An Ironman is the ultimate endurance test: 3.8km swim, 180km bike, 42.2km marathon run. You'll be racing for anywhere between 8 and 17 hours, burning 8,000-12,000 calories, and pushing your body to its absolute limits.
At this level, nutrition isn't just important - it's a fourth discipline. That 30-50% of Ironman DNFs are nutrition-related. What you eat in the days and hours before the race determines whether you'll cross that finish line strong or struggle from halfway through the bike.
Here's the comprehensive, evidence-based approach to pre-Ironman nutrition.
The Scale of the Challenge
Your body stores approximately 2,000 calories of glycogen in muscles and liver - enough fuel for roughly 90-120 minutes of intense exercise. An Ironman takes 8-17 hours. The maths is brutal: you'll burn through your stored carbohydrate multiple times over.
This means:
- Maximum glycogen loading before the race is essential
- Your race nutrition strategy must be practised and dialled in
- Pre-race eating sets you up for success or failure from the first stroke
Research from the International Journal of Sport Nutrition and Exercise Metabolism shows that starting an Ironman with maximised glycogen stores improves performance in the later stages - when races are won or lost.
The Week Before: Carb Loading Done Right
Traditional carb loading (eating massive amounts of pasta) is outdated. Modern research supports a more strategic approach.
7-4 Days Before
Continue eating normally with a slight increase in carbohydrate intake. Aim for 5-7g carbs per kg bodyweight daily. For a 75kg athlete, that's 375-525g of carbs per day. Taper your training but maintain food intake - this naturally creates glycogen surplus.
3-2 Days Before
Increase carbohydrate intake to 8-10g per kg bodyweight. For our 75kg athlete, that's 600-750g of carbs daily. This is where proper loading happens. This approach can increase glycogen stores by 150-200% above normal.
The Day Before
Continue high carbohydrate intake (8-10g/kg) but reduce fibre and fat to minimise GI risk on race day. Eat familiar foods only. Stay very well hydrated - check urine colour is pale. Early dinner, early bed.
Race Week Carb Loading Schedule
5-7g carbs/kg/day. Normal training tapering. Example for 75kg: breakfast with oats, lunch with rice/pasta, dinner with potatoes/pasta, snacks of fruit and bread.
8-10g carbs/kg/day. Maximum loading phase. Add extra portions of rice, pasta, potatoes, bread. Reduce fat and fibre. You may gain 1-2kg of water weight - this is normal and good (glycogen binds with water).
Carb-rich dinner, but not enormous. Eat early (6pm). Familiar foods only. White pasta with tomato sauce, rice dishes, jacket potato. Avoid high-fibre, spicy, or gas-producing foods. Drink to thirst - don't over-hydrate.
2-4g carbs/kg bodyweight. Easily digestible, familiar breakfast. Final top-up of glycogen stores. Sip fluids but don't over-drink.
Good Carb Loading Foods
High-Carb Meals
White Pasta with Tomato Sauce
The classic. Low fibre, high carb. Keep sauce simple - no heavy meat or cream.
White Rice Dishes
Risotto, rice with chicken, sushi, rice pudding. Very well tolerated and carb-dense.
Jacket Potato
Skip the skin for lower fibre. Simple toppings - beans, tuna, or just butter.
Pancakes or Waffles
Carb-dense breakfast option. Maple syrup or honey for extra carbs.
High-Carb Snacks
Bagels with Jam
Dense carbs, easy to eat, highly palatable. White bagels for race week.
Rice Cakes
Very low fibre, pure carbs. Top with honey or jam.
Bananas
Quick carbs, potassium, easy to digest. Perfect race week snack.
Sports Drinks
Help with both carb loading and hydration. Useful if struggling to eat enough solid food.
Race Morning: The Final Fuelling
Most Ironman races start around 7am. You need to eat 3-4 hours before - that means a 3-4am alarm for breakfast. This is non-negotiable: your last chance to top up glycogen.
Race Morning Meal Guidelines
- Timing: 3-4 hours before race start
- Amount: 2-4g carbohydrate per kg bodyweight
- Character: Familiar, low-fibre, low-fat, easily digestible
- Hydration: Sip fluids, don't gulp. 500-750ml over the hours before start
Race Morning Breakfast Options
Porridge with Banana & Honey
Made with water, not milk (easier digestion). Well-cooked oats, mashed banana, honey for extra carbs.
White Toast with Jam/Honey
Simple, reliable, easy to eat at 4am. Multiple slices to hit your carb target.
Bagels with Peanut Butter
Dense carbs, some protein and fat for sustained energy. A little fat is fine 3-4 hours out.
Rice with Eggs
Some athletes prefer savoury. White rice, scrambled eggs, keep it simple.
Final Hour Protocol
In the final 30-60 minutes before start, many athletes consume a small amount of easily digestible carbs:
- Dates or natural energy gel 15-30 minutes before swim start
- Half banana
- Few sips of sports drink
- Some sweets (jelly babies work well)
This tops up blood glucose without adding volume to your stomach. Test this in training - some athletes do better without anything close to start.
What to Avoid Before an Ironman
- New foods: Race week is NOT the time to experiment. Only eat what you've tested in training
- High-fibre foods: Whole grains, bran, raw vegetables, beans can cause GI distress over 8+ hours
- High-fat foods: Slow digestion. No fry-ups, pastries, or cream sauces
- Excessive protein: Harder to digest, not needed for immediate energy
- Alcohol: Impairs glycogen storage and hydration. None in race week
- Over-hydration: Can cause hyponatremia. Drink to thirst, not beyond
- Large race morning meal: Too much food too close causes GI distress. Start eating early, finish 3 hours before
- Excessive caffeine: Some is fine, too much causes GI issues and dehydration
Hydration Strategy
Hydration for Ironman starts days before the race, not race morning. Proper hydration supports glycogen storage (glycogen binds with water) and ensures you start the race in optimal fluid balance.
- Race week: Drink to keep urine pale yellow. Don't force excess fluid
- Night before: Normal fluid intake with dinner. Stop drinking 2-3 hours before bed
- Race morning: 500-750ml in the hours before start. Don't drink excessively
- Final hour: Small sips only. Your race nutrition plan takes over from here
Caffeine
Caffeine is ergogenic for endurance performance. However, for an 8-17 hour race, timing matters. Many athletes save caffeine for the run (when fatigue peaks) rather than the swim start.
If using caffeine race morning, 3mg/kg bodyweight 60 minutes before start is the standard recommendation. But consider: if you're starting at 7am, you may want caffeine later in the race when you need it most.
Your Pre-Ironman Nutrition Summary
Week before: 5-7g carbs/kg days 7-4, increase to 8-10g/kg days 3-1. Reduce fibre and fat approaching race day. Night before: Carb-rich familiar dinner, early (6pm), moderate portion. Race morning: 2-4g carbs/kg, 3-4 hours before start. Porridge, toast, bagels - familiar foods only. Final hour: Optional gel or small carb snack 15-30 minutes before. Hydration: Start hydrating days before. Drink to thirst, not beyond. Urine pale yellow.
After your race, recovery becomes critical. Read our guide on what to eat after an Ironman to support your recovery.
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