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, one of the few things left to get right is what goes on your plate this week.

Here's the truth: carb loading works, the science is 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.

Why carb loading works

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

Carb loading fills the fuel tank you already built in training. Research since the 1981 Karlsson and Saltin study consistently shows athletes with fully loaded glycogen stores delay the bonk, hold pace longer, and finish faster. The effect is real and measurable — roughly 2–3% on your marathon time.

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

The target: 10 g/kg per day

Every major sports nutrition position statement (IOC, ACSM, ISSN) lands in the same range: 7–12 grams of carbohydrate per kilogram of body weight per day, for 1–3 days before race day.

The lower end (7 g/kg) suits shorter events or slower marathoners. The upper end (12 g/kg) is for elites who genuinely empty their tank. For 90% of runners targeting a 3:30–5:00 marathon, the sweet spot is 10 g/kg per day.

| 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 |

A 75 kg runner needs roughly double what they normally eat on a training day. This is the first thing to accept: carb loading is a deliberate, structured intake that requires planning.

The 3-day plan

Modern carb-loading protocols have moved away from the old 6-day depletion-then-load cycle. A simple 2–3 day load at high intake does the same job without the risk of bonking in a workout.

Friday — Day 1 (72 hours out): Target 10 g/kg. Normal-sized breakfast and lunch plus a carb-heavy dinner, with small carb-rich snacks between meals. Don't skip meals.

Saturday — Day 2 (48 hours out): Same target. This is not the day for a giant pasta dinner at 9 pm. Eat your biggest carb meal at lunch so it fully digests before bed. Dinner should be smaller, lower in fibre, and boring.

Sunday — Race morning: Target 1–2 g/kg, 3 hours before the gun. A bagel with honey and banana, or oats with maple syrup and a sports drink. Stop eating 2 hours before the gun and sip fluids until 30 minutes out.

The four mistakes that ruin a carb load

Mistake 1: Front-loading into one giant pasta dinner. One massive evening meal is closer to 200 g of carbs — not your 650 g daily target. Spread the load across 5–6 eating windows instead.

Mistake 2: Choosing high-fibre carbs. Brown rice, whole wheat pasta, beans, raw salads — all healthy, all terrible in the 48 hours before a marathon. Switch to white rice, white pasta, white bread, bananas, honey, and sports drinks. Boring and beige is the goal.

Mistake 3: Underestimating liquid carbs. A 500 ml sports drink is 30–40 g of carbs. A small bottle of Maurten 320 is 80 g in one sitting. For bigger runners trying to hit 850+ g/day, liquid carbs are often the only way to get there without feeling wrecked.

Mistake 4: Trying a new food for the first time. Race week is not the time to audition new inputs. Write down the foods you practised fueling with during long-run builds and carb-load with those only.

Where Sweatr fits in

10 grams per kilogram sounds clean until you're standing in the supermarket at 8 pm on Friday with no idea whether a jacket potato and a bagel get you halfway or 10% of the way there.

Sweatr turns your body weight, race distance, and weekly training load into a personalised carb-load plan — daily targets, hourly spread, and food suggestions timed to your race start. It pulls your weight and training history from Apple Watch or Garmin.

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

One last thing: trust the plan

The biggest psychological trap in race week is thinking more is better. It isn't. A carb-loaded runner feels totally normal — not stuffed, not sluggish.

If you hit 10 g/kg for Friday and Saturday, ate boring beige foods, drank your fluids, and woke up race morning feeling a bit lighter than expected — that's what "done correctly" feels like. 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 — starts to fade.

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