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Cy Wilson--? on protein metabolism

Cy,
Brian Batcheldor’s plan had one consume only protein for most of the first half of the day.

I’m wondering how the body determines whether those amino acids would be use for anabolism (entering the regular protein turnover cycle) or energy.

I know protein is used for energy via glucogenesis or ketogenesis. But when the glycogen stores aren’t empty, does the body just deplete them before it starts using dietary amino acids?

In other words, if you eat only protein for the first half of the day (and enough to promote anti-catabolism), does your body go into fat and carb stores as though it is in a caloric deficit?

Would like your expertise, please.

Here’s the way it works in a case of starvation. When glycogen stores are still full or “not empty” then that’s what the body is going to be using carbs, primarily as a fuel source. It’s not until the few hundred grams of glycogen in the liver and muscles are depleted that you begin to see a rapid increase in protein utilization for fuel.