OUTRN

Carb Loading for a Marathon: What the Evidence Says

Training | Published 2026-10-03 | By OUTRN Team

The OUTRN team goes through how much carbohydrate to load, who benefits, and how many grams an hour to take in a race, and where the studies disagree...

Carb loading is some of the most repeated running advice, and its trial evidence is thinner than you would expect. It does what it says on the tin, filling your muscles with glycogen. Whether that makes you faster in a race that you also fuel properly is a different question, and the answer is less tidy than the pasta party suggests...

The short answer: carb loading means eating about 10g of carbohydrate per kilogram of body weight a day for the one to two days before an event lasting longer than about 90 minutes. It raises muscle glycogen. Whether it improves race performance once you are also eating during the race is contested. This guide is for someone who runs at least 10km comfortably and is training for a longer event.

What is carb loading?

Eating extra carbohydrate and cutting back training before a long event, to fill your muscle glycogen stores. The old method started with a three to four day depletion phase of hard training and low carbohydrate, then three to four days of loading. The Australian Institute of Sport (AIS) said in a 2009 fact sheet that today, 1 to 4 days of exercise taper on a high carbohydrate diet (7 to 12g per kilogram of body weight) is enough, and that the depletion phase is no longer needed.

A note on sources: the AIS carb loading and carbohydrate fact sheets we cite are from 2009 and 2014, are no longer live on the AIS site and are cited through archived copies. Treat them as dated official guidance. The current AIS pages on gels and caffeine are live.

How much carbohydrate, and for how long?

About 10g per kilogram of body weight a day, for one to two days. The AIS 2014 table recommended 10 to 12g per kilogram per 24 hours for 36 to 48 hours before events lasting more than 90 minutes. The International Society of Sports Nutrition's ultramarathon position stand (Tiller and colleagues, 2019) endorses about 10g per kilogram per day in the 48 hours before the event, with care to avoid gut distress.

How long? One day may do the job. In 8 endurance-trained men (Bussau and colleagues, European Journal of Applied Physiology, 2002), muscle glycogen rose from 95 to 180 mmol per kilogram of wet mass after one day on 10g per kilogram of carbohydrate and did not rise further over days 2 and 3. It was a small, men-only study that measured glycogen, not performance, and the participants were asked to stay inactive.

The arithmetic is stark: at 10g per kilogram, a 70kg runner needs 700g of carbohydrate a day. We worked that out ourselves from the AIS figure.

What does 50g of carbohydrate look like?

The AIS 2014 table lists portions that provide about 50g of carbohydrate, and this is the best Australian food guide we found. Check labels, as brands vary.

FoodAmount for about 50g carbohydrate
Wheat biscuit cereal (such as Weet-Bix)60g (5 biscuits)
Rolled oats90g (1 cup)
Porridge made with milk350g (1.3 cups)
Bread100g (4 slices white or 3 thick wholegrain)
Rice, boiled180g (1 cup)
Pasta or noodles, boiled200g (1.3 cups)
Bananas2 medium to large
Sultanas and raisins70g (4 tablespoons)
Potatoes350g (3 medium)
Flavoured milk560mL
Muesli bar2.5 bars

Our arithmetic: 700g is 14 of these portions. That is why AIS says to cut back on fibre and make use of compact sources of carbohydrate such as sugar, cordial, soft drink, sports drink, jam, honey, jelly and tinned fruit for the loading days: too many high-fibre foods can cause stomach upset or be too bulky to eat. Expect about 2kg of body mass gain from extra glycogen and water, which is normal.

Does carb loading actually improve performance?

Maybe, and the evidence is contested. The older view, from a 1997 review (Hawley and colleagues), is that high carbohydrate diets improved performance over a set distance by about 2 to 3 percent and delayed fatigue in events over 90 minutes. The AIS repeats that estimate, and says anyone exercising continuously at a moderate to high intensity for 90 minutes or more is likely to benefit.

Two pieces of evidence push back. In a placebo-controlled crossover trial of 7 well-trained cyclists (Burke and colleagues, Journal of Applied Physiology, 2000), three days of 9g per kilogram per day raised muscle glycogen more than 6g per kilogram, but did not improve a 100km cycling time trial (147.5 against 149.1 minutes) when carbohydrate was eaten before (2g per kilogram) and during (1g per kilogram per hour) the trial. The authors suggested a placebo effect might explain part of the earlier benefit. A 2026 evaluation of 14 carb-loading studies (Jones and colleagues, International Journal of Sport Nutrition and Exercise Metabolism) found only two were double-blind and placebo-controlled, both of which found no performance benefit, and both gave carbohydrate during exercise.

The fair summary: loading raises glycogen, which is not in doubt. Its effect on performance, once the race is properly fuelled, is thinner than the advice suggests. We found no marathon-specific randomised trial.

Who benefits from carb loading?

Runners in events longer than about 90 minutes. The AIS said shorter exercise is unlikely to benefit because normal carbohydrate stores are adequate, and a classic 1981 study of trained runners (Sherman and colleagues) found no difference in a 20.9km run between regimens. No source says 10km runners should not carb load: that is our reading of the evidence, so we word it as "the usual stores are probably enough".

Women: the evidence is thin. The AIS says some studies suggest women may be less responsive to loading, partly because of how hard it is to eat larger amounts, and that more research is needed. A position stand on nutrient timing (Kerksick and colleagues, 2017) makes a similar point: total energy may need to rise.

How much carbohydrate should you eat during a marathon?

Somewhere between 30 and 90g an hour, depending on the source and how long you are out there. The AIS 2014 table suggests none under 45 minutes, small amounts for 45 to 75 minutes, 30 to 60g an hour for 1 to 2.5 hours, and up to about 90g an hour beyond that using multiple transportable carbohydrates (a blend of glucose and fructose, absorbed by different transporters). A 2014 review (Jeukendrup, Sports Medicine) proposed similar tiers (about 60g an hour for efforts of 2 to 3 hours, about 90g an hour for ultra-endurance events), though its author was affiliated with the Gatorade Sports Science Institute when it was written.

These sources disagree for a reason. The ISSN ultra position stand recommends 30 to 50g an hour within 150 to 400 kcal an hour from varied foods, and says 90g an hour "may be unrealistic" for ultramarathons longer than about 6 hours. It is written for ultras, where intensity is low and gut tolerance falls. On those numbers, a 3.5 to 5 hour marathon sits in the higher AIS band and a 12 to 30 hour ultra in the lower ISSN one. That is our reading, not a tested rule. In 51 cyclists and triathletes (Smith and colleagues, 2013), performance gains of about 1 to 4.7 percent appeared across intakes of 9 to 78g an hour, with diminishing returns above about 78g. That was cycling.

A 2026 umbrella review (Tarrit and colleagues) gathering 262 trials found 102 studies (reported as 44 percent) showed a significant benefit of carbohydrate during endurance exercise, mostly for efforts of 1 to 4 hours, though 73 percent of the included reviews were rated low or critically low quality.

What is in an energy gel?

A typical gel sachet gives 20 to 30g of carbohydrate, about 80 to 120 kcal, and is 65 to 70 percent carbohydrate, according to the live AIS gels fact sheet. Sodium is typically under 100mg. The AIS says most gels should be taken with water to reduce the risk of gut upsets, that above about 60g an hour multiple transportable carbohydrates are more effective than glucose alone, and that gels are not needed at every training session. People with fructose malabsorption should note that those blends contain fructose.

We did not compare brands, so we make no claims about which gel is best.

How well are they tolerated? In two 16km outdoor runs (Pfeiffer and colleagues, 2009; 34 and 48 runners), gut complaints stayed low at gel intakes of 1.0 to 1.4g a minute, but about 10 to 20 percent of runners had serious problems.

Can you train your gut?

Probably, and it is the most practical advice on this page. In a 2-week randomised trial of 25 endurance runners (Costa and colleagues, 2017), repeated carbohydrate feeding during runs cut gut symptoms by about 60 percent, and a 1 hour distance test improved 5.2 percent on gel and 4.3 percent on food, against a 2.1 percent decline on placebo. A systematic review of 8 studies (Martinez and colleagues, 2023) found gut discomfort fell by an average of 47 percent after two weeks in the two 2-week carbohydrate trials, though the studies are small and short.

More is not always better: in 18 highly trained runners (King and colleagues, 2022), two weeks of very high carbohydrate plus gut training did not beat guideline-level intake over a 35km run (36 and 39 seconds faster), and in 19 race walkers it raised upper gut symptoms. The takeaway: build up slowly, rehearse in training, and meet the guideline rather than chasing the highest number.

What about caffeine in gels?

Take what you have tried in training. The live AIS caffeine page says small doses of 2 to 3mg per kilogram (about 200mg), taken before and during endurance exercise, are effective, that some people do not respond or respond badly, and that the half-life is about 5 hours, so watch your sleep. The ISSN says 3 to 6mg per kilogram is consistently beneficial and 9mg per kilogram causes more side effects without extra benefit. Gels with 25 to 100mg of caffeine exist, according to the AIS gels sheet. For repeated doses across a long race the ISSN ultra stand suggests lower amounts, about 1 to 2mg per kilogram.

Where does the evidence run out?

Should you carb load for your race?

If your event will take longer than about 90 minutes, it is a reasonable thing to try in a long training run first: eat higher-carbohydrate, lower-fibre meals the day before, and see how you feel. If your 10km takes under an hour, the usual stores are probably enough. Whatever you choose, the old coaching rule holds: nothing new on race day.

Inspired? Practise your fuelling with a crew

If you want a long day to rehearse your plan, our Blue Mountains trip has one: the Sunday Six Foot Track run, about 28 to 30km with more than 1,000m of climbing on the return, or a shorter 15 to 16km turnaround. It is not a race and there is no race clock, just a turnaround time so everyone is off the hill in daylight. Dates are to be confirmed and the price is an estimate of about $1,650 per person. Booking is not open yet, so register your interest and we will contact you first.

Register interest in Run the Blue Mountains

Further reading

References

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Frequently asked questions

Does carb loading work?

It reliably raises muscle glycogen, but its effect on performance is less certain than the advice suggests. A 1997 review and the Australian Institute of Sport estimate a 2 to 3 percent improvement over a set distance, but a placebo-controlled trial in 7 cyclists found no improvement in a 100km time trial when carbohydrate was eaten before and during it, and a 2026 evaluation of 14 studies found the two double-blind placebo-controlled studies both showed no benefit.

How many grams of carbohydrate should you eat to carb load?

The Australian Institute of Sport's 2014 table recommended 10 to 12g per kilogram of body mass per day for 36 to 48 hours before events over 90 minutes, and the International Society of Sports Nutrition's ultra position stand endorses about 10g per kilogram per day in the 48 hours before an event. For a 70kg runner, 10g per kilogram is 700g of carbohydrate a day.

How long before a marathon should you carb load?

About one to two days. In 8 endurance-trained men, muscle glycogen rose from 95 to 180 mmol per kg wet mass after one day on 10g per kilogram of carbohydrate and did not rise further over days 2 and 3. The old method of a depletion phase followed by loading is no longer needed.

How many carbohydrates per hour should you take during a marathon?

The Australian Institute of Sport's 2014 table suggests 30 to 60g an hour for efforts of 1 to 2.5 hours and up to about 90g an hour beyond that, using multiple transportable carbohydrates such as glucose and fructose. A 2019 position stand for ultramarathons suggests 30 to 50g an hour, and field studies it cites found amateurs around 30g an hour and elites around 70g. Practise in training and build up gradually.

Do you need to carb load for a 10k or half marathon?

The evidence suggests not. The Australian Institute of Sport said shorter-term exercise is unlikely to benefit because normal carbohydrate stores are adequate, and a classic study found no benefit over a 20.9km run. Carb loading is aimed at events lasting longer than about 90 minutes, which includes a marathon and the slower end of a half marathon.