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.
1 Like
Yes it was interesting. Maybe sheâs really unfit! Iâll watch the latest video (how did Manon do at the Steamboat Gravel) tonight to find out 
1 Like
Iâm 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 20â25 %, so you can infer total energy expenditure. These are hard numbers that have a very clear and direct meaning, and you donât need to know anything else.
3 w/kg LT1
1 Like
Kolie Moore, on a recent podcast, had a great quote on this. âWhen youâre doing VO2max intensities you churn through carbs like a Gatling Gunâ
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.
1 Like
Maybe I wasnât clear, but these werenât random numbers. One set was taken straight from the GCN video.
The second set I took them from last Saturdayâs 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 thatâs in lower Z2 for me.
For the OP thatâs also Z2, albeit a on the higher side (IF = 0.69 at FTP = 300 W).
The energy expenditure doesnât 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 thereâs a happy medium - I.e. Work out the max carbs you think you can stomach, work out the minimum carbs you think are needed, thereâs 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.
Iâd say the flattish first half of the bike sounds like the OP could push towards maximum carbs they can stomach as itâs fairly low risk doing that on the bike when thereâs minimal climbing, temperatures are cool, body and stomach are still fairly fresh. Also the best time to take on any solids if he doesnât want to fuel the whole race on liquids and gels. Second half of the bike with time above threshold itâs 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 itâs hottest and your body and stomach are getting tired. If youâve 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.
2 Likes
Ahh, my question was more about contributions from fat/carbs at low intensity rather than nutrition advice 
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 wasnât 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
1 Like
So to answer your original question, my guess (and weâre all guessing a bit!) is that with ~8-10 hours/week of cycling plus presumably running and swimming on top of that, youâll have pretty decent fat utilisation. And at 200W itâs reasonable to assume youâll therefore be getting maybe half your energy from fat, maybe a bit more. But the way to look at that part of the race when youâre cruising at 200W is not an opportunity to ânot pound sugar every secondâ but the opposite - thatâs the time when you can take on a lot of fuel and set yourself up well for later in the race when itâs going to be harder to fuel. E.g. At 200W youâre burning a bit over 700cals per hour, assume half of that comes from fat, so ~360 cals from carbs which is 90g per hour. You can probably replace all of those carbs, maybe even a little more, without pushing your limits. Which means youâre going into the hilly section and then the run with your glycogen stores still pretty full.
2 Likes
Great way of looking at it cheers, il make sure i have the flapjacks on board and ready to go for the first half! Itll be busy on the roads too, 3000+ people, and descending, so chances are there will be loads of opportunities to reach for the food
Its IM Wales
I have been dipping in and out but sometimes it can be hard to follow. IF i do another il choose a flatter one with a lake swim (IM Talinn in Estonia looks lovely) rather than the one in my own country that happens to be one of the harder in the worldâŚ
Complete disclosure i started to learn to swim in March/April, so its been a learnign curve for sure
1 Like
Iâm chuckling here, sorry. Yes, Wales is a tougher one.
Riding up hills below your FTP is an acquired skill, as is keeping the power on the descents, and is essentially the antithesis of all road cyclists Iâve ever met. It took me a few years to be honest, but doesnât have to. And it reaps huge rewards in the second half of the bike leg.
Come and say hi over here:
Weâve got two others down for Wales at the moment
1 Like