The most commonly leveled criticism of fasting is that in its efforts to survive self-imposed starvation the body metabolizes vital tissue, not just fat, and therefore, fasting is damaging, potentially fatally damaging.  People who tell you this will also tell you that fasters have destroyed their heart muscle or ruined their nervous system permanently.  But this kind of damage happen only when a person starves to death or starves to a point very close to death, not when someone fasts.

There is a huge difference between fasting and starvation.  Someone starving is usually eating, but eating poorly and inadequately, eating scraps of whatever is available such as sugar, white flour, rancid grease, shoe leather, or even dirt.  Frequently a starving person is forced to exercise a great deal as they struggle to survive and additionally is highly apprehensive.  Or someone starving to death is confined to a small space, may become severely dehydrated too and is in terror.  Fear is very damaging to the digestive process, and to the body in general; fear speeds up the destruction of vital tissue.  People starve when trekking vast distances through wastelands without food to eat, they starved in concentration camps, buried in mind disasters, they starve during famines and starve while being tortured in prisons.

Until water fasting goes on past the point where all fatty tissues and all abnormal deposits have been burned for fuel and recycled for the nutritional elements they contain, vital muscle tissues and organs are not consumed.  And as long as the body contains sufficient nutritional reserves, vital organs and essential tissues are rebuilt and maintained.  In fact the body has a great deal of intelligence that we don’t give it credit for.  It knows exactly which cells are essential to survival, which ones are not.  The body knows which cells are abnormal deposits, and it goes to work to metabolize them first.  For example, the body recognizes arthritic deposits, cysts, fibroids, and tumors as offensive parts of the landscape, and obligingly uses them for foods in preference to anything else.  A starving (not fasting) body also knows precisely in what order of priority body cells should be metabolized to minimize risk of death or permanent disability.

After a starving body has reached skeletal condition, or where some small amount of fat remains but nutritional reserves (vitamins and minerals) are exhausted and there is insufficient nourishment forthcoming, the body begins to consume nutrient-rich muscle and organ tissue in a last-ditch effort to stay alive.  Under these dire circumstances, the least essential muscles and organs from the standpoint of survival are metabolized first.  For example, muscles in the arms and legs would be consumed early in the process, the heart muscle used only toward the very end.  The very last part of the body to be metabolized when one is starving and as has come very close to death would be the brain and the nervous system.