If you are going to run a Half-Marathon (13.1 miles) or a Marathon (26.2 miles), you have probably heard that you need to “eat a lot of carbs” in the days leading up to the race. The famous carbo-loading is one of the most validated nutritional strategies in sports science for delaying fatigue. But the lingering question is: how do you fill the energy tank without gaining weight that will hurt your pace?
The secret is not simply eating more, but understanding the physiology of how the body stores this fuel.
The Physiology of Loading: Why Does the Scale Go Up?
The first thing you need to accept is: if carbo-loading is done right, you will gain weight on the scale. But relax, this is not fat.
When you consume carbohydrates, your body stores them in the muscles and liver as glycogen. The golden rule of physiology here is: every 1 gram of stored muscle glycogen carries with it about 3 grams of water. And unlike creatine, which works during short bursts of activity, carb-loading sustains prolonged effort.
Therefore, if you manage to store an extra 400g to 500g of glycogen before the race, it is perfectly normal and expected for the scale to go up by 1.5 kg to 2 kg (3.3 to 4.4 lbs). This extra weight consists of pure energy and intracellular hydration — exactly what you will burn and sweat out during the race.
The Biggest Mistake: Eating “More” Instead of Eating “Differently”
The gastric discomfort and lethargy reported by runners are almost never the fault of the carbohydrates, but rather how the diet is structured. The most common mistakes include:
- The Pizza and Fast-Food Party: Increasing carbs by eating pizzas, burgers, and pastas with heavy sauces adds an enormous amount of fat to your diet. Fat delays gastric emptying, causing that bloated feeling.
- Excess Fiber: Eating large volumes of whole foods, oats, and salads the day before and especially right before the race creates residue in the gastrointestinal tract. This residue can cause discomfort (and even urgent bathroom trips) during the run.
- Loading Only on the Night Before: The body needs time to synthesize glycogen. The famous giant plate of pasta at 9:00 PM the night before the race will increase your glycogen, but it would be much more efficient if you had been working on this increase throughout the entire previous day, not just at dinner.
The Efficient Protocol: How to Do It in Practice
To optimize your stores without overloading your stomach, follow the 36 to 48-hour carbo-loading protocol before the starting gun:
- Adjust the volume, not just the calories: To reach the recommended 8 to 10 grams of carbohydrates per kilogram of body weight, you will need to reduce your protein and fat intake. The plate is no longer balanced and becomes predominantly composed of carbohydrates.
- Swap complex for simple: The day before the race, swap brown rice for white rice (if you use brown), whole wheat bread for white bread, and reduce salads and leafy greens. Refined carbohydrates digest quickly, leave less residue in the gut, and fill your stores cleanly.
- Use liquid calories: Sports drinks, 100% grape juice, and even supplements like maltodextrin throughout the day help you hit your carbohydrate goal without having to chew mountains of food.
| Food Category | AVOID (36h Pre-Race) | PREFER (Easy digestion) |
|---|---|---|
| Carbohydrates & Grains | Whole grain carbs like brown rice, whole wheat pasta, and oats | Refined carbs (white rice, white bread, regular pasta) |
| Fruits & Vegetables | Raw salads and high amounts of vegetables | Cooked options in reduced amounts |
| Proteins | Fatty meats and heavily aged or high-fat cheeses | Leaner proteins like chicken breast, low-fat meats, and fish |
| Snacks | Fatty snacks like cookies and biscuits | Fruit jam, dulce de leche, honey |
Practical Verdict
Acute weight gain during carbo-loading is a sign that the strategy worked: you are full of water and glycogen.
To avoid bloating, ditch the excess fat and fiber in the 48 hours pre-race, hydrate adequately, and distribute the intake of easily digestible carbohydrates throughout the day. The real weight that hurts your pace is the weight of a poorly executed digestion.
By Gabriel Bueno Bocchi, Registered Dietitian / Sports Nutritionist (University of Campinas – UNICAMP)
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