A Runner’s Guide to Carbo-loading

If you are training for a half marathon, marathon, or any race lasting longer than 90 minutes, the meals you eat in the final 36 to 48 hours before the gun can matter almost as much as the miles you logged in training. That window is built for carbohydrate loading — a well-researched strategy that maximizes the glycogen stored in your muscles and liver so you can sustain your pace for longer before fatigue forces you to slow down.

Carbohydrate is the primary fuel for moderate- to high-intensity running. Your body stores it as glycogen, but only in limited amounts — roughly enough to power a hard effort for about 90 minutes to two hours. Once those stores run low, your pace collapses. Runners call it “hitting the wall,” and it is a direct consequence of glycogen depletion, as researchers modeling marathon metabolism have documented.

Why carb-loading matters

The goal of carbohydrate loading is to “supercompensate” your glycogen stores above their normal level, pushing back the moment you run out of fuel. According to the Gatorade Sports Science Institute, athletes seeking supercompensation should consume 10 to 12 grams of carbohydrate per kilogram of body mass per day for 36 to 48 hours before competition. The Victorian Government’s Better Health Channel and the Australian Sports Commission’s AIS nutrition team both echo similar targets of 8 to 12 g/kg/day for endurance athletes. Done correctly, a full load can raise muscle glycogen well above baseline and has been shown to improve endurance capacity — meaning you can hold race pace for longer before exhaustion sets in.

It is worth being clear about what carb-loading can and cannot do. It will not make you faster over a 5K, and it will not magically eliminate cramps or gastrointestinal issues. Its benefit is specific to sustained efforts lasting longer than about 90 minutes, where glycogen — not oxygen delivery or raw speed — is the limiting factor.

How to do it properly

Timing — 36 to 48 hours out: Begin the load a day and a half to two days before your race, paired with a taper in training. Resting legs combined with high carbohydrate intake is what allows glycogen stores to rise above their normal level.

Amount — 8 to 12 g of carbs per kg of body weight: For a 70 kg (154 lb) runner, that is roughly 560 to 840 grams of carbohydrate per day — far more than a normal diet. Most of your calories in this window should come from carbohydrate.

Swap to low-fiber, refined carbs: Trade whole grains, beans, and bulky vegetables for white rice, white pasta, white bread, and peeled potatoes. Lower fiber means faster gastric emptying and less bloating, which keeps your stomach settled on race morning.

Keep fat and protein moderate: Fat slows digestion dramatically, so keep meals lean with chicken breast, egg whites, or tofu. Save richer, celebratory meals for after the finish line.

Stay hydrated: Every gram of glycogen stored binds roughly three grams of water, so expect a small, normal weight gain of 1 to 2 kg. Sip fluids and electrolytes throughout — that water is part of what makes the load work.

Race morning

Two to three hours before the start, top off liver glycogen with 1 to 4 grams of carbohydrate per kilogram of body weight from something easily digested — white toast with honey, a banana, or a small bowl of rice porridge. Then stick to your usual in-race fueling plan. Carb-loading does not replace gels and drinks during the event; it simply gives you a fuller tank to start with.

Rehearse it in training

The single most important rule: nothing new on race day. Carb-loading is a skill, and your gut needs to practice it just like your legs practice the long run. Try your full loading protocol during a peak long-run weekend four to six weeks before your race so you learn which foods sit well and which cause trouble. Some athletes experience GI distress from the sheer volume of food, and a small number feel sluggish from the extra weight — both are reasons to test it under training conditions first, not on race morning.

Who should (and shouldn’t) carb-load

Carb-loading is built for events lasting longer than 90 minutes — half marathons, marathons, long triathlons, and ultramarathons. For a 5K or 10K it offers little benefit, and the extra weight may even slow you down. If your event is under an hour, focus instead on your normal balanced diet and a light pre-race meal.

Done right, carbohydrate loading is one of the few nutrition strategies with decades of solid evidence behind it. Load for the right race, rehearse it in training, keep your meals low-fiber and high-carb, and you will give yourself the best possible chance of crossing the finish line strong instead of hitting the wall.