This week we explore the physiological differences between amateur and elite Ironman athletes and how it’s recommended they eat to best prepare for race day.
A 2018 paper by Maunder et al named ‘Different Horses on the Same Courses’ has highlighted the following key considerations:
1) Top Ironman triathletes finish the ultra-endurance race in ~8hrs. The amount of energy used per minute increases as the competition level increases. i.e. elite athletes burn more calories compared to amateur athletes. Approx. 9,500kcal for the entire race (~20kcal/min).
2) The capacity for storing carbohydrates within the muscle and liver is finite – this amounts to ~3,000kcal/750g carbs. Where fat storage, even in lean athletes can be in excess of 70,000kcal.
3) Carbohydrates are needed to sustain medium-high intensity performance where their requirement increases as the intensity increases: Low intensity = greater fat usage, mod-high intensity = greater carb usage.
4) Therefore elite athletes need a very high amount of carbohydrate to fuel higher intensity performances. To offset depletion and poor work rates, they need to investigate methods to ‘spare’ their fuel source.
5) Elite ironman triathletes may wish to adopt a ‘train low’ approach during certain training phases to improve endurance adaptations. i.e. training under low carbohydrate availability activates certain pathways to upregulate the amount of mitochondria you have in the muscle. Therefore, a greater mitochondria content increases fat utilisation at competitive intensities and therefore spares glycogen stores.
6) Elite athletes should periodise their carbohydrate intake by contrastingly training under periods of high carbohydrate availability to (1) train the gut, and (2) ensure their ability to breakdown glycogen for fuel during higher intensity efforts.
7) When elite athletes use a combination of both approaches, they’ll have a greater ability to spare glycogen stores, and use the high amount of carbohydrates ingested during a race to support high rates of energy expenditure. Therefore, ensuring carbohydrate availability remains as high as possible throughout, especially for higher intensity phases of the race #NoCarbohdyrateDeficienciesHere.
8) Amateur athletes compete at lower intensities, take longer to complete the race (~14-15hrs), and expend less energy (~13kcal/min).
9) In low-performing amateur athletes whose aim is to simply complete the race, the smaller energy cost per minute may almost be met through fat oxidation when fat adapted, and therefore have less reliance on carbohydrates. Knowing this, it’s been suggested that a low carbohydrate or ketogenic diet may be an appropriate nutrition strategy for such low intensity, high duration races (see image above).
As the papers title suggests ‘Different Horses on the Same Courses’ – Preparation for the event needs to be individualised to the person based on their level of performance.

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