If you’ve ever hit the marathon start line running on empty because you treated the pasta dinner the night before as the whole strategy, you already know why carb loading matters. What happens in the 24 to 48 hours before a long race — and what you eat once the gun goes off — is often the difference between a strong final 10K and a walk to the finish. As a dietitian who’s spent years working with endurance athletes, I still see the same two mistakes on repeat: eating too little carbohydrate beforehand, and having no plan at all for during the race. Here’s what the evidence actually supports, and how to put it into practice without wrecking your stomach.
How Much Carbohydrate You Actually Need
The American College of Sports Medicine recommends 10–12 grams of carbohydrate per kilogram of body weight per day in the 36 to 48 hours before a race. For a 65kg (roughly 143lb) runner, that works out to 650–780 grams of carbohydrate a day — noticeably more than most people eat on a normal training day. Some guidance for marathon and half-marathon runners uses a slightly gentler range of 7–10 g/kg, which is a sensible starting point if you’re new to loading or tend to run into gut trouble. The goal isn’t to eat until you’re stuffed; it’s to consistently hit a high-carbohydrate target across every meal and snack so your muscles and liver can top off their glycogen stores, which is the fuel your body draws on once easily accessible blood sugar runs low.
Why the Week-Long Depletion Diet Is Out
Older marathon guides described a full week of carb "depletion" followed by carb "loading" — deliberately under-fuelling for several days before flooding the system with carbohydrate. Current sports nutrition guidance has moved away from that. A single day, or at most two, of consistently high-carbohydrate eating with minimal exercise is now understood to load glycogen stores about as effectively as the older multi-day approach, with far less disruption to training, mood, and daily life. Practically, that means your loading window is the day or two directly before the race, not the whole week — train and eat normally until then, then shift your plate composition.
What to Actually Eat in the Loading Window
The food choices matter as much as the total grams. Easily digestible, lower-fibre carbohydrates are the safer bet in the final 24–48 hours, because heavy fibre and fat both slow digestion and raise the odds of race-morning GI distress. Think white rice, pasta, white bread, bagels, bananas, and clear juices rather than the big bowl of lentils or beans you’d normally reach for. It’s also worth deliberately increasing fluid intake alongside the extra carbohydrate, since glycogen is stored in the muscles together with water — part of why runners often feel a bit heavier and more hydrated in the day or two before a race, which is expected, not a sign something’s gone wrong.
| Loading priority | Better choices | Use more caution with |
|---|---|---|
| Grains | White rice, white pasta, white bread | High-bran cereals, whole-grain-heavy meals |
| Fruit | Bananas, ripe fruit, fruit juice | Large portions of high-fibre fruit (e.g. whole apples with skin) |
| Protein/fat | Small portions, lean | Large, high-fat meals close to race day |
| Fluids | Water, electrolyte drinks | Excess caffeine or alcohol |
Fuelling During the Race: Grams Per Hour, Not Guesswork
Loading before the race is only half the plan — for anything longer than about 90 minutes, your body needs incoming carbohydrate during the effort too, because stored glycogen alone won’t carry you the full distance at pace. The International Olympic Committee’s consensus statement on sports nutrition puts the target at 30–60 grams of carbohydrate per hour for exercise lasting one to two and a half hours, rising to as much as 90 grams per hour — ideally from a glucose-and-fructose blend, which the gut absorbs faster than glucose alone — for anything longer. In practice, that intake usually comes from sports drinks, gels, chews, or whole foods like dried fruit or pretzels, and it should be spread out rather than taken in one large hit.
| Roughly how long you’re racing | Target carbohydrate intake |
|---|---|
| 1–2.5 hours | 30–60 g per hour |
| Over 2.5 hours | Up to 90 g per hour (glucose + fructose blend) |
| Sub-3-hour marathon pace | Often 75–90 g per hour, gut-trained |
| 4-hour-plus marathon pace | Often comfortable at 45–60 g per hour |
Practising Your Gut Like You Practise Your Pace
None of these numbers are useful if race morning is the first time you’ve tried them. Gut tolerance for carbohydrate during exercise is trainable, and it should be rehearsed on long training runs well before race day — the same way you’d practise your goal pace. If you know you’re prone to stomach upset, it’s reasonable to cap your loading intake closer to 7 g/kg rather than pushing to 12, and to keep your during-race carbohydrate at the lower end of the range until you’ve built tolerance. First-time carb loaders in particular should start conservatively and increase in future training blocks once they know how their gut responds, rather than trying a maximal protocol for the first time the week of a goal race.
The Bottom Line
Carb loading before a marathon isn’t about one big pasta dinner — it’s a one-to-two-day shift toward 7–12 g of carbohydrate per kilogram of body weight, built from easily digestible foods, paired with extra fluids, and followed by a planned 30–90 g of carbohydrate per hour once the race starts. Get the loading window right and rehearse your during-race fuelling in training, and the last miles of your race become a pacing problem, not a fuel-tank problem.