Why a Nutrition Plan for a Half IRONMAN Triathlon Can’t Just be Doubled for the Full

Allow me to take the opportunity to spoil the plot of this article from the very beginning. Your nutrition plan for a full iron distance triathlon will be less than double what it would be for the half.

There, you can leave now. However, for the inquisitive at heart let me digress as to why this is the case.

Disclaimer: I will only be focusing on calorie consumption (and to a lesser degree carbohydrates) in discussing this topic. Your “triathlon nutrition plan” is a term painted by a broad brush. Glucose, liquid intake, sodium intake, protein, and so on could all have articles devoted to each topic separately. So for the sake of simplicity we will only concentrate on pure caloric consumption for a half iron versus a full iron triathlon.

Firstly, the primary difference between your experience in the half IRONMAN as opposed to the full is going to be your intensity level. Obviously intensity drops significantly during the full, resulting in a decreased requirement of calories per hour.

Asker Jeukendrup, one of the leading sports nutrition scientists in the field today and a major authority in IRONMAN nutrition, published a relevant thesis in 2014 titled A Step Towards Personalized Sports Nutrition: Carbohydrate Intake During Exercise. In his work, Jeukendrup finds that, “When the exercise intensity is low and total carbohydrate oxidation rates are low, carbohydrate intake recommendations may have to be adjusted downwards.” This is because an increase in muscle glycogenolysis and an increased plasma glucose oxidation always contributes to greater energy demands.

In other words, as your muscles push harder and your blood pumps faster you need more fuel. As a result, you should be consuming more calories (carbs) per hour (especially on the bike) during a half iron triathlon than you would during a full iron.

Remember that the goal of triathlon nutrition is to consume just enough calories to sustain your goal intensity for the entire duration of the race. Too many results in cramping and too little means a nice, hard bonk (ouch). Therefore, don’t get excited during a full iron triathlon and think you actually need more calories per hour in order to finish the race. You don’t!

After all, the body can only absorb so many calories per hour anyway (some people max out around 300-400). So there’s no need to push beyond that.

Let’s look at an example. Say you’re able to hold a 20 mph average for the 56-mile bike portion of a half iron triathlon. This exertion may suggest that you burn around 1,000 calories per hour. As a result, there’s a chance you’ll need to consume at least 300 calories per hour to account for the loss.

In our 112-mile bike leg example, however, let’s imagine you hold a 17.5 mph average. At this speed (effort level) you might only be burning 700 calories per hour. Therefore, why consume the same hourly amount as you would during the half? Your body doesn’t need the extra carbs. Drop it down to 225-250 calories per hour to avoid cramping and to stay efficient.

As with all aspects of triathlon, training is needed before executing theory in a race. Be sure to experiment with your nutritional hourly caloric consumption depending on intensity level. If you need advice on where to start, check out my post on What, How Much, and When Should You Nutrition During a Triathlon?

Apply the basics but just remember that a longer race doesn’t necessarily mean more food by proportion.


A half IRONMAN is a more intense race than the full while still lasting beyond 3-4 hours. Therefore, scientifically speaking, you should be consuming more calories per hour in the half as opposed to the full. Thus, your nutrition plan for the half iron triathlon can’t simply be doubled for the full.


In general, what is your hourly caloric consumption on the bike and run for a half triathlon? What about the full?

JARED MILAM is a professional triathlete, TriDot coach, and member of the Tri4Him Pro Team. He has 16 years of competitive running experience and 11 years of competitive triathlon experience with a half Iron PR of 3:59 and a full Iron PR of 8:30. Coaching under the TriDot system since 2011, Jared loves working with aspiring triathletes of all ages and performance levels.


Jeukendrup, Asker. “A Step Towards Personalized Sports Nutrition: Carbohydrate Intake During Exercise.” Sports Medicine. Sports Medicine, 3 May 2014. 44(Suppl 1): 25–33. Accessed 28 Nov 2016. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4008807/#

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