What I think I know, from reading random blogs on the net:
- When exercising at low to moderate intensity levels, the body mostly burns fatty acids
- As intensity increases, the body burns more and more glycogen and glucose
- 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:
- 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?
- 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?
asked byjdege (253)
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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...
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
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.