Fasting breaks down muscle: breakdown of muscle tissue happens only at extremely low levels of body fat (4%). The body will preserve muscle mass until fat stores become so low that it has no other choice.
The brain needs glucose to function: the body uses ketones as a major fuel source during prolonged fasting.
Fasting will cause our basal metabolism to decrease (aka burn fewer calories as a normal act of life): Daily caloric restriction does lead to decreased metabolism, so people have assumed that this effect would be magnified as food intake drops to zero. Decreasing food intake is matched by decreased energy expenditure. However, as food intake goes to zero, the body switches energy inputs from food to stored fat. This significantly increases the availability of food, which is matched by an increase in energy expenditure. Caloric-restricted diets that do not involve fasting inhibits hormonal adaptations. Adrenalin is not increased to maintain total energy expenditure. Growth hormone is not increased to maintain lean muscle mass. Ketones are not produced to feed the brain. Persistent exposure to decreased calories results in the body adapting by reducing energy expenditure. The intermittent nature of fasting does not allow for this to happen. As mentioned earlier, hormones act in accordance with fasting to maintain muscle, ramp up metabolism, and burn ketones for energy. (Obesity Code, 245)
