How to Carb Up Before a Marathon or Ultra: A Practical Guide to Carb Loading (Plus the Science, If You Want It)
- 5 days ago
- 13 min read
Carbing up is simpler than the internet makes it sound. Here's how I actually do it for marathons, 50Ks, 100Ks and multi-day ultras, what the science says, and how the team behind the first sub-2-hour marathon used the same principles.
Carb loading, at its heart, is one of the simplest ideas in endurance sport: in the days before a long race, eat a bit more of the right stuff so your body has plenty of fuel to draw on when it matters. That's it. The rest is detail.
I've definitely over-thought it and way over-complicated it in the past, when carbing up is, in reality, pretty straightforward. You're simply adding fuel to the tank ahead of a long drive, or a long run in this case. Put in even simpler terms: you eat a little more in the days before a big event, the right things.
If you've followed my journey from your first half marathon, through your first marathon, and into your first 50K, this is the natural next piece of the puzzle. Train smart, then fuel smart.
What's in this guide
What carb loading actually is (the very short version)
Your body stores carbohydrate as glycogen in your muscles and liver. Glycogen is your fastest, easiest fuel, and it's what powers a hard, long run. You hold roughly 90 minutes to 2 hours' worth at marathon effort before the tank starts running low. Run out and you "hit the wall": pace falls off a cliff, your brain fogs, your legs feel like concrete. Carb loading just means starting the race with the biggest possible tank, so that depletion either never comes, or comes much later than it otherwise would.
That's the whole concept. Everything else in this article is the how.
How I carb up: real strategies for real races
So what I'm hoping to achieve with this article is, first, not to over-complicate it. Second, to bring you my personal experience and the strategies I've used for runs like the Tour du Mont Blanc, the 170-mile Land's End Run, and numerous 100Ks. Third, a little science, kept to its own section so you can read or skip. Then there are some meal ideas, and I'll drop in a few useful resources too.
How I carb up differs depending on what I'm doing.
For a long run in a training block, anything over 20 miles, I'll eat a little more in the 48 hours prior. Nothing dramatic, just consciously bigger portions of the carb-heavier elements on my plate.
For an ultra event like a 50K, I'll maybe start four days before. And for a 100K, around the same lead-in but with a bigger increase in volume, balancing carbs against fats, protein and all the other good-for-you elements. I look at my plate and ask, what is this giving me? That question keeps me honest. It's easy to think "carb loading" means white bread and beige food until you can't fit any more in. It doesn't. It means thoughtfully tilting your plate toward carbs while still feeding the body everything else it needs.
Multi-day events take a little more thought. A marathon or ultra distance a day, often with significant amounts of elevation change, demands something different of your fuel stores. I'll think carefully about my intake in the week ahead, increasing quite a bit in the 48 hours before and on the morning of, and then eating all day and all night during the event itself. (Though fuel intake during an event I'll save for the follow-up article, which I'll come back to at the end.)
The point I want to land here is that there's no single number. The "right" amount depends on the distance, the terrain, how many days of effort, and you. The official guidelines (which we'll get to in the science section) give you a useful upper anchor, but most of us don't need to hit them to race well. We just need to be deliberate.
How Maurten fuelled the sub-2-hour marathon
If you want proof that the underlying principles work, look no further than the most significant marathon performance in history. On 26 April 2026, Kenyan runner Sabastian Sawe became the first person to run an official marathon in under two hours, finishing the London Marathon in 1:59:30. He held 2:50 per kilometre for the full 42.2 km.
Behind that performance was a year of work with Swedish sports-nutrition company Maurten, whose staff made six trips to Kenya across twelve months. They left, in their own words, no stone unturned: stable isotope testing to measure carbohydrate absorption, doubly labelled water for energy expenditure, VO2 max, lactate, running economy, blood sampling, body composition, food logs, the lot.
The headline number from race day was extraordinary. Across the full marathon, Sawe averaged 115 grams of carbohydrate per hour. To put that in perspective, for years the accepted upper limit was 60 g/h, then 90 g/h. Sawe doubled the long-standing ceiling and broke the two-hour barrier while doing it.
For the carb-loading bit specifically, the relevant detail is what he did in the two days before the race: he used Maurten's Drink Mix 320, a high-carbohydrate fluid, to top up his glycogen stores. Then on race morning, a light breakfast, the Maurten Bicarb System at 6:45 a.m., and sips of Drink Mix on the bus to the start. One gel five minutes before the gun. That's it for the pre-race plan.
What he did during the race, mapped tightly to specific kilometre markers and almost entirely fluid-and-gel based, is its own masterclass. That's a story I'll come back to in the follow-up article on in-race fuelling. For now, the takeaway is this: the man who broke the two-hour marathon barrier carb-loaded with a drink. Not a pasta mountain. A drink. We'll come back to why that matters.
The practical timeline: what to eat and when
Here's the simple version, with the science-backed numbers built in so you have a target without having to chase them down yourself.
Three to four days out (longer ultras and multi-day events). Start nudging carbs up gently. For most recreational runners, around 7–8 grams of carbohydrate per kilogram of bodyweight per day is a sensible target across this window. For a 70 kg runner, that's roughly 490–560 g per day. You won't feel much different, but you're setting the table.
Two days out (the main loading window for marathons). Aim for the higher end. The research-backed target is 8–10 g/kg/day, climbing to 10–12 g/kg/day if you can tolerate it. For a 70 kg runner, that's 560–840 g of carbs per day. Most of us land at the lower end of that range, and that's fine. Switch toward lower-fibre, easily digested carbs as you go: white rice, pasta, bagels, potatoes (skin off), bananas, ripe fruit, jam, honey, sports drinks. Save the wholegrains, lentils and large salads for after the race.
24 hours out. Smaller, more frequent meals beat two enormous ones. Hydrate well. Keep fat and protein moderate rather than maximal. Expect the scales to creep up by a kilo or two: each gram of stored glycogen binds 2.7–4 grams of water with it, so a successful load adds weight. That's not fat. That's fuel and the water that comes with it. Don't panic and try to "cut" it.
The night before. Eat earlier than you normally would, and a touch smaller than you might think. A big late dinner sits heavy. Familiar, well-tolerated carbs are king here. Now is absolutely not the time to try the new restaurant.
Race morning (3–4 hours before the start). A pre-race meal of roughly 1–4 grams of carb per kilogram of bodyweight, scaled to how long you've got. Rule of thumb: about 1 g/kg per hour of digestion time. Eating 3 hours before the gun? A 70 kg runner targets around 210 g of carbs. Eating 2 hours out? Around 140 g. This meal matters more than people realise, because liver glycogen drops by roughly a quarter overnight while you sleep. The morning meal tops the tank back up.
30 to 60 minutes before the gun. Optional final top-up of 25–30 g of carbs if your stomach tolerates it. A banana, a gel, a few sips of sports drink. Some runners love this, others find it gives them a blood sugar dip at the start. Test it in training, never debut it on race day.
Five minutes before the gun (the elite trick). A single gel, taken right before you cross the line, lands in your bloodstream just as you're getting going. This is what Sawe did before the sub-2 attempt. It won't transform an average runner into a record holder, but it's a free, easy edge worth knowing about.
Sample meal ideas
What does this actually look like on a plate? Here are practical examples that hit the numbers without forcing you to weigh out 800g of pasta and feel sick.
A loading-day breakfast (around 120–150 g of carbs): large bowl of porridge made with oats, banana, honey and a splash of milk; a slice of toast with jam; a glass of orange juice.
A loading-day lunch (around 150–180 g of carbs): big bowl of white rice with chicken or salmon, light on the veg; bread roll on the side; ripe fruit or a smoothie.
A loading-day dinner (around 150–200 g of carbs): generous serving of pasta with a tomato-based sauce (skip the heavy creamy ones), garlic bread or a roll, a small pudding such as rice pudding or a sweetened yoghurt.
Snacks to fill the gaps: bananas, dried fruit, malt loaf, crumpets with honey, plain bagels, energy bars, sports drinks, smoothies. Liquid carbs are especially useful when solid food tolerance runs out, which it will.
Race morning, 3 hours before the gun: porridge with banana, honey and a drizzle of maple syrup; a slice of white toast with jam; a coffee if that's your habit (but only if it's already your habit); a glass of orange juice or a sports drink. Roughly 150–200 g of carbs, easily digested, familiar.
Carbs from fluids (and gels): the strategy I use
This is where I want to spend a moment, because it changed my fuelling completely and it's a strategy I genuinely recommend.
You don't have to load and fuel from solid food alone. You can take in a meaningful chunk of your carbs as fluid or gel: drink mixes, sports drinks, smoothies, even diluted juices, and increasingly, the high-carb gel packs designed exactly for this kind of intake. I do this routinely, and the reason is simple. Solid food has a digestive cost. Past a certain point, especially on race morning or in the final hours of loading, your stomach starts to complain. Liquid and gel carbs sidestep that. They go down easily, they hit your system faster, and they're far less likely to cause GI distress on race day.
My go-to during a loading phase is a Precision Fuel and Hydration PF90 gel pack, a resealable pouch containing 90 grams of carbohydrate. You'll often see me walking around with one, or sitting in a meeting quietly sipping at it, working through it over a morning or an afternoon. That's a third of a recreational runner's daily loading target in one easy, portable pack, taken in steadily without forcing down another bagel. It works for me, and I've stuck with it.
This isn't a budget option or a recreational shortcut. Look again at what Sawe did. He carb-loaded with a drink. His race-morning fuel was a drink. On the bus to the start, a drink. The world record holder for the marathon distance took on a huge chunk of his pre-race carbs as fluid because that's the cleanest, fastest, lowest-risk way to do it. Maurten's hydrogel technology is the eye-catching commercial example, and Precision's high-carb gels work on the same principle: deliver carbohydrate with less GI burden than equivalent solid food.
If you've got a sensitive stomach, if you find big race-morning meals leave you feeling lead-bellied, or if you struggle to hit the loading numbers from solids alone, fluid and gel carbs are your friend. As ever: practise them in training before you race with them.
Common mistakes runners make
A few honest pitfalls to avoid, most of which I've made myself at some point.
Going wholegrain in the loading window. Brown rice, wholemeal pasta, lentils, big salads. Brilliant 51 weeks of the year. Not in the last 48 hours before a long race, when fibre slows digestion and can leave you bloated on race morning.
Trying something new. New gel, new drink mix, new restaurant the night before, new breakfast on race morning. All bad ideas. The week of a big race is the worst possible week for novelty.
Panicking about water weight. A successful load adds 1–2 kg of "weight" that's actually stored glycogen plus the water it binds with. That's exactly what you want. Don't try to undo it.
Going so hard on day one that you can't face food on day two. Loading is a steady ramp, not a feeding contest. Five or six smaller meals across the day beats two enormous ones.
Forgetting the morning of. Some people load brilliantly across two days, then skip a proper race-morning meal because they're nervous. Liver glycogen drops overnight. Eat the breakfast.
Ignoring hydration and electrolytes. Carb loading and water go together. You can't fully store glycogen without water along with it. Drink steadily, salt your food normally, don't go overboard either way.
The science bit, if you want it
Feel free to skip this section. The advice above stands on its own. But for the curious, here's where it comes from.
The foundational work goes back to Bergström and Hultman in 1967, who first showed that a very high-carbohydrate diet before exercise could "supercompensate" muscle glycogen stores and extend endurance capacity. Six decades and thousands of studies later, that basic principle still underpins every carb-loading protocol in use today.
The numbers most often quoted come from the 2016 joint position stand from the American College of Sports Medicine, the Academy of Nutrition and Dietetics, and Dietitians of Canada (Thomas, Erdman & Burke, Medicine & Science in Sports & Exercise, 48:543–568). They recommend 10–12 g of carbohydrate per kilogram of bodyweight per day for the 36–48 hours before events lasting longer than 90 minutes, plus a pre-race meal of 1–4 g/kg in the hours before the start. That's the source of essentially every "carb loading guideline" you'll read.
More recently, the Seville Marathon 2025 study (Sánchez-Díaz et al., Sports Medicine - Open) tracked 160 runners at a real marathon and found that those who met in-race carbohydrate intake recommendations of 60–90 g/h were significantly more likely to finish. Carb strategy isn't just about going faster: it's about making it to the line.
For ultras specifically, work by Viribay, Urdampilleta and colleagues (published in Nutrients in 2020) showed that 120 g/h of carbohydrate during a mountain marathon reduced markers of exercise-induced muscle damage and improved recovery compared with 60 or 90 g/h. That research, alongside Maurten's work with Sawe, has pushed the ceiling of "useful" carbohydrate intake significantly upwards.
A couple of honest caveats. The 10–12 g/kg figure was derived largely from elite and sub-elite athletes; for most recreational runners, 7–8 g/kg per day across 2–3 days achieves most of the benefit and is far more sustainable. The glycogen "advantage" also narrows during the race itself, because with more glycogen on board you tend to burn it slightly faster: a 20% head start doesn't translate to a 20% surplus at mile 20. The load is still worth doing. It's just not magic.
And there's a small but credible body of research on fat-adapted approaches, in which highly trained ultra athletes following low-carb diets perform reasonably well on lower carbohydrate intakes. The mainstream consensus remains that for most runners at most race intensities, carbohydrate availability is still the limiting factor, but the "one-size-fits-all" version of carb loading is no longer dogma. There's room for individual variation, and the only way to find what works for you is to practise.
What's next in this series: in-race fuelling
This article covers getting fuel into the tank before the gun. The next piece in the series covers what happens once you're moving: how to actually take on carbohydrate while running a half, a marathon, an ultra, or a multi-day event. Gels versus drinks versus real food, what works at which distance, when to switch, and the kilometre-by-kilometre detail of how Maurten fuelled Sawe to that world record. Sit tight for that one.
Frequently asked questions
Do I really need to carb load for a marathon?
If your race will take longer than about 90 minutes, you'll benefit from carb loading. For a fast marathoner, that's almost guaranteed. For a recreational marathoner taking four hours or more, it's even more important, because glycogen depletion is what causes "the wall" at mile 20. You don't need to hit elite-level numbers; just be deliberate about increasing carbs in the 48 hours before the race.
Will I gain weight from carb loading?
Yes, and that's the point. A successful load adds 1–2 kg of bodyweight, made up of stored glycogen and the water that binds to it. That's not fat, and you don't lose anything by carrying it to the start line. You'll be lighter again within a couple of days of the race. Don't try to "cut" the weight, you're cutting your fuel.
Can I get all my carbs from drinks?
Most of them, yes, and you can extend that to high-carb gel packs too. Sawe carb-loaded with a drink mix and broke the two-hour marathon barrier. I personally use a Precision Fuel and Hydration PF90 (a resealable gel pack containing 90g of carbs) sipped through a morning or afternoon to top up easily. Fluid and gel carbs are easier on the stomach than solid food and faster to digest, so they're an excellent way to hit your numbers without feeling stuffed. Most runners will still want some solid food for satiety and routine, but liquid and gel carbs can absolutely make up a meaningful portion of your loading and race-morning fuel.
What if I have a sensitive stomach?
Lean harder on low-fibre, well-tolerated foods (white rice, white bread, bananas, plain pasta), spread your meals across more, smaller portions, and shift a larger share of your carbs to liquids. Avoid anything new, anything spicy, and anything you don't already eat regularly. If a single food has ever upset you, it doesn't go anywhere near race week.
Does the morning of the race matter as much as the two days before?
Yes, and it's the bit most runners skimp on. Your liver glycogen drops by roughly a quarter overnight while you sleep, so a successful 48-hour load can still leave you starting in a small deficit if you don't eat properly on race morning. Aim for 1–4 g of carbohydrate per kg of bodyweight, 3–4 hours before the gun, then optionally a small top-up 30–60 minutes before. Familiar food, low fibre, low fat, low novelty.
About the author
Andy Hood is an ultra and endurance runner, Runna ambassador and testicular cancer survivor based in Devon. After cancer took him out of his running shoes in 2021, he rebuilt through endurance running, beginning with a 170-mile run along the South West Coast Path to Land's End, an ultra a day across five days. Since then he's run events around the world, from the rolling trails of the North Downs 50 and London2Brighton to the Tour du Mont Blanc and the mountains above Nice at UTMB.
Alongside the racing, Andy designs bold, attention-grabbing charity ultras (a 50K on a shopping-centre treadmill and his Krispy Kreme London runs among them) and has raised over £27,000 for cancer charities to date. He's a loud advocate for men's health, encouraging guys to check their testicles for the signs of testicular cancer, a message he carries on his shorts at every race and through his Check Ya Balls underwear range.
You can follow Andy's journey, read more of his training and race writing, and find his merch at runningwestwardho.co.uk, or say hello on Instagram.
Sit back, relax and enjoy. And don't be like me: don't over-complicate it.
Coming next in the fuelling series: how to actually take on carbs while running a half, marathon, ultra or endurance event, plus the kilometre-by-kilometre breakdown of how Sabastian Sawe fuelled the first sub-2-hour marathon.
Subscribe to the blog so you don't miss it.
Article includes affiliate codes or links, I may receive commission from any product or services you purchase from any of the companies, at no cost to you. This helps to run the blog, bring you interesting articles and support my charity fundraising with over £27,000 raised to date.
Comments