Ironman Nutrition Plan: Full and 70.3 Fueling by Hour

Ironman is a fueling exam as much as a fitness exam. You can be trained to a peak and still blow up at mile 15 of the run because you under-fueled on the bike. The numbers in this guide reflect current sports science on carbohydrate oxidation, sodium losses, and caffeine pharmacokinetics for events lasting 8 to 17 hours.

How many carbs per hour for Ironman?

Full Ironman athletes should target 90g of carbohydrate per hour on the bike and 60g per hour on the marathon run. 70.3 athletes should target 75 to 90g per hour on the bike and 60g per hour on the half-marathon run. These numbers assume a trained gut. If you have not practiced high-carb fueling in training, start lower and build up.

Race distance	Bike target	Run target	Total carb load (36h)
Full Ironman	90g/h	60g/h	10 to 12g per kg body weight
70.3	75-90g/h	60g/h	10g per kg body weight

Reaching 90g per hour requires using a 2:1 maltodextrin-to-fructose ratio in your nutrition. The gut can only absorb about 60g per hour of glucose-based carbs. Adding fructose opens a second absorption pathway and allows total absorption to reach 90g per hour or more. Most purpose-built endurance nutrition products (Maurten, SiS Beta Fuel, Precision Fuel) are already formulated at this ratio.

Use the carb loading calculator to set your exact 36-hour carbohydrate target by body weight.

The 36-hour carb load

Do not show up to an Ironman without a 36-hour carb load. At this distance, the carb load is what separates finishing strong from finishing on fumes. The target is 10 to 12g of carbohydrate per kilogram of body weight over the 36 hours before the race.

What that looks like in practice for a 75kg athlete (750 to 900g of carbs over 36 hours):

Meal	Food	Carbs
Breakfast (day before)	White rice, honey, banana	120g
Lunch	Large white pasta, tomato sauce	130g
Afternoon snack	Rice cakes, honey, sports drink	100g
Dinner	White rice, chicken breast (small), sports drink	130g
Evening snack	White toast, honey	60g
Day 1 total		~540g
Race morning (3-4h before)	White toast, honey, banana, sports drink	100g
60-90 min before swim	2 gels, 250mL sports drink	55g

Stick to white rice, white pasta, plain bagels, and sports drinks. Avoid vegetables, high-fat foods, and high-fiber items that stay in the gut. See the 36-hour carb loading food list for the complete list of what to eat and what to avoid.

Race morning breakfast

Eat 2 to 3g of carbohydrate per kilogram of body weight three to four hours before the swim start. For a 75kg athlete, that is 150 to 225g of carbs. White toast with honey, a banana, and a sports drink is the standard, easily digested option.

Do not skip race morning eating. Liver glycogen depletes overnight. A race morning meal restores liver glycogen even when muscle glycogen is already loaded from the carb load.

Hours before start	What to eat	What to avoid
3-4 hours	Toast, honey, banana, sports drink (150-225g carbs)	Fat, fiber, protein-heavy meals
60-90 min	1-2 caffeine gels or plain gels	New foods
30 min	Sip electrolyte drink	Large volumes of liquid

On-course fueling: bike

Front-load your carbohydrate intake to the bike leg. It is physiologically easier to eat on the bike than on the run, and glycogen depletion late on the run is the most common cause of DNF and pace collapse at Ironman. Start eating at minute 15, not mile 1.

Target by bike hour:

Bike hour	Carbs	Sodium	Fluid
Hour 1	60-70g	600-800mg	500-750mL
Hour 2	80-90g	800-1000mg	500-750mL
Hour 3+	90g	800-1000mg	500-750mL
Final 45 min	60g (easy to digest)	600mg	400mL

Slow down carb intake in the final 45 minutes on the bike to give your gut time to clear before the run.

See how many carbs per hour for the absorption science and gut training protocol.

Caffeine timing for Ironman

Take your first caffeine dose at hour 3 to 4 of the bike, not before the swim. Taking caffeine before the swim wastes most of its benefit during the swim when the perception benefit matters least. Caffeine at hour 4 peaks around the time you begin the run, which is when you most need the effect.

Recommended protocol:

• Hour 4 on bike: 3mg per kg body weight via gel or chews
• Miles 10-13 of the run: 1.5 to 2mg per kg via caffeine chews or cola
• Miles 20-26: cola on course if needed (Coca-Cola is the classic late-Ironman fuel)

For a 75kg athlete, that is roughly 225mg of caffeine at hour 4 and 100 to 150mg at mile 10-13. Cap total race-day intake at 9mg per kg to avoid GI issues and heart-rate spikes. See caffeine timing for endurance athletes for the full dosing guide.

Sodium for Ironman

Target 800 to 1000mg of sodium per hour, scaling up to 1200mg per hour in heat above 25 C (77 F) or if you are a heavy sweater. Ironman run-course medical tents see far more hyponatremia than dehydration. Do not drink plain water exclusively; always pair it with electrolytes.

Use the sodium calculator to personalize your sodium target by sweat rate and body weight. Consider carrying a salt capsule like SaltStick for reliable dosing on the run when product varies by aid station.

For a detailed breakdown of sodium loss by sweat rate and temperature, see how much sodium do you lose sweating.

On-course fueling: run

Use real food early and gels late. Your gut sensitivity increases with fatigue. Chicken broth, watermelon, and orange slices available at Ironman run aid stations are often easier to tolerate at hour 10 than another gel.

Run mile range	Primary fuel	Sodium source
1-10	Gels, sports drink (60g/h)	Electrolyte drink on course
10-20	Gels + cola, broth	Salt caps or broth
20-26	Cola, gels only if tolerating	Broth at every aid station

Recovery nutrition

Within 30 minutes of finishing, take 1g of carbohydrate per kg body weight combined with 20 to 30g of protein. This window initiates glycogen resynthesis and muscle repair. A recovery shake is easier than solid food in the finisher area. In the first 24 hours, prioritize carbohydrate, protein, and fluid over micronutrients.

Frequently asked questions

How many calories do you need for an Ironman?

A full Ironman burns approximately 8,000 to 12,000 calories for most athletes. You cannot replace all of these during the race, nor do you need to. The goal is to supply enough carbohydrate to spare glycogen and prevent bonking, not to match total expenditure. Most athletes take in 1,500 to 2,500 calories during the race itself, relying on fat oxidation for the remainder. The pre-race carb load fills muscle and liver glycogen stores to maximize the substrate available for the effort.

What should I eat on the Ironman bike?

Target 90g of carbohydrate per hour using a mix of liquid nutrition (sports drink or custom mix in a bottle), gels, and chews. Solid food is fine early in the bike when your gut is fresh, but becomes harder to process as the effort accumulates. Products like Maurten 320, SiS Beta Fuel 80, and Precision Fuel PF 90 are purpose-formulated for this intake level with a 2:1 carbohydrate ratio. Avoid high-fat or high-fiber solid foods on the bike regardless of how normal they feel in training.

Can I use Gatorade and gels for an Ironman?

Gatorade Endurance on course contains approximately 200mg of sodium per 12oz serving, which is below most athletes' needs. Supplement with salt capsules or switch to a higher-sodium product on your bike. Standard Gatorade (not Endurance) is even lower in sodium and is not suitable as your primary electrolyte source at this distance. Gels are fine for carbohydrate, but check caffeine content: taking multiple caffeinated gels early in the race front-loads caffeine too early in the effort.

How do I train my gut for Ironman nutrition?

Practice race-nutrition volumes in every long training session over 2 hours. Start with 60g of carbohydrate per hour and build by 10g every two to three weeks. Long rides and bricks are the best venue for gut training because the bike intensity and duration most closely match race conditions. Never try a new product or a higher-than-practiced intake on race day. The tolerance you build in training is specifically adapted to your gut's trained microbiome and motility rate.