how are muscle glycogen stores affected by caloric deficit?

[KingGheedora]

I read somewhere that your body has something like 1800 calories worth of glycogen on hand for use (and this store running out is what causes runners to hit a wall around 20 miles into a marathon).

Side question: Carbs replenish glycogen stores, do dietary protein and fat do so as well?

So during a caloric deficit of several days to several weeks (say 500 cal / day), wouldn't you eventually run out of this glycogen?

 

[Buttzilla]

glycogen is stored in your liver and muscles (men have a bigger reserve then women). there is an order to energy metabolism, glycogen (or glucose) > carbs > fat > protein. you actually use up your glycogen storage fairly quick but as you eat throughout the day, it's storage gets replenished.

after a few days of not eating (and depletion of carbohydrate reserves) your body starts to break down fat in order to get the necessary building blocks for ATP (acetyl groups which gets converted into Acetyl CoA) the problem with this is the release of ketone acids (aka ketoacidosis, which is a problem with low carb diets). your body actually starts to break down fats earlier do to the glucose sparring effect. the nervous system and brain prefers glu as their main source of energy and your body will shift it's energy consumption around to reserve all glucose for the brain.

after 3-4 days of starvation and the depletion of carbs/fat reserves, the body will get energy via non carb/fat sources via gluconeogensis. primary sourc is through protein degradation breakdown down the polypeptide bonds to amino acid components. the amino acids are broken down in the liver and kidneys into pyruvate and enter the citric acid cycle.

carbs of made from polysaccharides, aka sugar building blocks
protein is made from polypeptides, aka amino acid building blocks
fat is made from triglyceride and free fatty acids

this is it in a nutshell, thanks for the good question, it's helping me reinforce and study for my dental admissions test next week

also, typical glycogen storage is 2k a day. the reduction of 500 calories from your diet is dependent on your Energy (in) vs. Energy (Out) so this varies from person to person.

 

[SociallyChallenged]

Just to quickly answer your side question - glycogen is just a complex of glucose. The body will break down proteins into amino acids and the glucogenic amino acids will then be converted straight to sugar and used. Glycogen is for storage. When your body has to use amino acids as energy, it's in a catabolic state, not an anabolic or storing state. It bypasses the whole conversion to glycogen and just goes to glucose.

 

[SP33Demon]

Yep, what SC said. You want to stay anabolic as much as possible. It's never good when you body is eating itself you're not going to maintain any muscle.