Carb Loading Before Marathon: What Works
Most runners eat a big pasta dinner the night before and call it carb loading. That's one meal. It's not a strategy.
By the time marathon pace starts to bite at 30km, you're no longer racing on fitness alone. You're racing on glycogen availability, fluid balance and how well you executed the final 48 hours. The runners who fade late — not from lack of training, but from lack of fuel — almost always share the same story: they thought they loaded, but they left it too late or didn't get the numbers high enough.
Carb loading is not a licence to eat everything in sight. It's a short, deliberate increase in carbohydrate intake — timed with reduced training volume — that tops up muscle glycogen before the gun. Done precisely, it gives you a bigger fuel reserve to draw from when the race gets honest.
"Before big races, the pre-race nutrition isn't something I improvise. Two days out, I start reducing fibre and fat, keeping carbohydrate consistent across every meal. The goal isn't feeling stuffed — it's arriving at race morning knowing every gram I've eaten over the last 48 hours has been working for me, not against me."
— Alistair Brownlee, Two-time Olympic Triathlon Champion and founder of truefuels
What carb loading before marathon actually does
Carbohydrate is stored in the muscles and liver as glycogen. During a marathon, glycogen is your primary high-intensity fuel source — particularly relevant if you're running at a pace that matters to you rather than just jogging to the finish. The more glycogen you can store before the start, the longer you can maintain output before fatigue, pace drift and poor decision-making stack up.
For most runners, the biggest gain from carb loading isn't magical speed. It's better protection against the late-race slowdown that comes when energy demand outpaces supply. If your training is in place, that margin matters.
Glycogen is also stored with water — approximately 2.6–4g of water per gram of glycogen. That means a proper carb load slightly increases body mass (normal) and improves cellular hydration going into the race. The scale going up the day before doesn't indicate a problem. It indicates the plan is working.
The 48-hour window that actually moves glycogen
Older protocols used a full week with depletion phases and extreme dietary swings. That approach is outdated. You don't need to empty the tank first. It adds stress and usually results in worse execution.
The effective window for most marathon runners is the final 36–48 hours before race day. During taper, training load is already reduced, which means more of what you eat can be stored rather than burned immediately. This is the leverage point.
Carb loading timeline
| Timeframe | Carbohydrate target | Fibre | Fat | Key foods |
|---|---|---|---|---|
| 48hr before (Fri for Sun race) | 8–10g/kg body mass | Reduce | Moderate | White rice, pasta, bread, banana, fruit juice, yoghurt drink |
| 24hr before (Sat for Sun race) | 8–12g/kg body mass | Low | Low | Toast, porridge, jam, white potato, cereal, sports drink, gels |
| Race morning (3–4hr before) | 1–3g/kg body mass | Very low | Very low | Porridge, white toast, banana, sports drink |
| 15–30 min before start | 20–30g top-up | None | None | 1 truefuels gel or carbohydrate drink |
For a 70kg runner, 8–12g/kg means 560–840g of carbohydrate per day across those 48 hours. That's substantially more than most people eat on a normal training day — which is precisely why carb loading so often fails in practice. Athletes say they loaded but never got close to the numbers.
Best foods for a marathon carb load
The best foods aren't the cleanest or most virtuous. They're the foods that help you hit carbohydrate targets with low fibre, low fat and low digestive risk. The final 24 hours are not the time to prove how disciplined your diet is.
Work well: white rice, white pasta, potatoes, bread, bagels, oats, pancakes, low-fibre cereals, bananas, fruit juice, yoghurt drinks, sports drinks, carbohydrate gels used as a low-volume liquid top-up.
Avoid: large salads, beans and pulses, high-fibre wholegrains, heavy sauces, large protein portions, anything new you haven't eaten before a long run.
The truefuels gel can be useful here specifically because it's water-mixable. When nerves suppress appetite in the final 12 hours, taking 40g of carbohydrate in a fluid form is easier than forcing another plate of pasta. It also removes the fibre and fat risk that comes with solid food.
Common mistakes runners make
Leaving it too late. A massive pasta dinner at 8pm the night before is not carb loading. It's one meal. If Friday was low carbohydrate, you've missed the primary opportunity.
Fibre miscalculation. Large portions of wholegrains, vegetables and pulses can make you feel like you're eating properly, but they leave gut residue, may cause bloating on race morning, and don't meaningfully raise glycogen. Reduce fibre, not carbohydrate.
Ignoring fluids and sodium. Glycogen storage and hydration are connected. If carbohydrate intake rises but fluid intake stays poor, you're not completing the preparation. Equally, excessive plain water without adequate sodium can leave you feeling flat. Include electrolytes with meals — particularly if travelling or racing in warm conditions.
Treating every runner the same. A lighter athlete with a sensitive stomach may need more carbohydrate from fluids and lower-bulk foods. A bigger athlete with a robust gut may tolerate larger meals comfortably. There's no one-size answer on exact foods — only on principle.
What to do race morning
Even after a proper carb load, liver glycogen drops overnight during sleep. Breakfast matters. It's not the main event, but it bridges the gap from dinner to the start.
Aim to eat 3–4 hours before the gun: familiar carbohydrate-rich foods, low fibre, moderate size. Porridge, toast, jam, a banana, sports drink. Then a small top-up — one gel or a carbohydrate drink — 15–30 minutes before the start if that's part of your established routine.
Keep dinner the night before moderate in size and familiar. Going to bed overfull, thirsty or uncomfortable is not a good sign. The goal is topped-up, not stuffed.
Does everyone need carb loading before marathon racing?
If you're racing the marathon seriously, most runners benefit from some form of structured carbohydrate preparation before marathon day. Longer finish times increase total exposure and make glycogen management even more critical — not less.
For runners with IBS, reactive gut symptoms or pre-race nausea, the answer is not to skip carb loading. It's to practise your approach in training and reduce complexity. Liquid carbohydrate, lower-residue foods and a disciplined schedule often work better than large mixed meals.
A practical approach that actually works
Two days out, start increasing carbohydrate across every meal rather than cramming it into one sitting. Lower training volume as planned. Reduce fibre and fat. Drink consistently and include electrolytes to match your normal habits.
The day before, same structure. Choose foods you know. Don't chase perfection with oversized meals.
Race morning: eat early, top up close to the start, and begin your in-race fuelling plan from 20–30 minutes in rather than waiting until you feel empty.
Carb loading gives you a bigger reservoir. It doesn't remove the need to use it well. The runners who execute the pre-race preparation calmly, without guesswork, are the ones whose training actually shows up in the final 10 kilometres.
