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Afaik skeletal muscles take glucose from the blood to store it localy as glycogen, to be used in the case of intensive exercise.

Does this still happen

  • if the calorie input is exclusively made up of fats and proteins?

  • while fasting / starving?

I know that muscles can run on fatty acids and ketone bodies. And that the liver can create glucose by gluconeogenesis.

But is this "backup"-glucose still used to refill the glycogen stores in the skeletal muscles in those two respective cases? (while resting)

If not, how would the performance of exercises like sprinting, that normally make use of those stores, be affected?

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Most of it isn't but a little bit is.

When your body has very little glucose and a lot of fat(Which can be the case if an overweight or obese person goes on a low carb diet) the ketones and fatty acids and glucose produced by the liver are used almost exclusively to keep the body alive although a little is synthesized into glycogen.

Specifically when you don't eat anything gluconeogenesis alone supplies enough for 1 day. Then the fat metabolism accelerates because gluconeogenesis is not enough. This is when you get into a state of ketosis(this can also happen in bodybuilders since increased muscle = increased fat metabolism). This ketosis causes fast weight loss from not just water like a lot of other diets but also from fat. This fat then eventually depletes and it metabolizes protein exclusively(during all this time it does metabolise some protein). However that can't continue forever even if the amino acids get reused over and over again because some are used to produce ATP. Thus you eventually would die from starvation with obese people surviving longer than underweight people.

Anyway as far as exercise assuming that you start off obese, getting your body into a state of ketosis makes your performance even better than on just glucose alone after a while of adjusting to fat metabolism since 1 molecule of palmatic acid(16 carbon fatty acid) produces 106 ATP compared to 32 ATP for 1 glucose molecule so your muscles can go longer without being exhausted. Theoretically someone who is 50 pounds overweight could exercise 24/7 for several days if he/she is in a state of ketosis.

Here is a reference to the low glycogen and increased muscle performance: http://www.bodybuilding.com/fun/glen12.htm

As to why it is 106 ATP it is because each NADH produces 2.5 ATP and each FADH2 produces 1.5 ATP and because of the activation step using 2 ATP just like how NADH transport from cytoplasm to mitochondria takes 2 ATP, 1 per NADH in glycolysis.

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  • $\begingroup$ interesting, can you add some references? I'm not convinced you can draw any conclusions about muscle performance from the molar ATP yield of fatty acids vs. glucose, those are two very different substrates ... Also, for the original question, is there some data showing that muscle does not synthesize glycogen when on low carb diets? I imagine low insulin would have this effect, but I'm not aware of a study. $\endgroup$
    – Roland
    Commented May 31, 2015 at 12:33
  • $\begingroup$ Well I looked at some websites and I was wrong about no glycogen being synthesized when the body is in a state of ketosis but there is a very low level of glycogen that is synthesized because of the low insulin. This low insulin also makes the fat be the primary metabolite since insulin causes there to be more fat in adipose tissue. $\endgroup$
    – Caters
    Commented May 31, 2015 at 20:13

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