Carb Loading Explained

Carb loading is a deliberate increase in carbohydrate intake in the two to three days before a race, aimed at maximizing glycogen stores in your muscles and liver before the start. It's genuinely useful for some race distances and mostly unnecessary for others, which is where a lot of the confusion around it comes from. The idea itself is simple: since glycogen is your body's primary fuel for sustained running, and a marathon typically takes long enough to run through normal stores, starting the race with fuller-than-usual stores gives you a bigger buffer before you have to rely entirely on fuelling during the race itself.

When carb loading actually matters

For a 5K or 10K, normal glycogen stores are generally more than enough, and carb loading isn't necessary; you'd finish well before depleted glycogen became a factor regardless of what you ate the day before. For a half marathon, a modest increase in carbohydrate the day before can help some runners, though it's optional rather than essential for most people, especially those finishing in under two hours. For a full marathon, where hitting the wall becomes a genuine risk without adequate glycogen stores, a deliberate carb loading approach in the final two to three days is considerably more worthwhile. Beyond marathon distance, carb loading becomes part of a broader fuelling strategy that also leans heavily on eating during the event itself, since no amount of pre-race loading covers that much time on feet alone.

How it works

Your muscles and liver store a limited amount of carbohydrate as glycogen, which is the primary fuel source for sustained running effort. Increasing carbohydrate intake while keeping training minimal in the final days before a marathon allows those stores to fill more completely than they would under normal training and eating. This is part of why carb loading is paired with tapering rather than done in isolation; if you're still running long, hard sessions in the same window, you're burning through glycogen about as fast as you're topping it up, which blunts the effect considerably. The combination of reduced training load and increased carbohydrate intake is what actually produces fuller stores, not the extra carbohydrate on its own.

A normal side effect: temporary water weight

Glycogen is stored in your muscles along with some water, so a common and entirely normal effect of carb loading is a small, temporary increase in body weight and a feeling of being slightly fuller or heavier in the day or two before the race. This isn't fat gain and isn't something to worry about or try to counteract by eating less; it's part of how the fuel is actually stored, and it typically comes back down once you start racing and depleting those stores. If you weigh yourself in race week and notice a small uptick, that's a normal sign the loading is working, not a reason to cut back.

A simple, practical approach

For most recreational marathoners, carb loading doesn't need to be complicated or extreme. A practical version looks like shifting the balance of your meals toward more carbohydrate-rich foods, rice, pasta, bread, potatoes, and fruit, for the two to three days before the race, while keeping portion sizes reasonable rather than dramatically overeating. In practice, this might mean a slightly larger serving of rice or roti at lunch and dinner, adding a banana or extra fruit as a snack, and choosing a carbohydrate-forward breakfast like poha, upma, or oats over a protein-heavy one. The goal is a moderate shift over several meals, not one enormous carbohydrate binge the night before, which tends to just leave you feeling heavy and sleeping poorly.

A sample 3-day loading plan

Seeing the shift laid out day by day can make it more concrete than a general instruction to eat more carbohydrate. This is one reasonable template; adjust portions and specific foods to whatever you already eat and know sits well.

• Increase the proportion of carbohydrates at each meal, rather than adding entirely new, unfamiliar foods
• Keep meals familiar; this isn't the time to try new dishes or cuisines
• Don't confuse carb loading with simply eating more of everything
• Spread the increase across two to three days rather than one large meal the night before
• Pair it with reduced training volume during tapering, since you're not burning through glycogen as fast

Common mistakes

Overeating in general, rather than specifically shifting toward carbohydrates, is a common mistake that can leave runners feeling heavy or bloated on race morning rather than well-fuelled. Trying unfamiliar high-carb foods for the first time during loading days is another; stomach issues from an unfamiliar meal are a bigger risk than slightly under-loading. A third common mistake is loading heavily on foods that are also high in fibre or fat, on the assumption that more carbohydrate automatically means more of everything; beans, very high-fibre wholegrain products, or rich, oily preparations can all cause the same bloating and discomfort as general overeating, even though they're technically carbohydrate sources too.

If carb loading upsets your stomach

Some runners find that even a moderate increase in carbohydrate, particularly from less familiar sources, causes bloating, gas, or looser stools in the day or two before a race. If this happens, the fix is usually to simplify rather than push through: drop back to your most familiar, plainest carbohydrate sources, rice, toast, banana, rather than experimenting with new ones, reduce portion sizes slightly, and prioritize feeling comfortable over hitting a specific target. A comfortable stomach on race morning is worth more than a maximally topped-up glycogen store achieved at the cost of feeling unwell at the start line.

How this fits into race week overall

Carb loading is just one part of a broader race week eating plan; see what to eat the week before a marathon for a fuller day-by-day approach, and how to fuel a marathon for what happens during the race itself. Getting carb loading right but ignoring hydration, sleep, or the rest of your race week routine still leaves gaps; treat it as one piece that works alongside tapering, rest, and your usual pre-race habits, not a substitute for any of them.