Metabolic Testing: Ironman Fuel Efficiency and Fat Burning Profiles

The results from my testing with Peter Byworth and Metabolic Solutions are in and I’ve had time to study them. The broad details from my initial post are all confirmed by the raw data so I’ve dug further with help from sports scientist Greg May. Greg’s already looking into this area and was keen to see my data, given the input I’m glad I shared!

I always knew this would be too late to have real influence on Kona. The concepts I’m learning need practice before I start racing with them. I’m reassured my fuelling strategy and pacing will comfortably see me through. What’s clear is modifying aspects of training and being more precise in racing will help take the next step.

The tests started by measuring my Resting Metabolic Rate (RMR). Basic information that’s main use is ensuring I’m more precisely aware of my daily calorific usage. This identified the 80:20 Fat:CHO utilisation ratio at rest which indicated that the Paleo diet is ideal were I a sedentary individual. As discussed training in high volume is a wildcard that demands significantly more carbohydrate.

Steady State Fuel Substrate Testing

Fuel Substrate Utilisation against Bike Power (Fat:CHO)

This colourful chart summarises the results of 20 minutes work and pain on a bike. We have the total Kcal being utilised per minute in yellow with the portion coming from fat in green. Along the x axis is the wattages I was holding at that point in time.

I warmed up from a low base of 100W through to 200W over several minutes not spend much time at any of the intensities so I never stabilised. Below 200W (and probably a little higher) I get comfortably over 60% of my energy from fat.

Beyond warming up the first step required me to hold 255W. I found a comfortable position with cadence around 90 (higher than I’d expected) and power on target. This continued for four minutes before we ramped to threshold and beyond. Greg would like longer spent in this region something to consider next time. It would give my body a better chance to stabilise at the intensity and more certainty with the resulting data. It sounds like I may need to suffer more next time!

We progressed the testing to threshold (around 300W) and held there for four more minutes (this is the one that’ll hurt for longer). From then on it was a ramp till failure a compromise to see roughly where that was and what was going on.

As you can see in the graph I’m still getting a lot of Kcal from fat at 255W and even at threshold don’t do to badly. Extend that all the way to failure and fat is still a contributing factor for me. From Greg’s perspective 2.5g CHO/min (10Kcal CHO/min) represents a key turning point for Ironman racing.

What’s special about 2.5g of Carbs?

As the results show with increasing intensity the body utilises it’s fuel sources in differing ratios. CHO becomes more significant as workload rises. The body contains a limited supply of CHO (as glycogen) in the muscles and liver and an effectively limitless supply of fat.

Fat is never likely to be the limiter working at low intensity I could last a long time on liver and muscle glycogen stores. Increase intensity and duration of work and there’s the potential to become limited by available CHO. The harder you work the quicker stored CHO is depleted. Which leads to nutrition.

Eating during training effectively tops up the glycogen available in the body. As you should expect there’s a limit to the rate we can absorb CHO though 1.2g CHO/min is a typical maximal figure. Consume CHO at a greater rate and the excess will sit in the gut whilst it’s digested.

You start a race with a store of glycogen and immediately begin to deplete it. Eating during the event allows some restoration of glycogen levels, but is limited by absorption rate. Overall levels of CHO fuelling the body will vary according to initial levels, food consumed and the intensity and duration of activity.

To race optimally you need to start with high levels of available glycogen. Intensity should be controlled to ensure you’re not getting through reserves too rapidly. Once it’s an option a fuelling strategy is needed to get yourself close to maximal CHO absorption. If this is achieved the limiter on performance won’t be nutrition.

Based upon this fuelling strategy we come to the figure of 2.5g CHO/min. With full glycogen reserves and optimal CHO intake if you work at an intensity requiring less than 2.5g CHO/min you’ll reach the marathon with sufficient reserves to keep going. You’ll need to fuel on the marathon too of course, but you will be in much less danger of hitting a low.

1.2g CHO/min Max?

The standard quoted limit for CHO absorption is 1.2g/min as mentioned, but Greg went on to allude to the potential to absorb more. Apparently as yet unpublished research suggests it’s possible to hit 1.9g CHO/min with the correct fuel strategy. Believe me I’d tell you what that was if I knew it myself! In terms of using the figure of 2.5g CHO/min you want to be absorbing more than 1.2g CHO/min if possible.

Interestingly this relates to an interview with Asker Jeukendrup I recently listened to on Competitor Radio. Definitely worth a listen if you’re interested in race nutrition. He alludes to a mix of CHO sources in your fuel enabling greater uptake. An optimised mix of glucose and fructose is the way to maximise fuel intake. There are already products out there doing this so whether that’s the strategy Greg referred to I don’t know!

Implications for Racing

From the data and Greg’s input I could potentially ride up to 255W and fuel myself successfully for an entire Ironman. Playing it safer we felt that 245W was probably a more reasonably target. With the correct fuelling strategy I should be able to hold that wattage and effectively run afterwards.

Fuel isn’t the only limiter though, there’s potentially other physiological factors that come into play. We don’t fail at a given intensity of work only when we’re low on fuel. Most recommendations for racing Ironman suggestion not going harder than 75% of your threshold power (FTP). That’s the top end of performance to race at and beyond that the risk of impairing your run increases

My FTP currently sits around 300W which gives me a 75% point of 225W. I rode Ironman Western Australia and New Zealand averaging about that last year (normalised was higher in NZ). I’m clearly approaching the physiologically sensible intensity to ride, but have a way to my fuel limit. There is room for improvement!

If my goal is riding at 245W for an Ironman next year doing so knowing I’m not limited by CHO I can work back to consider the FTP needed. For 245W to be a 75% effort FTP will need to be around 330W, but the higher the better. Racing at 75% has worked for me, but relies on my strength in running.

It’s not a performance predictor, but rather a metric for progress towards my racing goals next year. I have the metabolism to support a harder Ironman bike, but need to work to develop power further. It’s not surprising, but putting numbers to it acts as a strong motivator.

Implications for Training

Reiterating a point from my previous post training to enhance endurance and fat burning will yield minimal returns for me. The area where the biggest gains can be made is in my threshold. Raising this will take me closer to riding at that 2.5g CHO/min point. The aim is to remove the limiters that prevent me riding an Ironman at over 240W. What I know is that fuelling doesn’t have to be one.

Where I’ve relied on long hours of riding in the past I’ll look to reduce volume and focus more on intensive sessions. Being a coach I have the flexibility and time to train more than most so training load will remain high. Six hour rides will be rarer and when they occur the focus will be structuring them. They’ll be a couple of sessions working on threshold and then one or two at tempo or sweet spot.

Throughout this process I’ll be routinely testing fuelling strategies and race day pacing. When Austria comes around next year I will know the pace I can ride and how much I need to eat to do it!

Finally big thanks to Andy Bruce and the Polar Challenge team for introducing me to Peter Byworth and Metabolic Solutions for the testing work. Also thanks to Greg May for his input so far, hopefully he won’t mind further questions!

Ironman Training Library

From nutrition to pacing - a collection of CoachCox blog posts focused specifically on Ironman training and racing.


  • Thanks for sharing. Very nice numbers I must say.

    I am a little puzzled why you think you have to increase your FTP when the metabolic testing clearly seems to indicate you’re able to sustain the 245 W?

    It seems to be just because you’ve heard the number 75% of FTP and then working backwards you conclude you have to raise your FTP. But why? You seem to be way above average metabolically fit, why not use that and race at 80%? The numbers show you can.

    Or am I wrong?

    Whenever I hear something like that 75% figure it acts as a little red flag, it’s clearly a rule of thumb, and as such it will not apply to everyone, there will be outliers. And it seems to me you’re one.


  • Hi Morten,

    Training over the coming months will clarify a lot on my ability to ride at 245W. I wouldn’t hold to 75% as a fixed number (in fact according to the normalised power for IMNZ I rode at an IF of .78).

    My thinking is down to two points – firstly metabolism alone isn’t the only limiter. If we know I can fuel myself at 245W then why don’t I just do that – assuming you’re working at a stable level of glycogen in the system then metabolically you’re fine.

    But think of it this way an hour at FTP (300W) would require 3.4g CHO/min so over 60 minutes (definition of FTP) I would need 204g CHO to fuel the effort. The body can hold say 500g of glycogen so glycogen reserves are not the limiter riding at FTP. Other physiological (and psychological!) factors are in play leading to limitations on the effort. Similarly if I ride at 75% FTP for 5 hours I am not going to hit a fuel limit if I’m eating sensibly, but potentially over that time at that intensity I’m going to be limited by other factors involved in fatigue.

    Secondly 75% is only a guide number and you’re quite right to always question the validity of guidelines. This is where training and testing will come in to establish my capacity to work over an Ironman distance at increasing intensity.

    Whatever my actual maximum relative effort for an Ironman might be (whether it’s 75 or 80%) by raising my FTP the absolute power at those percentages increases. The aim is to have a fast bike that allows me to be in optimal run shape. So If I target 245W and think currently it’s 80%, but know I can get through a race like that I also know if I raise my FTP so 245W is 75% then I should be able to either ride faster and run the same, or ride at 245W and run better.

    Whatever I do I’ll be working to careful track performance. It’s also not an abandonment of long riding, but rather a periodisation. With winter approaching I’ll use the time for shorter more focussed bike sessions. When it’s spring I’ll expand my riding to involve longer sessions. There will be less just riding though and more riding with purpose.

    Hope that makes my viewpoint clearer?


  • maryka

    Just curious, what was your kJ output for Kona last year (and WA and NZ, the rides you said you did at ~225w?) Both LP and Kona for me were right around 3100kJ which if you assume 25% efficiency — a bit on the high side perhaps — means I burned ~535cals/hour. I haven’t done metabolic testing so I don’t know what percentage of that comes from CHO and what comes from fat.

    Your theoretical chart above shows at 255w you burn approximately 8kcal/min fat and a further ~10kcal/min CHO (1080kcal/hour). So the output from that (255w * 3.6) is 918kJ. Consumption of 1080kcal = 4320kJ, output = 918kJ, meaning your efficiency is only 21.25%?

    So I just wondered what your numbers were like from an actual race. If that 21.25% holds true with the chart above (guessing at 225W at it’s 13kcal/min total, 4 from CHO and 9 from fat), then with an output of 810kJ and a consumption of 3120kJ/hr you’d be at nearly 26% efficiency which seems way off.

    I guess the chart above isn’t exactly accurate, or is there something I’ve missed?

  • Don’t have Kona numbers as I didn’t race with power.

    For WA numbers were AP 215W, NP 219W and 3695Kj for a 4:52 bike split (okay I rounded more from memory on this one!). For NZ numbers were AP 222W, NP 232W and 4045Kj for a 5:04 bike split (the final 42K into a headwind had AP 232W, NP 242W and 1063Kj).

    Greg felt I really needed a longer period at 255W in my test to be sure I’d properly stabilised there. So there’s the possibility for some skew in the data. Also note I don’t have the raw power data for the session. Rather we have blocks marked at the intensity I was working at. In reality during that period there were fluctuations around the 255W mark, but I couldn’t give a precise answer how much.

    Below 255W we definitely didn’t stay long enough to be really sure of average values. Closest data point to 225W is 200W for which I’ve a small amount of data. That suggests 13.5kcal/min from 4 CHO and 9.5 fat. That gives your 810kj/hr so we would expect it higher at 215W through to 225W. So I’m probably not 26% efficient!

    As a first test it gives a number of points to carry forward and think on. Certainly in subsequent testing I’ll be looking to hold longer periods of time sub-threshold and won’t be worrying about ramping up higher. Should give a clearer picture and hopefully I can get hold of power data to precisely match to the consumption. As it is there’s enough margin for error that I think it’d be hard to make precise deductions about efficiency.