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How to fuel during an IRONMAN?
How to Fuel

How to Construct an IRONMAN Fueling Plan

My name is Craig Alexander… my friends call me Crowie.

I’m a former professional triathlete from Australia who won the Hawaii IRONMAN World Championship 3 times, and the 70.3 IRONMAN World Championships twice. In 2011, I broke the 15-year-old course record in Kona. Despite being 38 years old, I went 8:03:56. I consider that performance to be one of the highlights of my career.

A major contributor to my success was my ability to construct an effective nutritional plan that matched the particulars of each race and reflected the current state of my fitness. Now I’ll share my method with you.

What Your Body Needs

When considering a nutrition plan for any race, there are 3 basic components that require your attention:

  • Water
  • Sodium
  • Calories

In this guide, we’ll focus on how to determine the proper amount of calories needed for a long-distance triathlon, such as an IRONMAN. The topics of water and sodium are important, too, but I’ve found that athletes have the most trouble determining their caloric needs for the race, so let’s deal with that first.

The Two Most Common Mistakes

Eating and drinking too little

Consuming too much, too soon

Event-Specific Factors to Consider When Building Your Nutrition Plan

The first step in constructing your race day fueling plan is to analyze the specific conditions and attributes of your target event. Here are 4 variables that I always consider:

1. Race Distance

The distance of your event influences the duration of your fueling window and the intensity at which you’ll compete. I’ve found that events longer than 70.3 IRONMAN can be especially tricky to properly fuel because you’ll be out on the course for so long.

We rarely train for more than 6 hours at race intensity, so most triathletes haven’t adequately trained their guts for the unique stress of full-distance events. This guide is specifically designed for those long-distance triathlons (i.e., longer than 7 hours), especially full-distance IRONMAN triathlons.

2. Course Topography

A flatter course results in a steadier effort throughout the day, which can make for more predictable fueling.

In contrast, hilly courses cause intermittent spikes in our heart rate and intensity. Just like when you stomp on the accelerator in your car, each of these efforts burns fuel (in this case, glycogen) at a very high rate. This can make it more complicated to construct a successful fueling plan.

3. Environmental Conditions

Extreme environmental conditions have a direct and substantive impact on your race day nutritional needs and are often not given enough attention.

  • High heat and humidity – common in Kona, Asia, and Australia – can wreak havoc on any race nutrition plan. Hot, humid conditions promote overconsumption of water or sports drink that distends the stomach and inhibits the absorption of much-needed carbohydrates.
  • Severe winds – like those in Kona and Lanzarote –have a surprising impact on your fueling plan. Prolonged headwinds can cause an increase in sustained intensity during the bike, which must be compensated for when calculating your hourly caloric consumption.
  • Higher altitude – like in St. George, UT– impairs your aerobic capacity, lowers cardiac output, and creates metabolic stress that impacts your nutrition strategy. You’ll work harder at higher elevations for the same power output. You’ll also be vulnerable to dehydration.

4. Your Racing Intensity

My last key consideration when determining an event nutrition plan is the realistic estimation of racing intensity. The higher your average intensity, the faster you’ll burn calories (especially carbohydrates).

My first tip is don’t do what the pros do. Those at the very front of the race are performing at a much higher intensity than you! Their caloric demands are much different than yours.

Most age group triathletes race at a lower average intensity. This tends to be an aerobic effort (Zone 2), where your body performs at a relatively comfortable and familiar effort. This will make designing your nutrition plan a bit easier.

As mentioned above, hilly courses can cause spikes in racing intensity and the first 400m of the swim is typically performed at a higher-than-average intensity. You can train to anticipate and accommodate these temporary course-specific intensity surges.

After analyzing these event-specific factors months before your race, you can begin to plan, practice, and refine your nutrition strategy

Train with What’s on Course

While we all have our personal favorite sports nutrition products, smart triathletes train with what’s served on the course. Doing so allows you to carry only a minimum number of calories and prevents you from ingesting unfamiliar foods during the race.

As your event date approaches stock up on what will be served at the aid stations, and train with those products during your demanding brick sessions and race simulations.

How to Confidently Calculate your Caloric Needs During The Bike

One of the secrets to properly fueling your IRONMAN is to nail your caloric consumption on the bike. The bike leg is the longest discipline of the race and comprises approximately 50% of your total race time. You’re not going to get much nutrition in during the swim (!!!), and it’s more challenging to consume and utilize calories during the run. So, if we can correctly calculate the number of calories to ingest during the ride, then we’ll lay the groundwork for a properly-fueled race.

Where Do You Start?

Fortunately we have the ideal tool for calculating the number of calories we burn when cycling: the power meter. Using our power meter on a couple 3-hour training rides at our projected race pace, we can quickly determine an accurate starting point for the appropriate number of Calories needed during the bike leg of a long-distance triathlon. For our purposes, we’ll use the layman’s rule of thumb (validated by some rather complex science and real-life experimentation):

1 Kilojoule = 1 Calorie

So, if we know our average watts per hour and the number of hours ridden, we can determine the number of Calories burned per hour.

Let’s take a practical example and do the math together: Imagine that your realistic estimate for an IRONMAN ride is 210 watts per hour. Using that power target during a 3-hour training ride, we can calculate the following:

210 avg watts x 3 hours x 3.6 = 2268 total Calories burned during the ride (3.6 is the constant used to convert joules into kilojoules)

Divide 2268 Calories by 3 hours = 756 Cal/Hr burned

Okay…stay with me.

We now know how many Calories per hour we’ll burn on this ride. The key question, however, is how many Calories should we consume per hour? Our goal is not to replenish what we burn Calorie for Calorie, but to keep our engines fueled enough to manage our increasing caloric deficit, and to maintain our desired intensity for the entirety of the race.

From years of personal experience, which has also been validated by scientific research, it turns out that a reliable target for replenishment during the race is 35% of total Calories burned. So, in our example: 756 Cal/Hr burned x 35% = 265 Calories consumed per hour during the ride


I hope that you found this Framework for Constructing an Effective IRONMAN Fueling Plan to be useful as you look ahead to your next triathlon. By incorporating these methods – and practicing your fueling under conditions similar to what you expect on race day – you’ll set yourself up for an exceptional performance. You’ll also arrive at the race with the confidence that comes from knowing you have the “fourth event” mastered.

Remember: train with what’s on course by ordering exactly what you need from TheFeed.com. It’s your best option for sports nutrition products.

Be sure to download your own copy of Crowie's free eBook:

Framework For Constructing An Effective Race Fueling Plan

A practical & proven guide for the “fourth event” by 5x IRONMAN World Champion Craig Alexander

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The Feed. / Friday, November 11, 2022