What would be considered an "average" fat oxidization?

So, generic question.

Is there a decent baseline level of fat oxidization to work from, when calculating fueling for long rides?

I have my Ironman in 3 weeks, by no means am i aiming to restrict fueling, but i also dont want to be pounding the sugar every second. Aim is to sit close to 200w on the flat and keep things controlled on the 7500ft of elevation. After a course ride last weekend, my prediction on the day will be about 40mins over threshold on the day, purely because im heavy and there are some steep grades in places. Second half of the course is the hilly part!

When im around 180-190w (low z2) everything feels neutral, doesn’t feel like im having to push the pedals, heartrate infinitely stable, breathing is not much more than a brisk walk etc. So i have used that as my presumption that carb usage is minimal at that point, and fat oxidization is highest. On the TR plans, my Z2 ends around the 230w mark. My FTP is variable, but currently 300-305 for reference. For context, about 10hrs riding a week of which 7-8hrs Z2/longer, some tempo and some above threshold, basically the full IM plan + extra outdoors

Would that be a fair assumption, Or am i off the mark?


At the risk of being pedantic: there are two meanings for minimal, and I want to double-check which one you have in mind:

Meaning 1: minimal as in lowest value.
Meaning 2: minimal as in close to zero.

Good question, I hadn’t considered a difference!

Under 20g per hour usage at 180-190w would be my answer :slight_smile:

You also have to be very careful about carb usage: strictly speaking, fat utilization does not mean that your muscles are burning fat. Your muscles burn the glycogen stored in your muscles, and lipolysis replenishes the glycogen in your muscles.

Generally, it is not recommended that cyclist even try to become fat-adapted. Becoming fat adapted takes longer than 3 weeks, so even if you wanted to, I don’t think it is in the cards. But I have read feedback from ultra endurance athletes (where becoming fat-adapted might make the biggest sense) that in their opinion, it is a very bad idea. If you want, I can try to dig up some of these posts.

Please don’t. For a triathlete that sounds like a recipe for misery, bonking and failure.

For optimal performance you should always fuel with plenty of carbs. At your FTP it is virtually impossible to overfuel. Rather than trying to increase fat utilization, you should help out your body and give it as much of the fuel it actually needs. That’s triply true if you are speaking about a race.

20 g/h is dangerously low. The common recommendation is 80–120 g/h. During very long races, it is hard to hit those numbers without training your gut. On 5±hour rides, I typically max out at 80 g/h, during training I can do 100–105 g/h. If you are smaller or a woman, the value can be a bit lower, but way, way above 20 g/h.

I once heard the claim that triathlon is a sport of 4 disciplines: swimming, cycling, running and nutrition.


There’s a GCN video that showed Manon going to a sports lab and getting tested for a long race fuelling strategy, her carb use was surprisingly high.

Check it out here: (you can pause on the graph and see % ftp etc)

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Ahh sorry, i think I phrased the question badly!!

I was just meaning how much of my calorie expenditure at the point which is find my pedalling the easiest (about 180w) would be coming from fat.

Don’t worry I’m not thinking of fuelling with 20g an hour! My working amount was going to be 80-100g/hour

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If we take the numbers in the GCN video as a rough estimate, at low intensity (<= 50 % FTP) 40 % of the energy (= calories) came from fat and 60 % from carbs. At 180 W (which is conveniently 2x of Manon’s power) you are expending about 800 kCal per hour. 60 % come from carbs, i. e. 480 kCal. This corresponds to approximately 120 g of carbs. So you were off by 100 g.

I also had a look at a recent endurance ride I did. Average power was 181 W over 2:23 hours and I am shown an energy expenditure of 1,572 kCal. So I’m only getting 657 kCal/h. At a 40:60 ratio of fat-to-carbs, that’d be 100 g/h in carbs alone.

Phew, I was worried you wanted to become a fat-adapted athlete three weeks before the race :rofl: :+1:

If an N=1 from some lab testing helps, my FTP is ~340, my tipping point from predominantly fat to predominantly carb oxidation is around 250W (I.e. At 250 I’m about 50:50 carbs:fat), at 200W I’d be very heavily using fat, from memory around 90% of energy. The guy running the tests did say I was pretty well fat adapted, but not in my case from a keto diet, just from years of endurance training. So those fat oxidation numbers are maybe a little higher than average.

Interesting thing for me was how quickly things became very carb dominant above 250W. At 300W it had swung to nearly all carbs, again >85% from memory. I don’t really do super long endurance events like gravel or tri at the moment, but if I got back into them would definitely cause me to have a rethink about fuelling and pacing.


Oh wow, that’s a pretty huge energy demand. Il watch the video when I get home. I honestly didn’t think there would be such a carb demand at relatively low wattage

I did a 4hr z2 ride yesterday (after 1hr swim and 1.5hr run), took 4ltrs of fluids, with 350g carbs split across fluid/bars). Felt at the end, but dwindling. But, I’m at the end of my build and pretty pooped!

Why it is dangerous? It actually happens to be my usual amount for Z2 (IF ~0.63-0.65): 320kcal/750ml + 1 electrolyte tablet/750ml per every 4h. So far longest ride have been 24h, never felt hunger or anything. FTP 250-280W, stable weight for couple years ~67kg.

Am I naturally well fat-adapted? Definitely not trained specifically for it :man_shrugging:

I mean, I have tried to consume more but at this rate, stomach feels full and not sure if pushing for more improves anything?

While a nice little excerise in maths none of these numbers mean anything in reality. Not even as a rough estimate. It depends on the individual to the extent you can extrapolate next to nothing meaningful.

EVENT FUEL AND BONKING CALCULATOR :fuelpump: - CyclingApps.net (part of Fastfitness Group)

this is a pretty cool calculator to see calorie needs and your risk of bonk.

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If you’ve trained for 24 hour rides then you’ll be pretty well fat-adapted from that. Just doing decent volumes of endurance training improves your ability to oxidise fat, even if you don’t do any fasted training or follow a keto diet.



Probably because he will be swimming 3.8km before and running a marathon afterwards. I’ve seen a lot of people collapsed and or heaving at the side of the roads who get the nutrition wrong.


Just assume 100% Carb for a race is the safest opinion imo.

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Good luck, you’re nearly at the start line :slight_smile:

I don’t have an answer to your specific question, which I’m interpreting as being “how much can I get from my spare body fat?”, I get the sentiment behind it.

Let’s take the question from another angle - I’m fairly heavy for a triathlete and I will burn around 600cals in the water, 7000cals on the bike and 4000cals on the run, so lets say 11600cals for the day. I can’t eat that, I/you will be in a deficit, so the question is not “how much can I get from my spare body fat?” but rather how much can I limit the deficit by ‘pounding sugar every second’.

With three weeks to go ideally you would have already tried various fuelling options on the bike, and have a good idea of how much liquid you’ll need in the expected heat of race day.

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4hrs at target IM pace I assume, 4ltrs water and 350g during the bike would be good for me.

I would want to shove in a gel before and after the swim, add another litre of water and 3 gels for your 1.5hr run.

But its your body and not mine, with a higher BF% than most I think I need more water than your average Ironman and less energy on the run because I’m running slower than average.

You might want to ask yourself if you first needed the toilet or needed a drink at the end of your 1/4/1.5 training session?

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That’s not at all safe. Means taking on probably double the carbs you actually need. I.e. 200 watts done on 100% carbs means taking on about 180g carbs per hour. That’s a huge amount and best case scenario trying to take that much on will restrict your intensity to an artificially low speed, worst case you’ll have GI issues which could cause a DNF. If you’re training for an ironman or similar duration event you’re going to have developed pretty decent ability to use fat as a fuel source along the way.

I.e take the maximum you can stomach. That is the safest bet. Context is important here. That is not going to happen (double the carbs), it is an IM. I was talking about the race not a Z2 training session. Ref to Joes posts.


Pretty much, it was a little lower than target pace, but that was the plan. I’ve done some over pacing rides and under pacing, the under pacing mainly being kit tests and logistics etc.

It was about 5deg warmer yesterday than the average temperature of previous years events, so that should also go in my favour. Yes, I was thirsty after I finished!

From swim exit to T1 is approximately a 1km run, so I was going to have flapjacks/sandwiches, fluids etc for the 7-8min waddle to t1 also, get a good head start on solids

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