How hard can you go and still use fat?

If you’re a muscle of a rat, really darn hard…so hard that initially almost a quarter of all energy is derived from non-oxidative metabolism, and force falls by ~30% in 5 min and ~50% by 20 min:

If you’re a trained human, at least up to 85% of VO2max, and possibly even harder (methods for quantifying fat oxidation aren’t really valid during even higher intensity, shorter duration exercise so it’s hard to say for certain. Based on the rodent study cited above, however, it seems likely that there is still at least some small contribution from lipids even during exercise well in excess of 100% VO2max):


I’m a bit of an idiot, does this mean that 1/4 of energy during high intensity exercise comes from glycolysis in this example and the rest would be from fat?

Also, if you had a throw some ballpark numbers out there when you’re at ~threshold what is the breakdown for where energy is coming from fat vs other sources? 60-40? 90-10? or do you have a different way to answer this to help someone without a physiology background understand.

Roughly one-quarter from non-aerobic glycolysis (i.e., lactate production), with muscle glycogen being the primary source of carbohydrate, as well as a much smaller contribution from depletion of phosphocreatine stores.

The other three-quarters of energy was derived via oxidative phosphorylation, with both carbohydrates and lipids providing the requisite acetyl CoA. (I could refresh my memory by rereading Larry’s study, but it’s rat muscle, so not sure how interested/interesting that might be).

As for carbohydrate-fat balance at “threshold” (however that is defined), generally speaking it’s going to favor carbohydrate over lipid, but the exact mixture will depend upon habitual diet, nutritional state, exercise duration, and possibly sex, age, and exercise modality.

Big picture, “carbohydrate is king” - the only time lipid would clearly dominate over carbohydrate would be during lower intensity, longer duration exercise in the fasted state, and/or when consuming a low carbohydrate diet (when glycogen stores are low). Otherwise, your RER is likely to be in the mid 0.8 range or above (with 0.85 being 50/50).


Was tempted to post something snarky however I’ll be serious… What are your thoughts on “training fat burning” and the basis for people making those statements?

My bias - my training seems to be focused on either training muscle fibers or the aerobic system (or both in some cases).

Endurance exercise training increases the absolute rate of fat oxidation at the same absolute intensity (i.e., power output or metabolic rate, i.e., VO2). It also increases the absolute, and often the relative, rate of fat oxidation at the same relative intensity, i.e., the same percentage of VO2max.

HOWEVER, these adaptations are NOT the result of oxidizing more fat during training, which is what many mistakenly assume (and then attempt to maximize). Rather, they are primarily the result of having more mitochondria, which allows you to better balance ATP demand and production, thus minimizing the activation of carbohydrate utilization. The increase in mitochondrial respiratory capacity with training is driven by these energetic perturbances, as well as calcium release from the SR.

The $64k question then becomes, how to maximize the increase in muscle mitochondrial respiratory capacity with training? That’s something that science can’t really answer, at least at the level of the individual, but 1) like all muscular adaptations, they are restricted to the muscles and even the motor units recruited during exercise, and 2) the largest increases reported have been found in individuals training very intensely (not just long).

TL,DR: Train for performance and let your physiology sort itself out.

ETA: Here is a classic review articles discussing this issue:

If you haven’t heard of John Holloszy, he was essentially the OG of exercise metabolism, and is widely recognized as being the first person to demonstrate that endurance training increases muscle mitochondrial content:


Thank you!

I thoroughly appreciate these. Evidence > everything.

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I went about this hard:
(Fat Ox went to 0 about 340-350W, but was steady until about 300W)
From this test I got a VT2 of 337W, which was not really obvious to me just from looking at the graphs

Xert’s idea of fat/carb utilization is described in an article here. To me there seem to be at least some loose correlation to what you describe.

Calculated fat oxidation. At some point, hyperventilation with respect to CO2 production invalidates the use of indirect calorimetry to quantity substrate oxidation. Based on Spriet’s study that I previously posted, it’s possible that small amounts of IMTG are being utilized even at intensities >100% of VO2max.


I was Keto for a year or two of my life. Anecdotally, not fast enough. Z2 was awesome, and I could ride a long time fasted. Same with slow runs. But trying to put down any power sucked. I much prefer being a carb burner on the bike.

I can just add my results from testing in a lab this weekend, my FatMax was 210w. LT1 at around 250w and LT2 at around 310w.

It was cool to see that even at 120w I was burning around 1g/min of carbs…

My FatMax was around 0.55g of fat per minute.