Carb Loading Before Marathon Strategy: Science-Backed Guide

Why Your Muscles Need Fuel: Understanding Glycogen Depletion

Think of your muscles as a car with a fuel tank. During a marathon, that tank runs on glycogen — the stored form of carbohydrate packed into your muscle fibres and liver. The problem is, the tank isn’t very big. Most runners carry enough glycogen to sustain roughly 90 minutes to two hours of hard effort before the reserves start running critically low.

That’s why so many runners hit the wall around mile 20. It’s not a mental failing or a fitness problem — it’s a fuel problem. Your body, now starved of its preferred energy source, starts breaking down fat and protein instead. Fat burns slower, your pace drops, your legs feel like concrete, and suddenly the finish line feels very far away indeed.

A well-executed carb loading before marathon strategy directly addresses this. By systematically increasing carbohydrate intake in the days before your race, you can push glycogen stores beyond their normal resting levels — a process researchers call supercompensation. Studies published in sports nutrition journals suggest that fully loaded muscles can store up to 20% more glycogen than their baseline capacity, which translates to a meaningful delay before fatigue sets in.

Crucially, this isn’t just for first-timers shuffling around in four-and-a-half hours. Even well-trained runners who can sustain a sub-3:30 pace benefit from arriving at the start line with fully topped-up stores. The faster you run, the more glycogen you burn per minute. Elite runners aren’t immune to the wall — they’re just better at managing when it arrives.

Getting your marathon pre-race nutrition right starts with understanding that glycogen depletion is the enemy, and carbohydrate is your defence.

The Timing Question: When Should You Start Carb Loading?

The traditional approach, popularised in the 1960s, involved a depletion phase — several days of very low carbohydrate intake followed by aggressive loading. It worked, but it also left runners feeling dreadful mid-week and occasionally compromised their immune systems right before a big race. Thankfully, sports science has moved on.

The modern consensus is that a 36 to 48-hour carbohydrate loading protocol is both effective and far more pleasant to actually follow. Increasing your carbohydrate intake two days out from race day — while simultaneously reducing fat and protein to keep total calories manageable — is sufficient to achieve meaningful glycogen supercompensation, provided your training taper is already underway.

That last point matters. The taper is doing half the work for you. As you reduce your training volume in the final two weeks, your muscles naturally begin retaining more glycogen. The carb loading phase accelerates and maximises that process rather than doing it from scratch.

Some runners, particularly those racing shorter marathons or who are highly trained, find that even a 24-hour loading window produces noticeable results. Others need the full three days to feel properly fuelled. Individual variation here is real, and it’s one of the reasons Runner’s World UK recommends trialling your carb loading approach during training — specifically, eating more carbohydrate and less fat and protein two days before your longest long run, so you arrive at the start of that session with a genuine sense of how your body responds.

The worst time to experiment with your carbohydrate loading protocol is the week of your actual race. If you’ve never practised it, you won’t know whether it leaves you feeling energised or bloated and sluggish at 6am on race morning. Test it. Adjust. Then commit.

Getting the Portions Right: How Much Carbohydrate Do You Need?

Vague advice like “eat more pasta” isn’t going to cut it here. The research is specific, and your targets should be too.

During the carb loading phase — the 36 to 48 hours before your marathon — the target is 10 to 12 grams of carbohydrate per kilogram of body weight per day. That’s a substantial amount, and it’s worth calculating your personal number rather than eyeballing it.

Here’s what that looks like in practice for different body weights:

• 60kg runner: 600–720g of carbohydrate per day
• 70kg runner: 700–840g of carbohydrate per day
• 80kg runner: 800–960g of carbohydrate per day
• 90kg runner: 900–1,080g of carbohydrate per day

Those numbers look alarming until you realise that a large bowl of cooked pasta contains around 80–90g of carbohydrate, a medium jacket potato has roughly 50g, and two slices of white bread add another 30g. It adds up faster than you’d think, especially when you’re spreading intake across four or five meals and snacks.

A more moderate loading protocol — around 8g per kilogram — is appropriate for runners who find aggressive loading causes digestive discomfort or unwanted weight gain from water retention. (Glycogen storage pulls water into the muscles at a ratio of roughly 3g of water per gram of glycogen, so some temporary weight gain is entirely normal and actually a good sign.)

The key shift during loading is reducing fat and protein intake proportionally. You’re not eating more total food — you’re restructuring the balance so carbohydrates dominate. Swapping a chicken and vegetable stir-fry for a large bowl of rice with a modest protein portion is the right direction of travel.

What to Eat: Building Your Pre-Race Nutrition Menu

The ideal carb loading menu prioritises foods that are high in carbohydrate, low in fibre, and easy to digest. This is not the moment for brown rice, lentil soup, or a virtuous kale salad. Your gut will thank you for keeping things simple and familiar.

Practical high-carbohydrate staples that work well for UK runners include:

• White pasta with a light tomato sauce
• White rice — plain or with a small amount of lean protein
• Jacket potatoes or boiled new potatoes
• White bread, bagels, or crumpets with jam or honey
• Porridge made with oats and banana (a gentler fibre load than most people assume)
• Sports drinks, fruit juice, or squash for additional carbohydrate without bulk
• Rice cakes, pretzels, and cereal bars as snacks

Hydration runs alongside carbohydrate loading rather than separately from it. Because glycogen binds with water, your fluid requirements increase during the loading phase. Aim to keep urine pale yellow throughout. Sports drinks can serve double duty here — providing both carbohydrates and electrolytes without requiring you to force down another plate of pasta when you’re already full.

Foods to reduce or avoid during the loading phase are equally important. High-fibre vegetables, beans, and wholegrains increase the risk of GI distress on race day. High-fat foods — takeaways, cheese-heavy dishes, creamy sauces — slow gastric emptying and compete with carbohydrate absorption. Excess protein has the same effect, and your muscles aren’t building new tissue in the 48 hours before a marathon anyway.

Keep meals familiar. The night before a race is not the time to try the new Thai restaurant that’s just opened near your hotel.

Race Week Strategy: Combining Training Taper with Carb Loading

The final week before a marathon is where the carbohydrate loading protocol slots into a broader picture of reduced training and careful preparation. Getting this week right can make a genuine difference to how you feel at mile 20.

A practical day-by-day framework for the final seven days looks something like this:

1. Seven days out: Last moderate-length run (8–10 miles). Normal balanced diet, slightly higher carbohydrate than usual. No need to load yet.
2. Six days out: Easy 4–5 mile run or rest. Continue eating normally but prioritise carbohydrate-rich meals.
3. Five days out: Short easy run or cross-training. Begin shifting the carbohydrate-to-fat ratio in your favour.
4. Four days out: Rest or very light movement. Carbohydrate intake creeping up, fat and protein reducing proportionally.
5. Three days out: Short 20–30 minute easy jog if desired. Carbohydrate focus continues.
6. Two days out: Begin full carb loading. Target 10–12g of carbohydrate per kilogram of body weight. Rest or very light walking only.
7. One day out: Continue loading. Keep meals early enough to allow full digestion. Rest. Prepare race kit. Sleep.

The most common mistake runners make during race week is panic-eating. Seeing the carbohydrate targets and trying to cram everything into the final 12 hours before race morning is a recipe for a bloated, uncomfortable start. Spread intake evenly across meals. Eat your last large meal at lunch or early evening the day before, not at 9pm.

If you’re looking for training partners to help you through the final weeks of preparation, running clubs across the UK are a brilliant resource — and the collective wisdom of experienced club runners on race week nutrition is often more practically useful than anything you’ll read online.

Race Day Fueling Plan: From Start Line to Finish

Your race day fueling plan is a separate operation from the carb loading phase, though the two work together. By race morning, your glycogen stores should already be full. The job now is to top them up after overnight fasting and maintain fuel availability throughout the race itself.

Aim to eat your pre-race breakfast two to three hours before your start time. This allows adequate digestion while ensuring you’re not running on empty. A practical breakfast might be porridge with banana and honey, or white toast with jam and a sports drink — roughly 100–150g of carbohydrate in total, low in fat and fibre.

Some runners add a small carbohydrate snack 30–45 minutes before the start — a gel, a banana, or a small handful of jelly sweets — to top up blood glucose without adding digestive burden.

During the race, the target is to consume 30–60g of carbohydrate per hour, primarily through gels, sports drinks, or energy chews. More experienced runners with trained guts can push towards 90g per hour using a mix of glucose and fructose sources. Start fuelling early — around mile 5 or 6 — rather than waiting until you feel you need it. By the time you feel the need, you’re already behind.

Hydration during the race should be guided by thirst, with water at most stations and an electrolyte drink every second or third station to replace sodium lost through sweat. Don’t overcomplicate it. Drink when you’re thirsty, take your gels on schedule, and trust the preparation you’ve done.

You can find marathon races happening near you to start planning which event you’ll be putting all of this into practice at.

Testing Your Strategy: Why Training Runs Matter

Everything above is useless if you try it for the first time on race day. The gut is a trainable organ, and your tolerance for gels, sports drinks, and high carbohydrate loads during exercise improves with practice. Runners who experience GI distress mid-race almost always have one thing in common: they didn’t rehearse their nutrition strategy during training.

Use your longest training runs — anything over 16 miles — as dress rehearsals. Eat the same pre-run breakfast you plan to eat before the race. Take gels at the same intervals you plan to use during the event. Practise your carbohydrate loading two days before these runs, exactly as outlined above.

Notice what works. Notice what doesn’t. Some runners find certain gel brands cause cramping. Others discover they can handle solid food mid-run where they assumed they couldn’t. You won’t know until you test it under real conditions.

The London running community, like running groups in most UK cities, regularly organises long group runs in the final weeks of marathon training — ideal opportunities to test race nutrition in a supported environment.

For more training advice covering everything from tempo sessions to recovery weeks, browse our running blog, and if you want the best UK running content delivered directly to you, subscribe to our newsletter. Your marathon preparation deserves better than guesswork — and so do your legs at mile 20.