogram of body weight per day for one to three days before the race while tapering your training to almost nothing. The depletion-and-deprivation phase from the original 1960s method is no longer recommended; modern studies show you reach the same maximal glycogen without it. Tapering is what frees up the glycogen, so the less you train during the load, the more carbohydrate stays banked.
Here is how the three main approaches compare:
| Protocol | Depletion phase | Loading window | Daily carb target | Best for |
|---|---|---|---|---|
| Classic (Bergstrom, 1967) | Yes, 3 days low-carb + hard training | 3 days | 8 to 10 g/kg | Nobody, now outdated |
| Modified (Sherman, 1981) | No | 3 days, taper | 8 to 10 g/kg | Most marathoners |
| One-day (Bussau, 2002) | No | 24 to 36 hours, rest | 10 to 12 g/kg | Time-crunched, well-trained |
The one-day finding is the practical headline: Bussau and colleagues showed that 10 g/kg for a single day, combined with rest, maxed out muscle glycogen just as well as the multi-day methods. You do not need to ruin a whole week.
How many carbs do you need to carb load?
Aim for 10 to 12 grams of carbohydrate per kilogram of body weight per day, which is far more than most athletes guess. A 70 kg runner needs 700 to 840 grams of carbohydrate per day, the equivalent of roughly 12 to 14 cups of cooked rice. Hitting this almost always requires deliberately replacing fat and protein on your plate with carbohydrate, plus liquid carbs like juice and sports drink.
| Body weight | Daily carb target (10 g/kg) | Daily carb target (12 g/kg) |
|---|---|---|
| 55 kg (121 lb) | 550 g | 660 g |
| 60 kg (132 lb) | 600 g | 720 g |
| 70 kg (154 lb) | 700 g | 840 g |
| 80 kg (176 lb) | 800 g | 960 g |
| 90 kg (198 lb) | 900 g | 1,080 g |
The carb loading calculator personalizes this to your weight, the number of loading days, and your race, and the 36-hour carb loading food list shows exactly which low-residue foods hit the target without GI distress.
When should you start carb loading?
Start one to three days out, not a week. For a Sunday marathon, begin the high-carb intake on Friday (a two-day load) or Saturday (a one-day load), and keep the day-before food low in fiber and fat so your gut is light at the start line. Carbohydrate eaten earlier than three days out is simply stored or burned as normal and does nothing to raise race-day glycogen. The taper matters as much as the food: heavy training during the load burns the glycogen you are trying to bank.
A typical timeline for a 70 kg athlete:
| When | Training | Carb focus |
|---|---|---|
| 3 days out | Short and easy | Normal-to-high carb, normal fiber |
| 2 days out | Very short shakeout or rest | Begin 10 g/kg, start cutting fiber |
| 1 day out | Rest | Full 10 to 12 g/kg, low fiber and low fat |
| Race morning | Warm-up only | 1 to 4 g/kg, 3 to 4 hours before start |
The race-morning meal is its own decision; see what to eat the morning of a race for the timed breakdown.
Why do you gain weight when carb loading?
Gaining 1 to 2 kg (2 to 4 lb) during a carb load is normal and expected: your body stores roughly 3 grams of water with every gram of glycogen. That extra weight is fuel and hydration, not fat, and most of it is used up during the race. Athletes who panic and cut carbs to avoid the scale change arrive at the start line under-fueled. If you load correctly you should feel slightly heavy and well-hydrated the morning of the race. That is the signal it worked.
Common carb loading mistakes
The biggest mistakes are loading for too short an event, not eating nearly enough carbohydrate, adding calories instead of swapping them, and overdoing fiber the day before. Each one either wastes the effort or leaves you bloated at the start. Carb loading is a precise tactic, not just "eat a big pasta dinner."
- Loading for a short race. Below about 90 minutes there is no benefit; you just carry extra weight.
- Undershooting the target. Most athletes eat 4 to 6 g/kg and think they loaded. You need 10 to 12.
- Adding carbs on top of a normal diet. Replace the fat and protein with carbohydrate; do not just pile more food on, or you will be uncomfortably full.
- High fiber the day before. Whole grains, beans, and raw vegetables add gut bulk. Switch to white rice, white bread, and other low-residue carbs in the final 24 hours.
- Trying new foods. Race week is not the time to experiment. Practice your load in training before a long run.
Once your tank is full, the next problem is keeping it topped during the race itself; see how many carbs per hour to eat while racing.
Frequently asked questions
What is carb loading?
Carb loading is eating a very high carbohydrate diet for one to three days before a long race to maximize muscle and liver glycogen. It raises your stored fuel above normal so you can hold race pace longer before depleting and hitting the wall.
How do you carb load properly?
Eat 10 to 12 grams of carbohydrate per kilogram of body weight per day for one to three days before the race, while tapering your training. Replace fat and protein on your plate with carbs, lean on low-fiber foods the final day, and skip the outdated depletion phase.
When should you start carb loading?
One to three days before the event, not a week. For a Sunday race, start Friday or Saturday. Carbohydrate eaten earlier than three days out does not raise race-day glycogen, and the training taper during those days is what lets the glycogen accumulate.
How do you carb load for a marathon?
For a marathon, target 10 to 12 g/kg per day for the two days before, taper to rest the day before, and keep the final 24 hours low in fiber and fat. A 70 kg runner needs about 700 to 840 g of carbohydrate per day. The marathon is the textbook case where loading pays off.
How do you carb load for a half marathon?
Most runners finish a half marathon in 90 minutes to 2 hours, right at the threshold where loading starts to help. A lighter one-day load of about 8 to 10 g/kg the day before is plenty; a full multi-day load is overkill for the distance.
Do you gain weight from carb loading?
Yes, typically 1 to 2 kg, because your body stores about 3 grams of water per gram of glycogen. This is stored fuel and water, not fat, and it is burned off during the race. Feeling slightly heavy and well-hydrated on race morning means the load worked.
Sources
- Bergstrom J, Hermansen L, Hultman E, Saltin B. Diet, Muscle Glycogen and Physical Performance. Acta Physiol Scand. 1967;71(2):140-150. doi.org/10.1111/j.1748-1716.1967.tb03720.x
- Sherman WM, Costill DL, Fink WJ, Miller JM. Effect of Exercise-Diet Manipulation on Muscle Glycogen and Its Subsequent Utilization During Performance. Int J Sports Med. 1981;2(2):114-118. doi.org/10.1055/s-2008-1034594
- Bussau VA, Fairchild TJ, Rao A, Steele P, Fournier PA. Carbohydrate Loading in Human Muscle: An Improved 1 Day Protocol. Eur J Appl Physiol. 2002;87(3):290-295. doi.org/10.1007/s00421-002-0621-5
- Burke LM, Hawley JA, Wong SHS, Jeukendrup AE. Carbohydrates for Training and Competition. J Sports Sci. 2011;29(sup1):S17-S27. doi.org/10.1080/02640414.2011.585473
- Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. J Acad Nutr Diet. 2016;116(3):501-528. doi.org/10.1016/j.jand.2015.12.006