Fasted aerobic training

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  • #9967
    Colin Simon

    This short “Science of Ultra” podcast ( ) explores the topic of fasted aerobic training for the purpose of fat adaptation. The author presents some information that conflicts with a lot of the literature on this site, so I figure it could make for an interesting discussion:

      “If you choose to train in a glycogen depleted state, the biggest take home recommendation of this episode is to make sure that you’re actually depleting the tissue you’re trying to stress. If you want to stress skeletal muscle’s fat burning, an overnight fast won’t do it. For that, you have to deplete muscle of its stored glycogen with a bout of high intensity or long duration exercise and then not replace the carbs until after the next workout.”

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    Anonymous on #9981

    There are many ways to train with depleted glycogen:

    No recovery food between workouts will target local muscular stores. Training first thing in the morning, from what I understand, targets liver stores.

    Colin Simon on #10009

    Scott, that seems to agree with the podcast I linked. But it also suggests that simply training while fasted is not going to yield significant developments in fat adaptation in the skeletal muscles, which is theoretically what most visitors of this site should be interested in.

    Anonymous on #10012


    Overnight fasting does leave you in semi glycogen depleted state (dependent on your state of fat adaptation). It will mostly affect liver stores and not much in the muscles (which have been fairly inactive all night). But when you go out for that longer (or high intensity) run first thing in the AM with the lowered liver glycogen levels the muscles’ source of glucose (the liver) is going to be compromised much quicker. That in turn results in depleted muscle stores. The muscles have to find energy some place. So, lipolysis enzymes up-regulate to give more Free Fatty Acids (FFA) which, through beta oxidation produce Acetyl CoA, the precursor for aerobic metabolism. If the glycogen stores are lower, for whatever reason, then you have to find the fuel some place and your body’s natural inclination is to shift to fat. The overnight fasting just allows you to reach that glycogen depleted state a bit sooner in the workout.

    From the article:

    As you begin your workout, skeletal muscle prefers its own fat and glycogen stores but gets some of its energy from fats and glucose in the blood. The liver breaks down glycogen to ensure that blood glucose levels are maintained to support all the cells of your body. During exercise, a person ‘hits the wall’ or ‘bonks’ when they run so low in liver glycogen that blood glucose levels drop. Muscle glycogen levels will also be low at this point because the muscle has been preferentially consuming it.

    These two systems (liver and muscle glycogen stores) are not separate and independent. Glucose liberated from liver glycogen gets used in the muscles. Anyone who has ever bonked hard and then eaten a candy bar can attest to the fact that his muscles suddenly had power again almost immediately. This happened because his blood sugar rose within minutes of eating and the muscles made use of that glucose in the blood. Deplete liver glycogen and muscle depletion will happen sooner.

    As far as I can see the above quote negates the author’s earlier assertion that training in fasted state will not improve fat adaptation via muscle glycogen depletion.

    This is exactly what we say in many places on this site. And, you do not have to “Bonk” to trigger a fat adaptation response. Muscle fiber glycogen depletion is perhaps the most powerful signal that starts the aerobic adaptation cascade. But……It is only those muscle fibers that become glycogen depleted that see this effect. This may not be 100% true as there is some evidence that nearby fibers give up their glycogen in times of dire need. So peripheral fibers may get some of this aerobic training effect.

    As for not eating till the next workout: While that will speed fat adaptation it is going to slow down recovery. In some cases significantly (by days).

    His final comments concerning doing a depleting high intensity workout in the AM followed up with a longer low intensity workout in the PM may be fine for one day. But how are you going to feel the next day or two. In my rather extensive experience I have never seen good results come from delaying fueling after a workout. As mentioned above, it will slow recovery and in the end lessen the amount of training the athlete can do.


    rando_luke on #10056

    Would there be a greater training effect if instead of doing a high intensity workout in the A.M. then waiting until later for the long-duration exercise if you did them back to back? Let’s say the first 15-20 minutes before a training hike or run you went at a fast pace (zone 3) or did sprints for a couple minutes beforehand? And if so would it be any greater than just fasting in itself?

    Anonymous on #10094

    Yes there would be a significantly greater training effect from stacking these two workouts back to back. I would suggest trying this in small doses to see who it affected your recovery.


    rando_luke on #10123

    Thanks Scott, I started to test something like that about two-three weeks ago so I’ll report a little about how I feel/felt it helps when I post an athlete story sometime next month. I didn’t have too much of an issue with recovery after those but I did it mostly on rather longish days (14+ hours of hiking with climbing above 10,000′) so I was resting the next two days or so and basically just started out hiking at a pace that I couldn’t talk and breathe for 15-20 minutes then started to slow down to an all-day pace.
    No issues with bonking for me but I was wondering a little about a purely climbing- oriented method to do the same thing, my buddy climbed the Nose for his first time IAD a couple weeks ago and while they made it in 19 hours they both bonked pretty hard below the last couple of pitches. He’s trying to figure out ways of preventing that short of quitting his job and moving to the Valley haha. Any ideas or insight on that would be awesome.

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