Endurance carb loading.

Carb loading is a pre-race nutrition strategy that maximizes muscle glycogen (your stored carbohydrate) before an endurance event lasting longer than 90 minutes. You eat 6 to 12 grams of carbs per kilogram of body weight per day for 24 to 48 hours before the race while reducing training volume. Modern sport-nutrition research shows a single day at the higher end (10 to 12 g/kg) is usually enough. The older 7-day depletion-then-load protocol is no longer recommended.

Yes. A marathon takes most runners 3 to 5 hours at moderate-to-high intensity, right in the range where muscle glycogen becomes the limiting fuel. Loading 8 to 10 grams of carbs per kilogram of body weight per day for 24 to 48 hours before the race gives your muscles enough stored fuel to hold pace deeper into the race and lower the risk of hitting the wall around mile 20.

For most recreational half marathoners finishing in over 90 minutes, a moderate carb load of 6 to 8 grams per kilogram of body weight the day before is worthwhile. If your finish time is under 90 minutes, a normal high-carb dinner the night before is usually enough and a full carb load is not required.

24 to 48 hours before the race. Older protocols asked for a full week of glycogen depletion followed by loading, but modern research (Fairchild 2002 and follow-ups) confirmed a single day of high carb intake with reduced training is enough to top off muscle glycogen. Two days is a comfortable window. One day works if you are short on time.

Lower-fiber carbohydrate sources are easiest on your gut and keep the race-morning bathroom stops down: white rice, potatoes, pasta, refined bread, bagels, bananas, sports drinks, fruit juice, and cereal. Cut back on beans, whole grains, and raw vegetables in the last 48 hours. Keep protein at your normal maintenance level (around 1.6 grams per kilogram of body weight) and lower fat so the extra energy comes from carbs.

For endurance events longer than 90 minutes, aim for 30 to 60 grams of carbs per hour for events up to 2.5 hours, and 60 to 90 grams per hour beyond that. In ultras over 8 hours, mix gels and drinks with solid food to keep your stomach happy. Fluid target: 400 to 800 mL per hour. Sodium: 300 to 800 mg per hour depending on your sweat rate. Practice this intake on long training runs before race day.

Yes, temporarily. Expect 1 to 3 kg (2 to 7 lb) of water weight from stored glycogen. Each gram of glycogen carries about 3 grams of water with it, so a full glycogen top-off shows up on the scale. It is not fat gain, and the extra weight is back to baseline within a day or two after the race. The water and glycogen are exactly the fuel your muscles will pull from during the event.