Fine tuning ironman nutrition with metabolic testing

It’s often said that nutrition is the 4th discipline in Ironman. Eat too much and you bloat, slowing you down. Eat too little and you bonk, slowing you down or even worse, stopping you in your tracks. In an ironman race I burn around 10,000 calories. The body stores about 2000 calories in the muscles and liver as glycogen, meaning that a minimum of 8000 calories need to come from food taken during the race, as well as body fat. But how much comes from body fat, and how much needs to come from “on board nutrition”? In the past I’ve worked out my nutrition based on good advice, followed but trial and error during training and racing; basically eating as many calories as possible without bloating or cramping. In an effort to get a bit more scientific about nutrition, I decided to experiment with a metabolic test.

During the test you wear a mask connected to a machine that analyzes the amount of oxygen and CO2 in your breath. This allows you to calculate the % of calories that come from fat vs carbs, as well as the total calories per minute. With input from Coach Coady I drew up a test protocol where I would ride at increasing intensities every 5 minutes. I would then be able to see how much fat vs carbohydrate is burned at each different intensity:

15 mins @ 220 watts (steady state effort / zone 2)
5 mins @ 230 watts (lower Ironman race pace)
5 mins @ 240 watts (upper Ironman race pace)
5 mins @ 250 watts (half Ironman race pace)
5 mins @ 275 watts (gradual hill in a race)
5 mins @ 300 watts (steeper hill in a race)
10 mins @ 240 watts (return to Ironman pace after a hill)
1 min @ 300 watts
1 min @ 320 watts (threshold)
1 min @ 330 watts
1 min @ 360 watts
5 min @ 220 watts cool down

The results were pretty interesting – or even slightly confusing! You can view an interactive chart of the test here and a static image with some additions/annotations below.


At 200W, it’s pretty much 50/50 carbs (yellow line) and fat (green line). Interestingly it’s not constant, but shifts up and down all the time.  It also seems that as I change efforts, the fat burn rate goes up for a bit. A good example is as I switch up to 275W, my % fat burned goes up to 63% – so for 2 mins after increasing effort, I was burning more fat than carbs. Very soon after that, however, I’m burning almost 70% carbs.


as I increase to 275W, the % calories from fat goes up!

As I step it up to 300W, the same thing happens. The % fat hits a lower peak, but still reaches 50% / 50% for a short while. Then the % carb rises sharply to 80%


I then returned to 240W, where I entered a good fat burning state again (60% fat 40% carb),  higher than the first time at 240W, despite my heart rate being a lot higher (167 bpm now vs 144 bpm then)240WThen I hit the fast ramps at the end (300,320,340,360 watts), and by the time I finish 1 min at 360 watts I’m burning 100% carb and 0% fat, at a rate of 1600 calories per hour!


 So overall it was an interesting test, yet it was not as conclusive as I imagined it would be. I got the info I was interested in, which was what I’m burning at Ironman race intensities. But I’d like to do some repeat tests to see the results at lower intensities, and maybe another time also go beyond 360W just to see what happens at the max (we hadn’t designed this as a VO2 max test). I’d also be interested in following this test with a run test to see what my metabolic rate is like at Ironman running pace. And maybe a test after 3 hours of riding to see if that makes a difference. In fact there are so many variations I’d like to do, maybe I should just buy my own testing rig 😉


8 thoughts on “Fine tuning ironman nutrition with metabolic testing

    • good question! I’d say the more fat the better since it’s pretty much unlimited. But a realistic goal might be 60/40 or 50/50 fat/carb. When you start hitting 70% carb then you can get into a deficit pretty quickly…

  1. Rob, Great info thanks…. I think Pacific Health Labs would be very interested in working with your stats. Your info validates Endurox Excel.

    I have used Pacific Health Labs products for years, but for 2013 added the product Endurox Excel. I understand it’s primary ingredient (herb) ciwujia, purpose is to enhance fat vs glycogen metabolism.

    Ironman Nutrition has been the biggest adventure and learning curve for me.
    I’m hoping this intro to PHL’ is not elementary for you. PHL’ products are a tremendous enhancement to my LongCourse training. And you should know my only relationship with them is from purchasing (retail) the products.

    Best of Luck,
    Ken / San Diego

  2. Wow – these are super-interesting stats. Do u mind if I use some of this stuff in class? Presume higher O2 in breath = burning fat and higher CO2 = burning carbs? Hope final few weeks of training go really well xxx

  3. Pingback: Ironman Los Cabos Nutrition Report |

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