Decision summary
| Decision | What matters |
|---|---|
| Body size is one input | Larger athletes may have different fluid and energy requirements, but body mass does not produce a complete prescription. |
| Carbohydrate needs rehearsal | Choose an hourly range that can be absorbed at race effort and progress it during long sessions. |
| Separate fluid from sodium | Sweat rate and sweat sodium concentration vary independently. |
| Duration changes the consequences | A small hourly deficit becomes large across a full-distance event. |
Body size is one input
Larger athletes may have different fluid and energy requirements, but body mass does not produce a complete prescription. Gut tolerance, intensity, heat and product concentration matter too.
Carbohydrate needs rehearsal
Choose an hourly range that can be absorbed at race effort and progress it during long sessions. More is useful only when it reaches the bloodstream without compromising the stomach.
Separate fluid from sodium
Sweat rate and sweat sodium concentration vary independently. Measure body-mass change across representative sessions and use qualified testing or clinical guidance when risk is high.
Duration changes the consequences
A small hourly deficit becomes large across a full-distance event. Build bottle, aid-station and contingency plans that remain executable when decision-making declines.
How to turn this into a repeatable plan
Nutrition is an individual system, not a copied hourly number. Separate carbohydrate, fluid and sodium decisions, rehearse them at representative duration and intensity, and record what was actually consumed. Change one variable at a time so a successful or failed session teaches something usable.
- 1. Body size is one input. Record the current baseline before changing the plan.
- 2. Carbohydrate needs rehearsal. Rehearse this under representative fatigue while keeping the remaining variables stable.
- 3. Separate fluid from sodium. Rehearse this under representative fatigue while keeping the remaining variables stable.
- 4. Duration changes the consequences. Review the response and write the next decision before adding load.
What can change the recommendation
| Variable | Why it matters |
|---|---|
| Conditions | Temperature, wind, water and surface can change the cost of the same pace. |
| Athlete response | Sleep, soreness, stomach, confidence and movement quality determine whether the planned load remains appropriate. |
| Official information | Current organiser rules, routes, start procedures and cut-offs override historical descriptions. |
Evidence and sources
Facts and recommendations are separated deliberately. Final operational details remain subject to the current organiser documentation.
Frequently asked questions
How many carbohydrates per hour should I take in an IRONMAN?
There is no universal number. Establish a tolerable range in training and adjust for product mix, intensity, duration and professional guidance.
What should I test in training?
Test “Body size is one input” first, then add “Carbohydrate needs rehearsal” while keeping the rest of the session controlled enough to interpret the response.
Is the analysis for “ironman nutrition body size race duration” observed or modelled?
The guidance is a planning framework, not a guarantee. Any observed statistic is published only with a source, sample size and evidence state.
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