Many of the most dramatic changes that occur in the body during fasting take place on the first three days of the fast. These occur as the body switches from one fuel source to another. Normally, the primary form of energy the body uses for energy is glucose, a type of sugar. Most of this is extracted or converted from the food we eat. Throughout the day, the liver stores excess Sugar in a special form called glycogen that it can call on as energy levels fall between meals. There is enough of this Sugar source for 8-12 hours of energy and usually, it is completely exhausted within the first 24 hours of fasting. (However, once the body shifts over to ketosis or fat as fuel, this new fuel is used to also restore the body's glycogen reserves.)
Once the liver's stores of glycogen are gone, the body begins to shift over to what is called ketosis or ketone production - the use of fatty acids as fuel instead of glucose. This shift generally begins on the second day of fasting and completed by the third. In this interim period there is no glucose available and energy from fat conversion is insufficient but the body still needs fuel. So it accesses glucose from two sources. It first converts glycerol, available in the body's fat stores, to glucose but this is still insufficient. So it makes the rest that it needs from catabolizing, or breaking down, the amino acids in muscle tissue, using them in the liver for gluconeogenesis, or the making of glucose. Between 60 and 84 grams of protein are used on this second day, 2-3 ounces of muscle tissue. By the third day ketone production is sufficient to provide nearly all the energy the body needs and the body's protein begins to be strongly conserved. The body still needs a tiny amount of glucose for some functions, however, so a very small amount of protein, 18-24 grams, is still catabolized to supply it - from 1/2 to 1 ounce of muscle tissue per day. Over a 30 day Water Fast a person generally loses a maximum of 1-2 pounds of muscle mass. This conservation of the body's protein is an evolutionary development that exists to protect muscle tissue and vital organs from damage during periods of insufficient food availability.