What would be considered an "average" fat oxidization?

I’m heavy too! Actually what you’ve said is a good way of looking at things, I’d calculated roughly 9000kcal for the day and yes, that would be a grim day eating the entirety

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We are all guessing. If I was invested in an Ironman? I’d budget a couple hundred on a gas exchange test to nail down fueling.


That was a really good watch, cheers! Really surprised at the carb/fat ratio at the lowest (90w) compared to the 12min effort. Seemed the fat contribution went up, but the carb contribution was relatively stable.

It was on the list of things to do over the last few months, but il be honest I never got around to it. I saw more value in getting some swim coaching so I spent the money there instead

I know INSCYD on-road power testing will estimate carb usage across power outputs.

Aerotune does something similar, and costs less.

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Yes it was interesting. Maybe she’s really unfit! I’ll watch the latest video (how did Manon do at the Steamboat Gravel) tonight to find out :smiley:

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I’m not quite sure I follow: energy is power over time, so given power data, you can compute the energy put out at the cranks. Cycling efficiency of humans is about 20–25 %, so you can infer total energy expenditure. These are hard numbers that have a very clear and direct meaning, and you don’t need to know anything else.

3 w/kg LT1

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But you dont know the percentage of fat vs carb for a given intensity which varies from person to person dramatically and between male and females. The only way to know is a Met test.

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Kolie Moore, on a recent podcast, had a great quote on this. “When you’re doing VO2max intensities you churn through carbs like a Gatling Gun”

Ye, after listening a lot to Peter Attia and Inigo San Milan I have tried to think as LT1 as my threshold rather than LT2/ftp/CP, which is pretty depressing.

Body weight is my biggest downfall (ex powerlifter/strongman) LT1 is about 2.5w/kg

I think you are conflating two separate arguments here. I know that fat utilization differs widely across individuals.

But @Peterreynolds asked a specific question: he conjectured that at low Z2 most of the energy came from fat and very little from carbs. You can still use the references to make a ballpark estimate. That does not seem to be the case, even at low intensity, you are relying quite significantly on carbs. Wether your personal percentage at 60 % FTP is 30 %, 40 % or 60 % is beside the point.

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Where is your evidence for this?

What are you defining as low Z2?

These are just nade up numbers you’ve quoted.

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Maybe I wasn’t clear, but these weren’t random numbers. One set was taken straight from the GCN video.

The second set I took them from last Saturday’s endurance ride: IF = 0.61, average power = 181 W, NP = 208 W and 1,572 kCal. My FTP at that time was 339 W, so that’s in lower Z2 for me.

For the OP that’s also Z2, albeit a on the higher side (IF = 0.69 at FTP = 300 W).

The energy expenditure doesn’t depend on the zone, just on the average power of the ride and the duration.

Just ran the numbers, it came back as viable but with a high chance of bonking (it gave me 210w as the upper range)

IM Wales is a tough course (didnt realise how tough when i signed up) so there will be a lot of sub/at/above threshold. Itll be a fine balance

Taking on the max carbs you can stomach is something I can agree with a lot more! Though I still think there’s a happy medium - I.e. Work out the max carbs you think you can stomach, work out the minimum carbs you think are needed, there’s likely a pretty decent window between the 2 and then depending on balance of performance vs just finishing, and depending how confident you are in either estimation, then figure out where in that window you want to sit. An experienced racer looking to maximise performance and with plenty of previous race and training data on how much they can stomach is going to be able to push the boundaries much more than somebody doing their first IM and asking nutrition questions on an Internet forum 3 weeks out.

I’d say the flattish first half of the bike sounds like the OP could push towards maximum carbs they can stomach as it’s fairly low risk doing that on the bike when there’s minimal climbing, temperatures are cool, body and stomach are still fairly fresh. Also the best time to take on any solids if he doesn’t want to fuel the whole race on liquids and gels. Second half of the bike with time above threshold it’s probably sensible to back it off a little as the max you can absorb will diminish at those intensities (though also worth looking at changing gearing to minimise time above threshold). The run is where things get tough as harder to process carbs while your stomach is bouncing up and down, plus at that point the day is at it’s hottest and your body and stomach are getting tired. If you’ve fuelled well during the bike then you should have enough buffer to reduce carb intake on the run to reduce chances of gastric distress and still not risk bonking.


I know, I never disagreed with this.

I understand this as well, I quoted what I disagreed with, the numbers saying x kcals from carbs, there should be no misunderstanding. You can’t take numbers from a video and randomly apply them to another individual. For a start they are the oppsite sex which makes a difference and as you agreed it is individual person to person anyway.

^ This, without the actual numbers it just a random guess based on a video of an Ex Pro Female which I dont think is relevant.


Yeah I agree with that.

I think it is worth remembering it is easier to take calories on the bike without GI distress… on the run it can get messy very quickly.
I’d suggest taking the max on the bike and the minimum you can get away with on the run might be a viable strategy.

Of course if you know the actual burn rate of carbs at a given intesity you can trade pace with the fueling strategy to successfully make the finish line with a near optimal finish time. Not put it well but hopefully get the gist.

I guess hence the original question.

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Ahh, my question was more about contributions from fat/carbs at low intensity rather than nutrition advice :smile: id like to think im ok on the nutritional side of things, but still learning all the time. I usually fuel with a honey/starch/maltodextrin/electrolyte mix indoors and use SiS rego outdoors as its more palatable! pockets+bento are stuffed with food options too

Im definitely not going to be pushing boundaries, but id like to do my best with my limited/average capabilities. Gearing wise, ive got sram 12sp with a 35/33 lowest gear, but even then with the route recce last weekend (2/3rds of the course) 27mins was spent over 325w, and i can assure you i wasn’t doing any heroics! just +/- 60rpm on the steep stuff, a fair few 15-18% kickers on the hillier loop that gets completed twice. Yep, more solids at the start, planning on a sandwich/flapjack/fluids from the swim exit to T1 which is approx a 1km run.

The run is never going to be fast, i have been training 90% z2 on running with only marathon pace intervals during parkrun, swim and bike have been my main focus.

Pretty much! But then, that also comes back to metabolic testing… so its chicken and the egg. My fault for not getting it sorted before

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