I can’t answer these definitively, but there are a few reasons why I think our bodies burn some muscle for fuel to meet glucose needs instead of solely relying on glycerol:
1. Glycerol simply may not be able to meet all of our glucose needs.
2. Muscle is made up of several amino acids that can be used to generate energy in more ways than glycerol. This may increase the demand for and use of amino acids when our body needs energy.
3. Muscle mass burns a substantial amount of energy. Less muscle means less energy used to maintain that muscle and more energy for biological processes necessary for our survival. In evolutionary terms, if we never burned off unnecessary muscle mass during times of food deprivation, our vital organs would run out of energy much more quickly and we would perish sooner.

“If someone fasts for a long period of time, why would muscle need to break down at all once ketosis begins if they have a lot of fat?”

Because ketones can only supply up to 50% of our basal energy requirements and 70% of our brain’s energy needs.

Here is an excerpt from our article on ketones that explains why:
“During fasting or carbohydrate restriction, gluconeogenesis kicks in to keep blood sugar at healthy levels and provide energy for the liver and red blood cells, while ketogenesis (burning ketones for fuel) is used to provide energy for the brain, heart, kidney, muscle, and other aerobic cells.”

If you’d like to dive deeper into this topic, feel free to check out the full article:
https://www.ruled.me/ketones-everything-need-know/

Thanks so much! I’m glad you enjoyed the article 🙂

We fixed it — thanks for pointing this out!