Glycogen or fat

So I hear and see information about training the body to burn fat as a way to keep fueling your body with longer rides. If my body is trained to burn fat what does that really mean? Does it mean if I have a full glycogen level in my liver I would still skew toward burning fat because Ive trained my system to burn fat? Or do I still depleat my glycogen level to a certain level before skewing more toward fat? Also once glycogen is depleated isnt it normal to burn fat, I dont quite follow the need to train to burn fat. Sorry for my ignorance and thanks for any clarity u can provide.

At various intensities, your body will burn a different ratio of glycogen to fat. If you train to increase your fat utilization, you simply skew that ratio more heavily towards fat.

For example, if you were 50/50 fat/carb when riding at 80% of FTP, and you trained up your fat utilization, you may be able to do that same power at 60/40.

My understanding is that yes, even if your glycogen stores are full, you’d not be draining them as much, because more of your effort would be fueled by fat. I don’t believe that the energy expenditure ratio changes based on how much glycogen you’ve burned, although I don’t have a source to cite on that either.

Once glycogen is depleted, you will bonk. If you continue to ride, you will be forced to do at low intensity, and based on fat metabolism. If you trained to burn more fat, you will be able to ride longer and/or harder, before you deplete your glycogen stores. It also becomes easier to keep up with carb intake on long rides, because you have a finite amount of carbs you can absorb per hour while riding. If your burn rate is slower, but your intake rate stays about the same, you can last MUCH longer before completely running out of fuel.


Interesting. Thanks for the reply

One would have to write a book to give you a thorough response :slight_smile:

One of the key adapations to endurance training is the shift towards more fat utilization at a given work load. See the following charts, carb and fat ox of trained and untrained subjects at repeated bouts around FTP:

Trained subjects can go faster while relying less on carbs to fuel the effort.

One of the reasons for this is this:

Endurance training makes some of your fast twitch muscle fibres more oxidative, e.g. fatigue resistance (more mitochondria and so). You simply have more muscle fibres available for an effort. This means you can go faster and especially longer. One of the aspects of fatigue is that muscle motor units run out of glycogen. If you have more motor units available you fatigue later.

Endurance training promotes molecular signalling in this certain type of fast twitch fibres, e.g. this growht factor PGC-1alpha. PGC-1alpha triggers/controls plasticity of muscle fibres, e.g. a shift towards more endurance type. This is one of the reasons why there is a lot of research on two-a-days and so which is assumed to promote PGC-1alpha exposure (see the other thread on “secres of a pro’s coffee ride?”)


There was a very interesting new research paper published on two-a-days as you mention just a week or so ago:

It’s out of David Bishop’s lab (who is a very well-respected and reputable scientists in the industry). Of course, this is just one study, but it supports what the current base of evidence already suggests:

“These results suggest that the elevated molecular signalling reported with previous train-low approaches can be attributed to performing two exercise sessions in close proximity rather than the reduced muscle glycogen content per se. The twice-a-day approach might be an effective strategy to induce adaptations related to mitochondrial biogenesis and fat oxidation.”

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What really puzzles me is the relevance of a low-glycogen state. Necessary? And is it necessary to withhold carbs between workouts? Studies show conflicting results, one is cited in the thread above.

My understanding is that it is possible that consciously training in a low-glycogen state strategically (ie. not all the time) might add the icing on the cake so to say. There is conflicting evidence as you say, and especially, I don’t think anybody has been able to show direct performance benefits.

Given this potential, if it can be done without negatively impacting important training or recovery, why not?

But necessary - no I definitely don’t think so. My understanding is that the majority of pros aren’t intentionally putting themselves in low-glycogen states.

But, the vast majority of improvements in substrate metabolism comes from actually training to become fitter (first) and training smart (the right metabolic systems, second). Put Chris Froome on an ice cream diet but keep him training the same way and his fat metabolism will still be at an insane level compared to average Joe’s.

studies by Marquet have shown direct perfomance benefits with “sleep low”.

However, what bothers me is how “performance benefits” are measured in all these studies. A short ramp test. And sometimes a 40min time trial but this is already the max.

I would expect these adaptions to be most benefcial for longer racing. When you actually run out of glycogen. And this is never tested in any of these studies.

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If it is beneficial at longer distances I think a 40 minute TT is a really good performance test and a difference should be visible simply based on the fact that aerobic endurance has improved. Because even though we want to increase our ability to oxidise fat, we don’t want to decrease the absolute amount of carb oxidation - just the relative one. If we do that, then aerobic capacity increases.

So I guess even a ramp test shows the same thing, but a 40-min TT is probably a bit better as it’s more purely aerobic.

Need to check those studies by Maquet out. The ones I’ve come across always tend to be the ones that only measure FatMax but no actual performance testing. Great that somebody has done it.