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High-intensity workouts when glycogen-depleted?

Answered on August 19, 2014
Created October 08, 2012 at 1:18 PM

What I think I know, from reading random blogs on the net:

  1. When exercising at low to moderate intensity levels, the body mostly burns fatty acids
  2. As intensity increases, the body burns more and more glycogen and glucose
  3. Even though the body burns a lower percentage of fat during high intensity workouts than at low, the total energy burned is higher at higher intensities, and the actual number of fat calories burned at higher intensities is greater than that at lower intensities.

From this, some come to the conclusion that high intensity works better for weight loss, since it's burning more fat calories as well as more calories total. Others seem to believe that high intensity workouts, by depleting glycogen, drive hunger, and thus pushes people to eat more calories than they burned.

So, my questions:

  1. Do people with metabolic syndrome burn fat at moderate intensity levels, or do their chronically high insulin levels block fat metabolism, and thus redirect the energy source to glycogen at lower levels of intensity than for health people?
  2. If you've eating a low-carb/ketogenic diet for enough time to fully adapt, and your glycogen stores are significantly depleted at normal times, what happens during high intensity exercise? If the glycogen isn't there, does the body simply not shift to glucose, and increase the use of fatty acids? Or does it start breaking down protein to create the glucose that it needs?

8af1e83ec3ea5a39f050baf362708a78

(253)

on October 08, 2012
at 07:50 PM

But how long would mitochondrial biogenesis take? In my own case, the increase in energy showed up after about two weeks. Would sufficient new mitochondria have been created to make a significant difference, in that short a time?

A968087cc1dd66d480749c02e4619ef4

(20436)

on October 08, 2012
at 05:51 PM

Another idea is that low carb diets ramp up AMPK which increases mitochondrial biogenesis. Which might be due to insulin (or lack thereof) - Dr. Mike Eades blogged about it here: http://www.proteinpower.com/drmike/inflammation/can-your-food-make-you-fit/

8af1e83ec3ea5a39f050baf362708a78

(253)

on October 08, 2012
at 04:53 PM

People with metabolic syndrome who go low-carb commonly report an increase in energy, and an improvement in their ability to exercise. A reasonable explanation is that in the presence of insulin, their ability to burn either fat or sugar is crippled, but that in its absence, their ability to burn fat is significantly improved. If this is true, cutting carbs would significantly improve their ability to exercise at moderate levels, but that would decrease as the intensity increased into zones where sugar would normally become more significant. But "reasonable explanation" does not mean "true".

B4e1fa6a8cf43d2b69d97a99dfca262c

(10255)

on October 08, 2012
at 01:52 PM

+1 for the awesome formatting job!

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3 Answers

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A968087cc1dd66d480749c02e4619ef4

(20436)

on October 08, 2012
at 02:19 PM

If you're glycogen stores drop near zero, you will soon be dead. Glycogen never really gets below about half full, due to the body's emergency processes to refill it - gluconeogenesis driven by epinephrine and cortisol.

I am not aware that depleting glycogen increases hunger. Rapidly dropping blood sugar can, however. Obviously there are linkages there.

People with metabolic syndrome have fewer mitochondria and more defective mitochondria than healthy, lean types. This is true for obese, pre-obese and post-obese people. Of course, intense exercise helps with mitochondrial biogenesis, but it makes that exercise more difficult.

Sorry, I didn't really answer either of your questions directly. But for #2, the scenario isn't plausible to me and as for #1, all energy burning is at a reduced effectiveness - it's not about the ratio of glucose to fat...

A968087cc1dd66d480749c02e4619ef4

(20436)

on October 08, 2012
at 05:51 PM

Another idea is that low carb diets ramp up AMPK which increases mitochondrial biogenesis. Which might be due to insulin (or lack thereof) - Dr. Mike Eades blogged about it here: http://www.proteinpower.com/drmike/inflammation/can-your-food-make-you-fit/

8af1e83ec3ea5a39f050baf362708a78

(253)

on October 08, 2012
at 04:53 PM

People with metabolic syndrome who go low-carb commonly report an increase in energy, and an improvement in their ability to exercise. A reasonable explanation is that in the presence of insulin, their ability to burn either fat or sugar is crippled, but that in its absence, their ability to burn fat is significantly improved. If this is true, cutting carbs would significantly improve their ability to exercise at moderate levels, but that would decrease as the intensity increased into zones where sugar would normally become more significant. But "reasonable explanation" does not mean "true".

8af1e83ec3ea5a39f050baf362708a78

(253)

on October 08, 2012
at 07:50 PM

But how long would mitochondrial biogenesis take? In my own case, the increase in energy showed up after about two weeks. Would sufficient new mitochondria have been created to make a significant difference, in that short a time?

1
3ce6a0d24be025e2f2af534545bdd1d7

(26217)

on October 08, 2012
at 02:21 PM

I can take a shot at #2.

According to, "The Art and Science of Low Carbohydrate Performance": Muscle glycogen levels are never totally depleted, even on a VLC ketogenic diet.

And the glycogen levels for the cells that require glucose can be replenished meet their needs through gluconeogenesis and moderate carb intake.

However, fully adapted low carb people have shown significant physiological adaptations that reduce the need for glucose in aerobic activities

So,

  • During anaerobic exercise, a VLC fat-adapted athlete would utilize the same glycogen stores as anyone else. If you were to attempt to perform anaerobic exercise with depleted stores you would show the same fatigue as anyone else. There may be a slight advantage to the VLC athlete in terms of fat processing, but likely negligible in the context of high-intensity exercise.
  • During aerobic exercise, a VLC fat-adapted athlete can convert fat to energy more efficiently than most. This has been shown to be true regardless of the aerobic capacity (V02 Max). Thus there is less need for taping into the glycogen stores (although all forms of energy available to the body will be burned.
  • You liver can produce about 200g glucose from GNG each day. And muscle glycogen levels are never totally depleted. Thus, even someone on VLC will replenish their stores daily (assuming they are eating a proper diet of fat and protein).

  • 0
    C60cd8de76ce21b7d09c8e54586f41db

    on October 08, 2012
    at 04:51 PM

    #1 the more physically fit you are, the higher intensity of exercise in which you will burn more fat. So someone with metabolic syndrome will probably burn more glycogen at lower intensity levels, but that will change as their health and physical fitness improves.

    #2 by being fat adapted you can burn more fat at higher intensities, however your peak level of anaerobic performance will be a bit lower than if you were eating a high carb diet. I was a long-distance runner on a keto diet and was able to perform very high intensity workouts in a fasted state. Part of the reason your peak performance suffers is I believe the use of glycogen even when available is down-regulated while in a fat-adapted keto state. So if you are 90% keto and occasionally carb-load, your peak performance won't be as good as if you are 100% high-carb. But you also have to consider that a low carb diet may allow people to stay at a lower weight easier by suppressing over-eating habits, which benefits their athletic performance as well.

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