Hi ST2008, it sounds like you are mainly worried about protein. We really don’t need very much protein on a daily basis as we only lose about 20g to 40g of protein a day depending on our body size. As we do a multi-day fast, the amount of daily protein lost declines. Most protein is recycled many times in our bodies. Proteins are broken down into amino acids and those are used to build new proteins or can be used as a fuel too.

When we are fasting our bodies are breaking down a lot of old or poorly functioning cells. As we burn off fat we also have a lot of cells that used to be just supporting that fat, so there is a lot of junk the body can work on before it has to go after muscle. In fact while fasting the body becomes protective of muscles. So while our bodies don’t warehouse protein there is plenty in our bodies to go a very long time, provide we have fat to burn. When fasting your muscles act more like you are a teenager than a mature adult. That is mainly because the hormone levels change a lot, for example large increases in HGH.

Over shorter fasts, a lot of protein comes from cells being shed in the intestinal track. Basically the lining of the intestines. These cells are replaced when we are eating. During a multi-day fast, apoptosis kicks into high gear and weak cells end up breaking down so they can be replaced or repaired. This is a major benefit of multi-day fasting and is also a source of protein.

There is a period while our bodies are shifting into ketosis that some extra protein likely does get consumed, but the actual amount is probably less than 50g, maybe a lot less. So there is a possibility that a small amount of lean body mass will be lost.

Compared to standard calorie restriction this loss is minor. Many people on calorie restriction end up losing more lean body mass than fat. That is because of how the hormone levels work. With just calorie restriction there isn’t really any protection for your muscles.

Anyway people have fasted for months and even over a year without issues with protein, at least as long as there is fat to burn the protein is safe. If you are underweight then you shouldn’t fast because there probably isn’t enough fat for the body to depend on. Most of us don’t have that problem.

Actually it is autophagy where cells are cleaned out and parts of them recycled. This increases a lot in fasting. Apoptosis is programmed cell death which also likely increases in fasting but is also an on-going process. These are sources of proteins while fasting. Also see https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2495396/ It is about a 27 year old man that water fasted for 382 days under medical supervision. He was super morbidly obese when he started.