Carb Loading for Endurance Events
Carbohydrate loading is one of the most evidence-based nutritional strategies available to endurance athletes. Done correctly, it can dramatically improve performance. Done incorrectly, it leads to digestive discomfort, bloating, and suboptimal glycogen stores on race day.
This guide covers everything you need to know, from the science behind carb loading to a practical loading chart for events ranging from a 5k to a multi-day ultra.
What Is Carb Loading and Why Does It Work?
Carbohydrate loading is the practice of increasing carbohydrate intake in the days before a race or high-intensity event to maximise glycogen stores in the muscles and liver. Given that glycogen is the primary fuel source for moderate-to-high intensity exercise, arriving at your start line with fully topped-up stores can delay fatigue and improve sustained performance.
It is a proven strategy to increase endurance performance — but it comes with a few considerations.
Which Events Benefit From Carb Loading?
Carb loading is relevant across a wide range of endurance disciplines:
- 5k / 10k races
- Half marathon
- Full marathon and ultra-distance events
- Cycling events
- Ironman triathlon
- Long training runs
- HYROX and functional fitness competitions
How Much Should You Carb Load?
The scientific literature broadly recommends 8–12g of carbohydrates per kilogram of body weight to fully restore glycogen levels. However, the optimal amount varies based on your training history, previous carbohydrate intake, and the length of the event you're preparing for.
A practical starting point for most athletes and shorter events is 7–9g/kg/bw. The higher end of the range (10–12g/kg/bw) is more appropriate for elite athletes or those who have completed multiple carb loading cycles previously.
The Top 5 Carb Loading Mistakes
Even experienced athletes regularly make these errors when approaching carb loading:
- Eating too much fibre — increases GI discomfort and bloating in the days before a race
- Not eating enough carbs — underloading means starting the race with partially depleted glycogen stores
- Eating too much fat — displaces carbohydrate calories and slows gastric emptying
- Only loading the day before — a single day is rarely sufficient for complete glycogen replenishment
- Carb loading for too long — extending the loading phase unnecessarily increases GI issues without additional benefit
In addition to these, failing to taper training properly in the lead-up to race week can also compromise preparation — even if your nutrition strategy is sound.
The Carb Loading Chart by Event
The following chart outlines the recommended carbohydrate loading protocol across different event types, working back from race day. Reducing intake the day before the event is advisable to minimise GI discomfort, as the effects of increased muscle glycogen storage can persist for 3–5 days.
- 5–10k Mon to Sat: 4–5g/kg/bw throughout the week
- Half Marathon / HYROX Mon–Thu: 4–5g/kg/bw | Fri: 5–7g/kg/bw | Sat: 4–5g/kg/bw
- Marathon Mon–Thu: 4–5g/kg/bw | Fri: 7–9g/kg/bw | Sat: 4–5g/kg/bw
- Ultra / Ironman Mon–Tue: 4–5g/kg/bw | Wed–Fri: 7–9g/kg/bw | Sat: 5–7g/kg/bw
- Multi-Day Event Mon–Tue: 4–5g/kg/bw | Wed–Sat: 7–9g/kg/bw
All amounts are daily totals expressed in grams per kilogram of body weight (g/kg/bw), spread across all meals.
Why Spread Carb Loading Over Several Days?
Spreading your carbohydrate loading over a longer period — rather than attempting to consume everything in a single day — is considerably easier on the digestive system and helps avoid GI discomfort on race day. The increased muscle glycogen storage effect can last 3–5 days depending on activity level during the taper period.
Final Word
Carb loading is a powerful tool when used correctly. Know your event distance, calculate your individual carbohydrate target, reduce fibre and fat intake during the loading phase, and begin the process early enough to allow full glycogen saturation without compromising digestive comfort on race day.
Pair your carb loading strategy with quality running fuel — including energy gels, bars, and recovery nutrition — to ensure your body is fully prepared to perform from the starting gun to the finish line.