Scientific analysis of the dangers of fasting (and discussion of partial fasting with protein supplementation)
While anyone can lose weight by fasting (temporarily stopping one's food intake), it is a dangerous practice. When concentration camp survivors, who involuntarily suffered famine as a result of horrendous living conditions, were examined by doctors, what little weight they had was mostly fat, with practically no muscle.
The muscle loss is partly due to the fact that the brain cannot rely completely on fat for fuel. The brain usually reserves ketones for lipid synthesis but will use ketones (from fat) for some energy once levels rise during carbohydrate shortages or starvation, but it must get at least 15 percent of its energy from glucose, and it takes a much greater percentage than this early in a fast before the switch to ketones for most energy needs. Glucose can only be synthesized from proteins, glycerol and carbohydrates.
The body stores carbohydrates as glycogen in the muscles and the liver; glycogen is used to make glucose. Glycogen stores (from carbohydrates) can only last a couple days (during starvation). (In fact, marathon runners experience a shortage of easily-available glycogen after only 2 hours, commonly called "hitting the wall" or bonking.)
Because fasts, very low calorie diets (VLCD), and low-carbohydrate diets restrict the intake of carbohydrates, glucose must be obtained from protein. In the event dietary protein is insufficient, internal sources will be obtained: autolysis and muscle wasting occurs. (The conversion of amino acids to glucose is called gluconeogenesis.)
A very low calorie diet that restricts all carbohydrates and non-essential fats, while providing just enough dietary protein to prevent muscle loss, is termed a "protein sparing modified fast" (PSMF). This type of diet is possible when dietary protein is sufficient to meet the body's glucose needs via gluconeogenesis conversion, thus sparing muscle protein. After experimentation, it was found that a protein intake of 1 to 1.5 grams of protein per kilogram of ideal bodyweight (lean body mass) per day prevented the loss of body protein. A somewhat "safer" intake of 0.8 to 1.2 grams of protein per pound of LBM per day is often recommended. Thus, a PSMF allows for rapid fat loss due to the severe caloric deficit that is created when nearly all carbohydrates and fats are removed from the diet. This extreme dieting technique has many potential hazards, such as hormonal changes and rapid metabolic slowdown. A PSMF is sometimes used by bodybuilders for "cutting" (losing fat to expose muscle) just before competitions