The night before a big race, every amateur endurance athlete does the same thing: they eat a massive bowl of pasta. Sometimes two. They've heard about carbohydrate loading. They know glycogen is the primary fuel for racing. What they're less clear on is whether what they're doing actually works, and the answer, based on current evidence, is: sometimes, and not in the way most people think.
Carbohydrate loading has solid scientific support for specific race distances and conditions. It's also one of the most commonly misapplied nutritional strategies in recreational sport. Understanding the actual physiology helps you decide whether to do it, and how to do it correctly.
What carbohydrate loading actually does
Your muscles store glycogen, the polymerised form of glucose, at rest. An average trained athlete stores approximately 400–500g of glycogen total: roughly 300–400g in muscle tissue and 80–110g in the liver. At moderate racing intensity (around marathon pace), glycogen provides the majority of fuel and at typical depletion rates, muscle glycogen runs critically low around the 90-minute to 2-hour mark of continuous exertion.
This is "the wall", glycogen depletion forcing a shift to fat oxidation, which is slower and produces noticeably reduced power output. The goal of carbohydrate loading is to expand glycogen stores above normal resting levels before a race, pushing the depletion point further into the event.
Well-executed carbohydrate loading can increase muscle glycogen concentration by 20–40% above normal training levels. In practical terms, this can delay glycogen depletion by 20–30 minutes in a race lasting 2+ hours, a meaningful performance advantage.
The original protocol vs. modern evidence
The classic carbohydrate loading protocol, developed in the 1960s, involved a 3-day depletion phase (very high training load, low carbohydrate) followed by a 3-day loading phase (rest, very high carbohydrate). The theory was that glycogen depletion would "supercompensate", causing muscles to store more glycogen than normal when carbohydrates were reintroduced.
Modern research has rendered the depletion phase unnecessary. Studies from the 1980s onward demonstrated that trained athletes can achieve equivalent glycogen supercompensation with 2–3 days of high carbohydrate intake plus exercise tapering alone, without the preceding depletion phase. The depletion protocol adds fatigue, muscle damage, and immune suppression in the days before a race, a significant cost for no additional benefit.
Current best-practice involves 36–48 hours of high carbohydrate intake (10–12g/kg/day) combined with training taper. The depletion phase is not recommended.
Who actually benefits from carb loading
Carbohydrate loading produces measurable performance benefits only for events lasting 90 minutes or longer at moderate-to-high intensity. The research is clear on this boundary.
For events shorter than 90 minutes, normal resting glycogen stores are sufficient to fuel the effort completely. Carb loading before a sprint triathlon, a 5km, or a 10km race adds calories without performance benefit. The extra glycogen simply isn't needed.
Athletes who benefit most:
- Marathon runners (particularly at paces where glycogen is the dominant fuel)
- Ironman and Half-Ironman triathletes
- Cyclists doing events of 3+ hours
- Ultra-distance runners (though fat adaptation strategy may apply here instead)
- Team sport athletes doing 90+ minute events (though application is more complex)
The 48-hour carb loading protocol
Based on current evidence, the most practical and effective protocol for non-elite athletes:
2 days before race day (D-2): Carbohydrate intake of 8–10g/kg. Training should be light (30–40 min easy, or complete rest). Focus on familiar, easily digestible carbohydrates. Total calorie intake will be significantly above normal, this is expected and necessary.
1 day before race day (D-1): Carbohydrate intake of 10–12g/kg. Rest completely or do only a 15-20 minute shakeout run. Distribute carbohydrates evenly across 4–5 meals. Reduce dietary fat and fibre to make room for the higher carbohydrate volume without exceeding stomach capacity.
For a 70kg athlete, 10–12g/kg means 700–840g of carbohydrates on the day before the race. This is a significant volume of food. Planning meals in advance is essential.
What to eat and what to avoid
Food choices during carb loading should prioritise digestibility and glycogen storage efficiency. The best options are high glycaemic index, low fibre, low fat, and low protein (relative to normal training days):
- Best choices: White rice, white pasta, white bread, bagels, potatoes (without skin), banana, sports drinks, rice cakes, pretzels, pancakes, porridge
- Moderate: Sourdough bread, sweetened yoghurt, fruit juices, sports bars
- Avoid during loading: High-fibre vegetables (broccoli, beans, lentils), whole grain options, high-fat foods (avocado, nuts, cheese, fried food), large portions of meat or dairy
The reason to avoid fibre is twofold: it slows carbohydrate absorption (reducing loading efficiency) and increases bowel content, raising GI distress risk on race morning.
The GI risk and how to manage it
Gastrointestinal distress is the most common side effect of carbohydrate loading, and it can be race-destroying. High carbohydrate intake increases water retention in muscles (glycogen is stored with approximately 3–4g of water per gram), leading to a bloated feeling and temporary weight gain of 1–2kg. This is normal and expected.
To minimise GI risk:
- Eat familiar foods you've used in training, race week is not the time to experiment
- Distribute intake across 4–5 smaller meals rather than 2–3 large ones
- Avoid foods with artificial sweeteners (sorbitol, xylitol), they cause osmotic diarrhoea
- Stay well hydrated, aim for pale yellow urine throughout the loading phase
- Avoid alcohol completely, it impairs glycogen storage and disrupts sleep
Race morning nutrition
The pre-race meal is distinct from carbohydrate loading. Its purpose is to top up liver glycogen (which is depleted overnight during sleep) and maintain blood glucose, not to add meaningfully to muscle glycogen.
Evidence-based pre-race breakfast: 1–4g/kg carbohydrates consumed 2–4 hours before the gun. Timing matters, eating too close to race start can cause reactive hypoglycaemia in some athletes. Familiar foods, low fat and fibre, moderate protein.
Classic options: oats with banana and honey, white toast with jam, bagel with jam, rice with fruit. Coffee is fine, and may improve performance directly through caffeine's ergogenic effects.
Common mistakes
The most frequent carb loading errors among amateur athletes:
- Loading for races that don't need it: Events under 90 minutes don't benefit. You'll race heavier and feel sluggish.
- Relying solely on the pre-race dinner: One large pasta meal the night before provides only a fraction of the glycogen increase achievable with a 48-hour protocol.
- Eating too much fat with the carbohydrates: A pasta dish loaded with cream sauce slows gastric emptying and blunts glycogen storage.
- Trying new foods: Race week is the worst possible time to discover a food intolerance.
- Panicking about weight gain: The 1–2kg of water weight from glycogen storage is fuel, not fat. It will be used during the race.
Carbohydrate loading, done correctly, is one of the few legal and highly effective performance interventions in endurance sport. The science is solid. The execution just needs to be deliberate.