Carb Loading for Your Spring Marathon: A 3-Day Plan by Body Weight

Boston is thirteen days away. London is eighteen. If you're racing this spring, the last long run is done, the hay is in the barn, and one of the few things left to get right is what goes on your plate this week.

And this is where most runners quietly panic.

You Google "carb loading" and get numbers that contradict each other. One article says 7 grams per kilogram. Another says 12. Some say start Monday. Some say start Thursday. One says pasta, one says rice, one says smoothies, one says "just eat more." And somewhere in the back of your head is the memory of a mate who ate so much pasta he couldn't sleep the night before his marathon and ran the worst race of his life.

Here's the truth: carb loading works, the science is actually quite clear, and the reason most runners get it wrong is that they treat it like an all-you-can-eat buffet instead of a precise plan.

This guide gives you the exact numbers — by your body weight — and shows you what that looks like on a plate.

Why carb loading works (and why it isn't about feeling stuffed)

Your muscles store carbohydrate as glycogen. A well-trained endurance athlete can store around 400 to 500 grams of glycogen across muscles and liver — roughly 1,600 to 2,000 calories of easy-access fuel. That's enough for about 90 minutes of marathon effort before you start running on fumes.

Carb loading doesn't give you new muscle. It fills the fuel tank you already built in training. The classic 1981 Karlsson and Saltin study, and every study since, shows that athletes who enter a marathon with fully loaded glycogen stores delay the bonk, hold pace longer, and finish faster than athletes who don't. The effect is real. It is measurable. It is worth roughly 2–3% on your marathon time — which, at 4:00 pace, is nearly 7 minutes.

The mistake is thinking that if 500 grams of glycogen is good, 1,500 grams must be better. Your muscles have a ceiling. Once they're full, extra carbs don't make you faster. They just make you bloated, heavy, and regretful on the starting line.

Carb loading is not about maximum. It is about full.

The science-backed target: 7 to 12 grams per kilogram per day

Every major sports nutrition position statement (IOC, ACSM, ISSN) lands in the same range for marathon carb loading:

7 to 12 grams of carbohydrate per kilogram of body weight per day, for 1 to 3 days before the race.

The spread looks confusing but it actually isn't. The lower end (7 g/kg) is for shorter events or slower marathoners who will finish well under 4 hours of glycogen-burning intensity. The upper end (12 g/kg) is for elite or fast amateur runners who will genuinely empty their tank.

For 90% of the runners reading this — people targeting a 3:30 to 5:00 marathon — the sweet spot is 10 g/kg per day. That's the number you want.

What 10 g/kg actually looks like on a plate

Here's where the generic advice falls apart. 10 grams per kilogram sounds simple until you try to convert it into breakfast, lunch and dinner. So let's do it for you.

| Body weight | Daily carb target (10 g/kg) | Roughly equivalent to | |---|---|---| | 55 kg (121 lb) | 550 g | 2 bowls oats + 2 bananas + large pasta dinner + 1 bagel + 600 ml sports drink | | 65 kg (143 lb) | 650 g | 2.5 bowls oats + 3 bananas + 1 bagel + large rice dinner + honey toast + 600 ml sports drink | | 75 kg (165 lb) | 750 g | 3 bowls oats + 3 bananas + 2 bagels + large pasta dinner + 1 jacket potato + 750 ml sports drink | | 85 kg (187 lb) | 850 g | 3 bowls oats + 3 bananas + 2 bagels + 1 jacket potato + large pasta dinner + 750 ml juice + 2 gels | | 95 kg (209 lb) | 950 g | 3 bowls oats + 4 bananas + 3 bagels + 2 jacket potatoes + large pasta dinner + 1 L sports drink + 2 gels |

If those numbers look like a lot, that's because they are. A 75 kg runner needing 750 g of carbs needs roughly double what they normally eat on a training day. This is the first thing to accept: carb loading isn't "eat a bit more pasta." It's a deliberate, structured intake that will probably require some planning.

The 3-day plan: Friday, Saturday, and race morning

Modern carb-loading protocols have moved away from the old "6-day depletion-then-load" cycle your coach's coach swore by. We now know you don't need to deplete first — a short, straightforward 2 to 3 day load at high intake does the same job without the risk of bonking in a workout. For a Sunday marathon, here's the clean version:

Friday — Day 1 (72 hours out)

Target: 10 g/kg. This is a normal-sized breakfast and lunch plus a carb-heavy dinner, with small carb-rich snacks between meals. Don't skip meals. Don't "save room." Ease your body into the load.

Saturday — Day 2 (48 hours out)

Target: 10 g/kg. Same target, same spread. This is not the day to eat a giant pasta dinner at 9 pm with a bottle of Nebbiolo. Eat your biggest carb meal at lunch so it is fully digested before bed. Dinner should be smaller, lower in fibre, and boring.

Sunday — Race morning

Target: 1 to 4 g/kg, 3 hours before the gun. For a 75 kg runner that's 75 to 300 g. Most runners land at 1.5 to 2 g/kg (110 to 150 g) and feel great — a bagel with honey and banana, or oats with maple syrup and a sports drink. If your race starts at 9 am, you're eating at 6 am. Set the alarm.

The four mistakes that ruin a carb load

This is where most runners go wrong. Every one of these is fixable if you know about it.

Mistake 1: Front-loading it all into one giant pasta dinner

This is the classic. Runner eats normally all day then sits down to 350 g of pasta and half a loaf of garlic bread at 8 pm. They feel stuffed, sleep badly, and wake up heavy. Worse, they didn't even hit their daily target — one massive meal is closer to 200 g of carbs, not 650.

Fix: Spread the load across 5 to 6 eating windows: breakfast, mid-morning snack, lunch, afternoon snack, dinner, evening snack. Small, steady, boring.

Mistake 2: Choosing high-fibre carbs

Brown rice, whole wheat pasta, beans, raw vegetables, salads — all healthy, all a terrible idea in the 48 hours before a marathon. Fibre sits in your gut, draws water, and delivers you to the start line with a stomach that's already angry.

Fix: Switch to white rice, white pasta, white bread, jacket potato without the skin, bananas, honey, maple syrup, sports drinks, and fruit juice. Boring and beige is the goal.

Mistake 3: Underestimating liquid carbs

A surprising amount of your carb-load can come from drinks. 500 ml of sports drink is 30 to 40 g of carbs. 500 ml of orange juice is 45 g. A small bottle of Maurten 320 is 80 g in one sitting. For bigger runners trying to hit 850+ grams a day, liquid carbs are often the only way to get there without feeling wrecked.

Fix: Keep a sports drink or juice with you throughout Saturday. Sip steadily. Count every 500 ml as part of your target.

Mistake 4: Trying a new food for the first time

This is a race-week mistake that shows up in every Strava group chat: "I've never tried sweet potato pancakes but this recipe looks great!" No. Absolutely not. Your gut has spent 16 weeks learning how to digest your training foods. Race week is not the week to audition new inputs.

Fix: Write down the foods you actually practised fueling with during your long-run builds. Carb-load with those. Nothing else.

Where Sweatr fits in

Here's the problem every spring marathoner runs into: 10 grams per kilogram sounds clean until you're standing in Tesco at 8pm on Friday with a basket and no idea whether a jacket potato and a bagel get you halfway or 10% of the way there.

Sweatr turns your body weight, your race distance, and your weekly training load into a personalised carb-load plan — daily targets, hourly spread, food suggestions mapped to products you can actually buy, and a race-morning countdown timed to your race start. It pulls your weight and training history from Apple Watch or Garmin, so you never have to type a number twice.

[Try Sweatr free in the App Store →] Build your carb-load plan in under two minutes.

What to eat in the final 24 hours

The day before the race is its own micro-phase and deserves its own rules.

• Breakfast (24 hours out): big and normal. Bowl of oats, banana, honey, toast, juice. This is your biggest meal of the day.
• Lunch (around noon): the biggest carb meal of the whole load. White rice or white pasta with a simple, low-fat sauce. Chicken or white fish if you want protein. Small portion of vegetables, cooked soft. No raw salad. No beans.
• Afternoon (2 pm to 6 pm): small, steady, boring. A banana. A bagel with honey. A sports drink. A rice cake. Think "grazing," not "feasting."
• Dinner (6 to 7 pm, no later): small and simple. A small bowl of pasta with olive oil and salt. A piece of plain bread. Water. Stop eating by 8 pm.
• Evening: one final sports drink or 200 ml of fruit juice if you feel hungry. Then stop.

The goal is to go to bed comfortable, not stuffed. Glycogen storage was mostly done by mid-afternoon. Anything you force-feed at 9 pm is just sitting there until 7 am, and it will feel like it.

The morning-of meal

Three hours before the gun. This is not carb loading — it's topping off liver glycogen, which drops overnight. Target 1 to 2 g/kg of easily digested carbs.

• 75 kg runner: 110 to 150 g. A bagel with honey and a banana, plus a coffee. Or a bowl of instant oats with maple syrup and a sports drink. Or — if you're a nervous eater — a 500 ml smoothie with banana, oats, honey and milk.
• Stop eating by 2 hours before the gun. Sip fluids until 30 minutes out.
• Caffeine, if you use it, goes in 45 to 60 minutes before the start. (Full post on caffeine timing coming next cycle.)

One last thing: trust the plan

The biggest psychological trap in race week is thinking that more is better. It isn't. A carb-loaded runner does not need to feel bloated, sluggish, or like they've eaten three Christmas dinners back-to-back. A carb-loaded runner feels totally normal.

If you hit 10 grams per kilogram for Friday and Saturday, ate boring beige foods, drank your fluids, and woke up race day feeling a bit lighter than expected — that's what "done correctly" feels like. Don't panic and eat a bagel at 6 am to "make sure."

Full glycogen stores don't feel like anything. They just quietly wait for you at mile 18, when the runner next to you — the one who ate pizza at 9 pm the night before — starts to fade.

That's the race you trained for. Go run it.

[Build your personalised carb-load plan with Sweatr →]