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Carb loading before a marathon: what the research says
A 2026 meta-analysis suggests more than 8 g of carbohydrate per kg of body mass per day for 36–48 h before competition to raise muscle glycogen. Whether that improves race performance is not settled.
Carb loading means eating a very high-carbohydrate diet in the days before a long race to raise muscle glycogen, the carbohydrate stored in your muscles.1 The International Society of Sports Nutrition (ISSN) position stand on nutrient timing describes it as protocols in the days leading up to prolonged endurance competitions, and calls its ability to raise muscle glycogen unquestioned.1 A 2026 meta-analysis suggests more than 8 g of carbohydrate per kg of body mass per day for 36–48 h before competition, and its authors call that recommendation uncertain.2 Whether loading makes you race faster is a separate question: a 2026 review says its true efficacy remains uncertain.3
Most of this research is not in runners. In the 2026 meta-analysis, 72% of the exercise protocols were in cycling, and only 74 participants, 12% of the total, were women.2 A 2025 review of glycogen after cycling or running had 319 participants, 271 men and 48 women, in studies published between 1966 and 2020.4 Both measured glycogen in the muscle, not race times.2, 4
Below is what carb loading is, how long and how much, whether it improves performance, a table to apply it, and what the research does not cover. The meal on race morning is a separate question: see what to eat on the morning of the race.
What carb loading is
The ISSN position stand places carbohydrate loading in the days leading up to prolonged endurance competitions.1 It encourages athletes to consider it particularly if their event will significantly deplete muscle glycogen.1 It calls the ability of loading to rapidly increase and maximise muscle glycogen currently unquestioned.1
How does carb loading work? The 2026 meta-analysis reviewed 63 trials with 641 participants, and found that muscle glycogen depends mainly on how much carbohydrate is eaten, with the length of the higher intake, training status and exercise also playing a part.2
It is not only a cycling result. The 2025 review found glycogen supercompensation, glycogen rising above its usual level, after both cycling and running, after 3–5 days on a high-carbohydrate diet.4 The rise was larger after cycling than after running.4
How long, and how much?
The amount
The 2026 meta-analysis concludes that to raise pre-competition muscle glycogen significantly, athletes should eat more than 8 g of carbohydrate per kg of body mass per day.2 It puts that at 36–48 h before competition.2 The authors say significant heterogeneity and confounding variables in earlier research make these recommendations uncertain.2 They add that more well-controlled studies are needed to find the true optimal intake.2
One study described in the ISSN position stand reduced training volume for three to four days while athletes ate a very high-carbohydrate diet, 8–10 g/kg/day, for one to three days.1 The position stand’s text does not describe who took part.
Loading inside the taper
In that study, the high intake came with less training.1 For a marathon, that points to the last days of your taper, when training is already reduced. The sources here say nothing about how long the taper itself should be; for that, see how long a marathon taper should be.
Women
The ISSN position stand says female athletes may need to significantly increase total energy intake over the loading days to get effects similar to men.1 The 2026 meta-analysis found that, when carbohydrate intake was matched, men and women appear to have the same capacity to increase muscle glycogen.2 Only 74 women took part across all its trials, 12% of participants, so that finding rests on few women.2
Does it improve performance?
The evidence is mixed. Raising glycogen is well supported: the position stand calls it unquestioned, and the 2025 review found it after cycling and after running.1, 4 Whether more glycogen means a faster race is a different question, and one 2026 review answers it with caution.
That review says carbohydrate loading has long been a cornerstone of endurance nutrition, yet its true efficacy remains uncertain.3 It notes that loading is commonly associated with improved time to fatigue and modest performance gains.3 Of the 14 studies it identified, only two were double-blind, placebo-controlled trials.3 Both included appropriate carbohydrate intake during exercise and reported no evidence of a performance benefit from loading.3 The review says it remains unclear whether the reported benefits reflect loading or placebo effects.3 Its authors call for a re-evaluation of whether loading in the 24–48 h before competition improves performance once placebo effects are accounted for.3
Some of that review’s authors also wrote the 2026 meta-analysis, so read it as one group’s reading of the trials, not an independent verdict. It does not show that loading fails; it shows that the best-controlled test of it is small. Each claim in this guide links to its source, and our editorial standards say how each one is checked.
How to apply it
Here is what the sources support for carb loading before a marathon, step by step.
| Step | What the sources support | Source |
|---|---|---|
| When to start | 36–48 h before competition, a recommendation its authors call uncertain | Jones 2026, meta-analysis2 |
| How much | More than 8 g of carbohydrate per kg of body mass per day; one study in the position stand used 8–10 g/kg/day for one to three days | Jones 2026, meta-analysis; Kerksick 2017, ISSN position stand2, 1 |
| Training | Less: that study reduced training volume for three to four days | Kerksick 2017, ISSN position stand1 |
| Race morning | A separate question: what to eat on the morning of the race | Our guide |
Try a one-day version before a long training run first, the kind a marathon training plan is built around, rather than meeting it for the first time in race week. Change one thing at a time so you can tell what made the difference. Keep the food on race day familiar: eat what you have already tried in training.
This guide is general information. For a personal nutrition plan, a joint position statement of the American College of Sports Medicine, the Academy of Nutrition and Dietetics and Dietitians of Canada says athletes should be referred to a registered dietitian/nutritionist.5
In RunFast
RunFast’s plans run through base, build, peak and taper phases, with one long run a week and about 80% of running easy. A marathon plan has three taper weeks before the race, and each session is shown with its target pace. Carb loading falls in the last days of that taper, and the plan shows the sessions of those days and the race itself.
What the research does not cover
The 2026 meta-analysis cautions that very high carbohydrate intakes can be hard to reach because of possible gut discomfort, and suggests low-fibre, high-glycaemic-index carbohydrate sources to help tolerability.2 The parts of these sources we could read say nothing on changes in body weight or water. We found no source on loading before a 5K, 10K or half marathon, little on women and little on recreational marathoners, and none of the abstracts and full texts we could read reports marathon race times after loading.
The 2026 meta-analysis’s authors say more well-controlled studies are needed to find the true optimal intake.2 The 2026 blinding review says it remains unclear whether reported performance benefits reflect loading or placebo.3
In practice, treat the amount and timing here as a starting point to test, not a rule.
Common questions
What is carb loading?
It is a set of protocols in the days leading up to prolonged endurance competitions, meant to raise muscle glycogen.1 The ISSN position stand encourages athletes to consider it particularly when their event will significantly deplete muscle glycogen.1
When should I start carb loading for a marathon?
The 2026 meta-analysis suggests more than 8 g of carbohydrate per kg of body mass per day for 36–48 h before competition.2 Its authors call the recommendation uncertain.2 That is a figure for raising glycogen, not a tested start time for race performance.
Does carb loading work?
For raising muscle glycogen, yes: the ISSN position stand calls that unquestioned, in mostly highly trained adults.1 For performance, the evidence is mixed: loading is commonly associated with improved time to fatigue and modest gains, but the only two double-blind, placebo-controlled trials one 2026 review found both reported no performance benefit.3
Should I carb load for a half marathon?
No source here studied loading before a half marathon. The ISSN position stand says prolonged exercise of more than 60–90 min at moderate to high intensity relies heavily on carbohydrate stores, and places loading strategies there.1 The reported performance gains from loading were in exercise lasting about 90–240 min.3 So whether your half marathon falls in that range depends on how long and how hard you run it.
This guide is general information, not medical advice. If you have an injury, a health condition or are pregnant, ask a doctor or physiotherapist before changing how you train or eat.
Related guides
- How long should a marathon taper be?Two to three weeks, with weekly volume cut and fast running and most running days kept, is what the research supports. Whether two or three is better is not settled.
- What to eat before a run: timing, carbs and the researchThe ISSN position stand says carbohydrate-rich snacks or meals are commonly recommended in the hours before longer, harder exercise. For shorter exercise, a meta-analysis found no performance benefit from eating first.
- How a marathon training plan is builtA marathon training plan combines rising weekly distance, a long run, easy and hard running, lighter weeks and a taper. The research backs some of these parts and says little about how to combine them.
Sources
- Kerksick CM, Arent S, Schoenfeld BJ, Stout JR, Campbell B, Wilborn CD, Taylor L, Kalman D, Smith-Ryan AE, Kreider RB, Willoughby D, Arciero PJ, VanDusseldorp TA, Ormsbee MJ, Wildman R, Greenwood M, Ziegenfuss TN, Aragon AA, Antonio J. International society of sports nutrition position stand: nutrient timing. Journal of the International Society of Sports Nutrition. 2017. doi:10.1186/s12970-017-0189-4
- Jones RO, Areta JL, Fortis HO, Pugh J, Louis JB. Dietary Carbohydrate Intake and Skeletal Muscle Glycogen: A Systematic Review and Meta-Analysis to Determine Optimal Carbohydrate Loading Strategies. Scandinavian Journal of Medicine & Science in Sports. 2026. doi:10.1111/sms.70379
- Jones RO, Louis J, Saunders B, Morton JP, Gejl K, Nielsen J, Areta JL. Beyond Belief? Absence of Double-Blinding and Limited Ecological Validity in Carbohydrate Loading Research. International Journal of Sport Nutrition and Exercise Metabolism. 2026. doi:10.1123/ijsnem.2026-0058
- Solem K, Clauss M, Jensen J. Glycogen supercompensation in skeletal muscle after cycling or running followed by a high carbohydrate intake the following days: a systematic review and meta-analysis. Frontiers in Physiology. 2025. doi:10.3389/fphys.2025.1620943
- Thomas DT, Erdman KA, Burke LM. American College of Sports Medicine Joint Position Statement. Nutrition and Athletic Performance. Medicine and Science in Sports and Exercise. 2016. doi:10.1249/mss.0000000000000852