The Marathon Diet Plan: Carb Loading to Race Day for 42.2 KM
- Shrey Aggarwal
- Aug 13
- 8 min read
A runner I worked with last year went through Mumbai's 2024 marathon on four gels and a bottle of water, then couldn't explain why she'd lost eleven minutes over the final 8 km despite feeling controlled at halfway. Her training was fine. Her fuelling plan was two-thirds of one.
A marathon diet plan has three phases: raised carbohydrate intake across the final 2–3 days (carb loading), a practised low-fibre race-morning breakfast 2–3 hours before the start, and 60–90g of carbohydrate per hour during the race for anyone out there longer than 2.5 hours. For 42.2 km, unlike shorter races, all three phases materially change your result.
Quick Overview
Why 42.2 km is a fuelling problem and 10 km isn't
Muscle glycogen stores hold roughly 400–500g of carbohydrate in a trained runner, which is somewhere between 90 and 120 minutes of hard running. A 10 km race finishes before that becomes a question, which is why the plan for a half marathon or 10K looks so different. A marathon does not. Everything the sport calls "the wall" is a description of a glycogen store running out with 10 km still to run.
That's the whole reason a marathon diet plan exists as a separate thing from ordinary good eating. You're trying to start with more fuel on board than usual, then slow the rate at which you spend it by adding carbohydrate from outside.
How much carbohydrate does carb loading actually require?
Carb loading means 7–10g of carbohydrate per kilogram of bodyweight per day across the final 2–3 days before the race, alongside a taper in training. The ACSM and Academy of Nutrition and Dietetics joint position statement puts it higher, at 10–12g/kg across 36–48 hours (Thomas et al., MSSE, 2016). I work at the lower daily figure over a slightly longer window because it's the version runners actually manage to eat, and a loading plan you complete beats a loading plan you abandon on day one.
For a 70kg (154lb) runner, 7–10g/kg is 490–700g of carbohydrate a day, which is well above what most people eat without planning it.
The number surprises almost everyone. A large plate of pasta is about 100g of carbohydrate. Three rotis with dal is about 60g. Two bananas is 50g. Getting to 600g means carbohydrate at every meal, plus juice, sports drink, white bread, jam, honey, and dried fruit filling the gaps between them. Most runners who tell me they carb loaded had one big dinner and hit maybe 350g.
Loading doesn't need to start a week out. The taper does, and carbohydrate intake tends to rise on its own as training volume falls, but the deliberate loading window is the last two to three days. Extending it further adds digestive load and heaviness without adding stored glycogen.
Expect the scale to go up by 1–2kg. That's water bound to stored glycogen, roughly 3g of water per gram of carbohydrate, and it's the sign the loading worked.
The final three days, meal by meal
Three days out, start raising carbohydrate and start dropping fibre at the same time. Brown rice, whole wheat pasta, salads, raw vegetables, beans and bran cereals all go down, and white rice, white bread, refined pasta, poha, upma, congee, plain potatoes, bagels and rice porridge come up. The aim is a full glycogen store and an empty large intestine on the start line.
Fat and protein don't disappear, they just stop competing for space. Keep protein moderate, keep fat lower than usual because it slows gastric emptying, and let carbohydrate take the volume.
The day before is where most people get it wrong in the other direction, eating an enormous unfamiliar dinner at 9pm. Spread the load across four or five smaller carbohydrate-heavy meals through the day and eat dinner early. A 550g-carbohydrate day eaten calmly beats a 700g day eaten in two sittings that leaves you awake at midnight.
Alcohol is worth skipping across those days. It interferes with glycogen storage and with sleep, and the marginal social benefit isn't worth either.
What should you eat on marathon morning?
Eat 1–4g of carbohydrate per kilogram of bodyweight, 2–3 hours before the gun, and eat something you've already tested in training (Thomas et al., MSSE, 2016). For a 70kg runner that's 70–280g, a wide range, and where you sit in it depends on how well your stomach tolerates food before running.
Practical versions of that breakfast: white toast with jam and honey plus a banana and a sports drink; plain poha with a banana; oats made with water rather than milk, with honey; rice porridge with sugar; two bagels. All of them land in the 100–150g region, which suits most runners.
The London, Berlin and Tokyo marathons start early enough that this means eating at 4:30am. Boston, which starts late morning, and the Indian winter races that go off at 5am need completely different breakfast timing. Work backwards from your actual start time rather than copying a schedule from a runner in another race.
Coffee is fine if it's normal for you. Race morning is the wrong day to introduce it, and the wrong day to introduce anything else.
How many carbs per hour during the race?
Aim for 60–90g of carbohydrate per hour once the race will take you more than 2.5 hours, using a glucose-fructose mix rather than glucose alone (Thomas et al., MSSE, 2016; Tiller et al., JISSN, 2019). Single-source glucose absorption saturates around 60g per hour; adding fructose recruits a second intestinal transporter and lifts the ceiling.
Where you sit inside that range depends on pace, not only duration. A 3-hour runner is oxidising carbohydrate fast enough to use the top of it. A 5-hour runner is working at a lower absolute intensity, burning less per hour, and should sit nearer 60g. Carbohydrate taken beyond what you're actually using doesn't get stored mid-race, it sits in the gut and causes the problem it was meant to prevent.
Runners who followed a structured in-race carbohydrate and fluid plan finished a marathon roughly ten minutes faster than runners eating and drinking freely (Hansen et al., IJSNEM, 2014). Ten minutes is more than most people gain from a training block.
There's a fashionable upper end at 120g per hour, which has been tested in elite mountain marathon runners and reduced markers of muscle damage (Viribay et al., Nutrients, 2020). Those were elite athletes who had trained their guts for months at that intake. It is not a starting point for a first marathon.
In practice, that's a gel every 25–30 minutes from around the 40-minute mark, or a combination of gels, chews and on-course sports drink. Start early. Fuelling at 30 km to fix a problem that began at 20 km never works, because absorption doesn't accelerate to catch up.
Check the on-course brand at your race and train with it if it agrees with you. Carrying your own is safer than discovering at 25 km that the race drink doesn't sit well.
Hydration and sodium across 42.2 km
Drink to thirst as a baseline, and adjust upward if you're running in heat. Sodium losses run 500–2,000mg per litre of sweat depending on the individual, so a heavy or salty sweater in a humid race in Chennai, Singapore or Houston has a very different requirement from someone running Berlin in October. Most sports drinks and gels carry some sodium already, which is often enough for a sub-4-hour race in temperate conditions, and the question of whether you need a separate electrolyte product comes down to your own sweat rate and the weather on the day.
Overdrinking is the risk that actually harms marathon runners, not underdrinking. Gaining weight across a race is a warning sign, not diligence.
The week you should have started practising
Every number here needs rehearsing in training, and the gut adapts to carbohydrate intake with repeated exposure. Two weeks of repeated carbohydrate feeding during running reduced both gastrointestinal symptoms and carbohydrate malabsorption in trained runners (Miall et al., Scand J Med Sci Sports, 2018). Use your long runs. Take the same breakfast, the same gels at the same intervals, the same fluids. By the third or fourth long run at race intake, most runners who thought they had a sensitive stomach find they don't, which is the whole argument for training your gut the way you train your legs.
Nothing in this plan is new information on race day. That's the point of it.
Fueletics Perspective on your Marathon diet plan
Most runners I see have trained for sixteen weeks and planned their nutrition for about forty minutes. The three phases each fail in a predictable way. Carb loading fails because runners eat one big meal instead of two full days. Race morning fails because it's untested. In-race fuelling fails because it starts too late and stops too early.
Your loading target, your breakfast size and your hourly carbohydrate intake all depend on your bodyweight, your finish time, your sweat rate and what your gut has actually been trained to handle. Those are four individual numbers, and guessing them is how a good training block turns into a bad race day. If you want yours worked out properly before your next start line, start with a Fueletics assessment.
Fix the fuelling and you don't get fitter. You just get to use the fitness you already built across all 42.2 km instead of 32.
Frequently Asked Questions
How many days before a marathon should you carb load?
Two to three days. Raise carbohydrate to 7–10g per kilogram of bodyweight for the final 2–3 days while tapering training. Starting a full week out doesn't add stored glycogen; the taper begins earlier, but the deliberate loading window is short.
What should I eat the night before a marathon?
A carbohydrate-heavy, low-fibre, lower-fat dinner eaten early: white rice, refined pasta, plain potatoes or white bread with a moderate protein portion. Keep it familiar and keep the portion normal, spreading your carbohydrate across the whole day rather than loading it into one meal.
Should I eat during a marathon?
Yes. Anyone racing longer than 2.5 hours should take 60–90g of carbohydrate per hour, starting within the first 40 minutes rather than waiting until fatigue arrives. Faster runners sit at the top of that range and slower runners nearer the bottom, since carbohydrate use per hour tracks pace.
Is pasta the best carb loading food?
No single food is required. Pasta is convenient and carbohydrate-dense, but white rice, poha, potatoes, bread, congee, juice and sports drink all work. The best loading food is whichever one you can eat a large quantity of without digestive trouble.
Research References
Thomas DT, Erdman KA, Burke LM. American College of Sports Medicine Joint Position Statement: Nutrition and Athletic Performance. Med Sci Sports Exerc, 2016. DOI: 10.1249/MSS.0000000000000852
Tiller NB, Roberts JD, Beasley L, et al. ISSN Position Stand: nutritional considerations for single-stage ultra-marathon training and racing. J Int Soc Sports Nutr, 2019. DOI: 10.1186/s12970-019-0312-9
Hansen EA, Emanuelsen A, Gertsen RM, Sørensen SSR. Improved marathon performance by in-race nutritional strategy intervention. Int J Sport Nutr Exerc Metab, 2014. DOI: 10.1123/ijsnem.2013-0130
Miall A, Khoo A, Rauch C, et al. Two weeks of repetitive gut-challenge reduce exercise-associated gastrointestinal symptoms and malabsorption. Scand J Med Sci Sports, 2018. DOI: 10.1111/sms.12912
Viribay A, Arribalzaga S, Mielgo-Ayuso J, et al. Effects of 120 g/h of carbohydrates intake during a mountain marathon on exercise-induced muscle damage in elite runners. Nutrients, 2020. DOI: 10.3390/nu12051367
