The changes that occur during a fast are listed in the FAQ at the top of the page, and a post I made about 3 years ago...............
In switching from glucose as the bodys main fuel source in the absence of nourishment the body switches to fat-burning or ketosis within the first 2 to 4 days.
Muscle tissue or mass does not kick-start ketosis because this is instigated when the reserves of glucose are exhausted or almost so. Muscle mass and tissue is preserved for the most part in a process known as protein-sparing in an effort by the body to conserve its vital organs and tissues.
The Liver is a major organ of elimination, along with the kidneys, skin, and lungs, and work to that end in detoxing throughout the fast. Some glucose is still required by the body throughout and is manufactured via the Liver as well.
If the pre-fast diet excludes carbs' then the body will be in partial but not full ketosis and where I mentioned previously muscle tissue is preserved in much the same way as are the other vital organs such as the heart, lungs, kidneys, pancreas, spleen and so on. The body has an innate ability in the process of autolysis to be very discerning about its fuel source in an effort to stave off actual starvation.
Hope that helps.