I think a good answer would be It depends
In order to give a better answer for your friend we would need context. (history in the sport, his objectives (races/weight/…), hours/week, ftp, time in the season,…)
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Carbs don’t inhibit fat burning. That’s not how your body works. Carbs provide a quicker source of energy either exogenously (in the bloodstream) as you ingest them during or in close proximity to your workout, or in the form of stored muscle glycogen. Carbs can be used to sustain anaerobic glycolysis OR to sustain aerobic metabolism through the creation of electron transporters. That process can happen using glucose or fatty acids.
What seems to be the determinant in which substrate you use for aerobic metabolism is DURATION and INTENSITY. So, the harder you ride, generally speaking the more carbohydrate you will preferentially use for aerobic metabolism as the demand for transporters goes up, as your muscles demand more ATP to do work. THAT is what inhibits fat burning - you simply demand more ATP than your body can supply using slowly mobilized fatty acids.
What you eat has less to do with substrate utilization than the demands you’re placing on your body. Denying your body exogenous sugar or riding in a glycogen depleted state probably has more of a detrimental effect on performance in that ride and subsequent recovery than it helps you become a better “fat burner”.
If you want to become a better fat burner, give your body the appropriate signal - meaning ride LONG durations at low intensity.
But if you can do more work while fueling your rides properly, odds are good you’re going to lose more weight. If you’re denying yourself exogenous sugars while riding and it has a detrimental effect on the amount of work you can do, you’re going to lose that calorie deficit battle every time.
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My $0.02 here:
For a 2.5-3hr endurance ride, you don’t need A LOT of carbs. I usually fuel these in the 60g/hr +/- range, whereas my longest rides I get up to about 90-100g/hr (or more?). (I don’t really do a lot of rides in this range that are strictly zone 2 anymore, so I’m often taking in more because I’m trying to fuel threshold/sweet spot efforts within this work).
Less than 2hrs, I often just do a bottle or two of Skratch or water, which amounts to 0-40g/hr of carbs.
The bottom line is: you don’t fuel a shorter endurance ride as much as you do a longer or harder session, but you should still fuel it. Chasing “fat adaption” or thinking you’re going to get better at burning fat by denying your body any in-workout sugar is a mistake, IMO, even if weight loss is a goal. Do more work, fuel that work, and then make sensible changes in how you eat AROUND the workouts.
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It doesn’t allow YOU to do more work, maybe. Perhaps because you’re a guy that goes out and does 12 hour rides. But I think you have the tail wagging the dog here - you’re not “fat adapted” because you don’t fuel, you’re “fat adapted” because of the duration and nature of the work you choose to do.
And I would challenge how you know you can’t do more work by not fueling it. Yes, you can certainly survive a 3000kj ride without fueling, but you’d probably perform and recover better if you adequately fueled it.
Finally, I’d caution making broad physiological statements based off of n=1.
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more work once you run out of carbs stored in muscles (and liver?). Which is why Kurt said “IF” you can do more work.
From field testing I’m good for roughly 3 hours / 2000kJ, and then
the low fuel signal starts blinking!
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This is also a pretty gross misconception. Your body wants to use fats preferentially provided that the ATP demand you are placing on it is appropriate for those fats. That’s how we are designed and indeed WHY we store fat in the first place.
Even at the lowest intensities, you’re still using carbohydrate. As intensity goes up, your body demands more ATP, so THAT is what causes it to burn more carbohydrate. In addition, if you are drawing on energy stores before your body has been able to mobilize fatty acids, you’re going to burn carbohydrate.
When the ATP demand exceeds the ATP that your mitochondria can produce using mobilized fatty acids, that is when your body starts drawing more on exogenous and stored glucose. The longer you generate that ATP demand signal, the more your body releases fuels in the form of fatty acids to compensate over the long term.
Substrate utilization depends more on the signaling your body is receiving from the work demand than it does on what is immediately available. A LONG, low intensity ATP demand will necessarily foster better fat oxidation; shorter, higher intensity or variable demands will increase carbohydrate utilization.
But both efforts should be fueled with carbohydrate to maximimize performance.
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100% Don’t diet on the bike.
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It is certainly possible you were overfueling the work you were doing. The recommendations of doing 100g/hr of carb for every ride regardless of duration and intensity is silly. That said, you seem to be swinging the pendulum all the way to the other end and saying fuelling with carbs at all is bad and unnecessary, and that’s just plain misguided.
This is a cynical viewpoint not really founded in physiological reality. I do agree that the TR podcast goes over the top in their fueling recommendations in terms of quantity, but their general guidance is not bad.
You should fuel the work that you’re going to do. A 1-hr sweet spot workout doesn’t need 100g of carbs, but many people will perform better with 20g worth of Skratch in their bottle during that workout. Changing your carb intake based on intensity and duration is a smart strategy. Underfuelling your work hoping to lose weight or chase improvements in fat oxidation as a performance gain is not.
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This is not my take-away at all. Having to eat 3 healthy meals that are around ~1,500cals is much harder than eating 3 healthy meals that are ~1000cals.
I will say that I’ve never struggled with keeping my weight low (in fact I have a hard time gaining weight) so my n=1 experience might be different than others. It’s very hard for me to eat enough-not the other way around. Eating while riding helps me maintain my weight/performance, if I didn’t eat while riding I would surely go into an unhealthy energy deficit and end up blowing up or having to reduce my training volume.
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It might be, if the target audience for the advice was broadly representative of Western society at large.
I can’t say I’ve ever heard the advice given in that context. The target audience has always (as far as I recall) been cyclists looking to manage body composition (a euphemism for “get lighter”) in order to maximise performance.
Those are very different audiences.
For context, I’ve been successfully experimenting doing longish (2.5 hr) Z2 trainer rides fuelled by the previous evening’s risotto, a decent breakfast and nothing else except water, so I’m not driven by ideology. 
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Counterpoint to all the non eating during rides people. I eat during everything over 90 min and have upped training to 20-25 hours a week after struggling to hit 12 hrs previously. Just eat and watch your weight. If it goes up eat a bit less off the bike. If it doesn’t you’re fine. People make this more complicated than it needs to be.
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and the same for recovery. No matter what I eat, at sixty with my work/personal life the max I can recovery from is 11 hours.
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That description is not one I have ever read anything about with respect to substrate utilization in any way. Central governor theory has to do more with conscious and subconscious regulation of effort relative to fatigue and glucose availability. And it’s a flimsy theory, at that, IMO.
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“Keto” athletes still burn sugar and are still capable of glycolysis. Increased fat oxidation has never been shown to be a performance enhancing effect, so again, I am not sure why there is this fascination with maximizing fat oxidation when the body has bioenergetic systems designed to use multiple substrates and does so exceptionally efficiently.
Chasing manipulation of substrate utilization is a waste of time in all but the most exceptional cases, IMO, and in my experience (I was “keto” and purposefully chasing fat adaptations for many months in 2013-14), it is performance limiting for most mainstream events and interests.
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Reminder that weight lifting burns a metric f*ckton of glycogen (probably a misquote) according to Kolie Moore. Something to consider for those lifting as well as doing judicious amounts of Z2.
Needs plenty of carbs to sustain all that work IMO. Of course shorter stuff <0.65IF could get a pass - just depends what sessions you have before and after.
The difference is relative vs. absolute: if you increase your energy demand by raising your FTP/lactate threshold by 10 %, your body will need to produce 10 % more energy. If you increase fat oxidation and glucose production by 10 % each, the relative fat and glucose oxidation rates will remain the same. That is, even though you have raised your fat oxidation rate by 10 %, you are not more fat adapted than before.
You are only more fat adapted if relative fat oxidation rates increase.
As above, you are being too vague, because you do not clearly distinguish between relative fat oxidation rate and absolute fat oxidation.
Sounds more like you did not adapt your off-the-bike nutrition properly. When I started fueling more, my relative power increased a tad, but since I weighed about 1.5 kg more, my absolute power season-to-season was higher. N = 1 anecdata, I know.
Yes, but should you? Is that good for your training in the long run?
Yup, for me it is about the same, I I start feeling empty after about 3ish hours. When I empty my tank, life gets really miserable, not just the rest of the ride, but also the next day. Not recommended.
I’m curious why people measure nutrition (on the bike) in kcal vs carbs when the assumption is kcal outside of carbs are less or ineffective when fueling an effort. Ie One could consume 300kcal in nuts which would not amount to much in regards to carbs.
Fats and protein need significantly longer to be digested and your body needs to divert more energy to your digestive system — not great during exercise. (Unless you are talking about an all-day effort or so.) That’s why people count carbs and calories.
You are jumping to conclusions: measuring your in-workout intake doesn’t mean you are measuring food intake outside of your workouts.
What you write here suggests to me that you have adapted your daily diet to fuel your workouts, and adding more fuel without altering your off-the-bike diet results in a calorie surplus. So your reported weight gain comes to the surprise of no one.
Yeah, so are others like myself. I eat what my family eats and what I cook, which is relatively normal food (including roasts, although I tend to make those myself).
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Exactly, The direct reference was to a comment that referenced kcal/hr and not carbs.
…eating about 400-700 kcals total on the 2 hour ride. Which I usually do as 2 Cliff bars (250 calories each),
I guess I’m not really sure why this is a response to me. I will go back to what I said earlier:
In other words, if you want to be a better fat burner, ride longer at lower intensity FIRST AND FOREMOST. You won’t become a great fat burner by doing 2 hour fasted rides. Simple as that.
Yes, you can manipulate fat oxidation with dietary change. The question is, why do you want to? My point was that you enter a ride with some kind of base metabolic rate of consumption of glucose and fatty acids, and the primary factor affecting substrate utilization is then intensity and duration. You can manipulate that base rate, and of course you can not take in any carbohydrate and theoretically burn more fat, but again, at a performance decrement to the point where most people are able to do LESS work at a higher RPE and recover LESS effectively by doing so.
This is why the theory-to-practice with fat-adapted athletes has never made it to the pro peloton: it simply doesn’t work as well as you’d think. Why that is, well, for performance, there are studies showing that the glycogen-sparing effects of high-fat diets are overblown from a performance perspective, even after carb loading in this case. This talks about sympathetic nervous system activation as well as neurological stressors associated with low glucose levels. I have no idea if that’s why, but it could be.
If ISM truly believes that, then I would expect that Pogacar and much of the pro peloton would be keto and fat-adapted. From a pure endurance perspective, the theory makes sense. In practice, however, it has not played out that way, and professional bike racers need to be able to perform both sub-threshold for long durations and at high power after that. Unfortunately, science and anecdotal evidence points to keto being suboptimal for performance. ISM knows that, too.
Again, I have argued only about performance here; my argument with you specifically is that going after fat oxidation as a performance gain is an exercise in futility. If you want to chase it because you think it helps you lose weight, go for it. My counter to that would be if you can do more work for longer, you’re better off fueling your rides, and most people can do more work for longer if they’re taking in adequate carbohydrate.
Back to the original points I made - your body always uses carbs at low intensity. Carbohydrates and fats are both used in aerobic metabolism. Fatty acids take time to mobilize, so say for the first 20 minutes of any ride, you’re usually using carbohydrate preferentially anyway. The longer you ride, the more signaling your body gets to use fatty acids, the more it mobilizes, and the better it gets at doing so and the rate of carbohydrate usage also declines regardless of the existence of exogenous glucose in your bloodstream.
There’s a reason most coaches don’t prescribe fasted rides that are of any significant length, or if they do, they tell you to go into the ride carb-depleted, but then fuel the ride as you go. Why is that? Because they know that underfueling rides to chase fat oxidation gains inhibits performance and recovery, and that detriment is not worth the gain in improved fat oxidation unless you’re racing ultra-distance events at very low intensities.
Here’s another pretty good read on all of this.
It seems that the main candidate for FA oxidation regulation is the muscle carnitine content. At high-intensity exercise, the rapid glycolysis provides the mitochondria with excess acetyl-CoA, which is buffered by free carnitine to form acetylcarnitine. Accordingly, a fall in muscle concentration of free carnitine may reduce CPT-1 activity, and thus the ability to transport FAs into the mitochondria, and therefore, also the rate of FA oxidation.
This is part of the reason why when you do high-intensity efforts early or throughout an otherwise long endurance ride, you’re compromising your ability to use fats as fuel until your cells can regulate carnitine content and get things going again. That one-minute KOM you chase can inhibit fatty acid oxidation for many minutes afterwards, requiring further carbohydrate use.
TL;DR:
So the best way to burn fat? Ride long at low intensity. You’re going to be better able to go longer if you fuel that ride.
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I made one claim here, and then present my hypothesis/speculation (AKA untested unproven theory).
Hypothesis: - a supposition or proposed explanation made on the basis of limited evidence as a starting point for further investigation.
Speculation: the forming of a theory or conjecture without firm evidence.
Research done by Iñigo San Millán (Tadej Pogacar Coach) and George Brooks.
In the Podcast where Inigo talks about this graph he mentions that they were forced to publish the data in Absolute Watts instead of the relative W/Kg which would have showed an even greater difference between ¶ Professional Athletes and MA (Moderately Active)
Conversely you can see that the levels of Carb oxidation is quite similar between Pro’s and Moderate Athletes.
Relationships between the average blood lactate concentrations and the average CHOox rates as a function of exercise power output in a international-level professional endurance athletes, b moderately active healthy individuals, and c individuals with metabolic syndrome. CHO carbohydrate oxidation
You seem to have a bad habit of not reading peoples words and then twisting them. Where do I say negative?
For the record I had no issue with your strategy and fueling choices…I just found that after years of doing exactly that strategy I was possibly missing some of the adaptations that I was looking for in those zone 2 rides. I took issue with your post because instead of just giving your recommendation you choose to shit on mine, and present your own as the superior scientific method without any evidence…not only that you seemed to jump to false conclusions on what I was conveying.