A full Ironman is not a race you finish on training alone. You train to give yourself the aerobic ceiling. You fuel to defend it for 10 to 14 hours. The two are not the same problem, and most DNFs and death-march marathons come from confusing them.
The physiology is unforgiving. You will burn 6,000 to 10,000 kilocalories on the course. You store roughly 500g of glycogen. Sodium losses can exceed 12,000mg in the heat. Nothing about that maths works unless you fuel like it is a job.
The two numbers that decide your finish
Carbohydrates per hour. Every extra 15 to 20g of tolerated carbohydrate per hour translates into meaningful pace preservation over the marathon leg. Target 70 to 90g/hr on the bike, 40 to 60g/hr on the run. Elite athletes with a rehearsed gut can go to 100 to 120g/hr on the bike.
Sodium per hour. Target 700 to 1,200mg/hr on the bike and 500 to 800mg/hr on the run. The single biggest driver of "the wheels came off" is sodium depletion compounding hour after hour. Learn your losses with our sweat rate estimator.
Everything else - caffeine, gels, aid station Coke, cold water sponges - is optional and personal. These two are not.
Race morning: the 3 hours before the cannon
3 to 4 hours before start: Familiar high-carb breakfast at 1 to 2g of carbohydrate per kg bodyweight. Oats with honey and banana, white toast with jam, or rice with eggs. Low fibre, low fat, no experiments. The night before, follow a structured carb-loading protocol.
25 to 40 minutes before your wave: One serve of HYDRAX PRIMER in 400 to 500ml water. 29g dual-carb, 1,313mg sodium. This pre-loads plasma volume before the first stroke of the swim. Skipping this window costs you sodium reserve you cannot rebuild once the race is under way.
The bike leg: 4.5 to 7 hours of fuelling opportunity
The bike is the only leg where you can comfortably take in high rates of carbohydrate. What you bank on the bike is what carries you through the marathon. Miss it here and no amount of on-run heroics recovers it.
Target: 70 to 90g of carbohydrate per hour (up to 120g/hr for rehearsed elite athletes) and 700 to 1,200mg sodium per hour, delivered mostly through HYDRAX CORE in 750ml bottles. Two scoops per bottle = 85g carb + 980mg sodium.
The dual-carb mechanism matters most here. A single carbohydrate source saturates the SGLT1 intestinal transporter at 60g/hr. Adding fructose activates the GLUT5 transporter, lifting total absorption to 90 to 120g/hr. This is why HYDRAX CORE uses the peer-reviewed 2:1 maltodextrin-to-fructose ratio. Deep dive in why dual-carbohydrate transporters matter.
Special-needs bag: pre-mix a spare bag of CORE powder to refill a bottle at the halfway point. Do not depend on aid station drink for your primary sodium.
Full Ironman fuel plan at a glance
| Segment | Carb Target | Sodium Target | HYDRAX Protocol |
|---|---|---|---|
| Breakfast (3-4h pre) | 1-2g/kg | , | Familiar high-carb meal |
| 25-40 min pre-swim | 29g | 1,313mg | 1 serve PRIMER in 400-500ml |
| Bike hours 1-2 | 80-90g/hr | 900-1200mg/hr | 1 bottle CORE per hour (2 scoops in 750ml) |
| Bike hours 3-5 | 70-90g/hr | 700-1000mg/hr | CORE + optional small solid food (banana, rice cake) |
| Last bike hour | 60g/hr | 700mg/hr | CORE only - set up for the run |
| Run hours 1-2 | 50-60g/hr | 600-800mg/hr | Dilute CORE at aid stations + carried flask |
| Run hours 3+ | 30-50g/hr | 500-700mg/hr | Small, frequent sips; drop carb concentration before dropping volume |
Tune the plan to your experience
The ranges above are the working envelope. Where you sit inside them depends on how much you have rehearsed high-rate fuelling in training. Do not race a number you have not trained. Build tolerance progressively with our gut training guide.
| Experience | Bike carbs/hr | Run carbs/hr |
|---|---|---|
| Beginner (first full Ironman) | 70-90g | 30-50g |
| Intermediate | 90-110g | 50-75g |
| Experienced (rehearsed high-carb gut) | 110-120g | 75-100g |
Aid stations: know them, don't rely on them
Read the athlete guide the week before. Know which brand of sports drink, gel and food is on course and at which kilometre markers. That is your plan B.
Plan A is self-sufficiency. Over 10 to 14 hours, aid stations run out, hand-ups get missed, and on-course drink concentration drifts. Carry enough HYDRAX CORE in bike bottles, jersey pockets and your special-needs bag to hit carbs and sodium end-to-end using only products you have trained with.
Pro tip: the T2 soft flask
Train the run with a soft flask carrying a concentrated serve of HYDRAX CORE - one full serve in 150 to 250ml of water. On race day, place the pre-made flask in your T2 bag. You leave transition with a full hour of carbs and sodium in your hand, independent of aid station timing.
Practise it on long bricks so the concentration and mouthfeel are familiar. It is the single highest-yield run-fuelling trick most Ironman athletes never rehearse.
Build a plan tailored to you
This guide is the framework. For a plan built to your body weight, sweat rate, race course and finish target, use the HYDRAX Fuelling Strategy Engine, estimate losses with the sweat rate estimator, and structure the 48 hours before with our carb loading protocol.
Ironman-day fuel is a system. PRIMER before. CORE during. Order now, ships Australia-wide.
The marathon: where the plan pays out
Running impact slows gastric emptying. Attempting to hit 90g/hr on an Ironman marathon is a fast route to a portaloo. Realistic target is 30 to 60g of carbohydrate per hour, delivered as small sips at every aid station, not as gulped-down 500ml top-ups every 3km.
Sodium remains critical on the run. If cramping starts, it is almost never a lack of carbohydrate - it is sodium. A dilute serve of HYDRAX CORE in a handheld flask, sipped every kilometre or two, keeps both taps open.
Rule of thumb: drop concentration before you drop volume. Water and sodium keep working even when the stomach is refusing high-carb loads.
The 5 most common Ironman fuelling mistakes
1. Waiting to feel hungry. By the time you feel hungry on the bike, you are already 30 to 45 minutes behind on carbs. Fuel on the clock, not the feeling.
2. Under-doing sodium. Standard sports drinks replace 15 to 25% of actual losses. Over 10 hours, that gap compounds into cramp, fog, and pace collapse.
3. Relying on aid station variety. Cola, gels, salt tabs, bars, banana chunks. Every added variable is a GI experiment. Pick one race-day plan, rehearse it, execute it.
4. Under-fuelling the first bike hour. The bike hours 1 and 2 are when the gut is at its most cooperative. Miss them and you cannot make it back.
5. Untrained gut. High-carb intake is a skill. If your training has not rehearsed 90g/hr under race pace, do not attempt it on race day. See gut training for endurance fuelling.
Related resources
- Ironman 70.3 nutrition plan
- IRONMAN Cairns fuelling plan
- IRONMAN Australia Port Macquarie fuelling plan
- IRONMAN Western Australia fuelling plan
- Carbs per hour for Ironman-distance events
- How much sodium endurance athletes actually need
- Carb loading in the 48 hours before race day
- Fuelling Strategy Engine
Ironman-day fuel is a system. PRIMER before. CORE during. Order now, ships Australia-wide.
Frequently Asked Questions
How many carbs per hour do I need for a full Ironman?
For most age group athletes finishing between 10 and 14 hours, target 70 to 90g of carbohydrate per hour on the bike and 40 to 60g per hour on the run. Elite athletes with rehearsed gut tolerance can push toward 100 to 120g per hour on the bike using a dual-carb source such as HYDRAX CORE.
How much sodium do I need for a full Ironman?
Sodium losses over an Ironman can exceed 8,000 to 15,000mg total for a high sweater in hot conditions. Aim for 700 to 1,200mg per hour on the bike and 500 to 800mg per hour on the run. HYDRAX CORE delivers 980mg sodium per serve, making it straightforward to hit those numbers.
When should I start eating on race morning?
Have a familiar high-carb breakfast 3 to 4 hours before your swim wave at 1 to 2g carb per kg bodyweight. Then take one serve of HYDRAX PRIMER in 400 to 500ml water 25 to 40 minutes before the swim start.
Should I eat solid food during an Ironman?
Some athletes tolerate small amounts of solid food on the bike (bananas, rice cakes, energy bars) in the first 90 to 120 minutes when gut motility is highest. From mid-bike onwards and throughout the run, liquid fuel such as HYDRAX CORE is generally better tolerated and absorbed faster.
How do I avoid bonking on the marathon leg?
Front-load carbs on the bike, hit your sodium target every hour, and do not let carbohydrate intake drop below 40g per hour on the run. Bonking on an Ironman marathon is almost always the consequence of accumulated under-fuelling from earlier in the race.
How does HYDRAX compare to the on-course drink at Ironman events?
Most on-course sports drinks are single-carbohydrate formulations with 30 to 60g carb and 200 to 500mg sodium per hour of intake. HYDRAX CORE is a dual-carb formulation with 980mg sodium per serve, engineered for the sodium and absorption demands of long-course racing. Athletes commonly carry HYDRAX in special-needs bags rather than rely on aid station variability.
You train for a year. You fuel for a day. Make the fuel count.
Disclaimer: This guide is general educational information based on endurance sports science, not medical, dietetic or coaching advice. Individual responses to carbohydrate and sodium intake vary. Consult a qualified sports dietitian, doctor or coach before making changes to your race-day nutrition, and always follow the mandatory warnings and directions on the product packaging.