there is a consensus of professional opinion that while fasting on water-only-according to thirst, muscle tissue is preserved to a large extent, as are the other organs of the body, otherwise the body would feed on these vital organs and we would die within a short time-frame.
Amino acids are not replaced through the diet while fasting, but the body needs to feed on some of these existing amino acids from muscle in its conversion into glycogen for vital organ function for the Brain and so on.
If we exercise while fasting, these amino acids are utilized, so muscle tissue is not replaced, but is in fact depleted.
The research work here is contained with Dr Fuhrmans book: Fasting and Eating For Health and listed at its rear.
Yes I know what "protein-sparing" means and your definition is a correct one, but this occurs only in a state of rest during ketosis and the use of ketone-bodies as a primary fuel-source, instead of glucose.
When one is fasting, the person is consciously diverting energy from the digestive system to the immune system.
In addition, there is a reduction in core body temperature. This is a direct result of the slower metabolic rate and general bodily functions. Following a drop in blood Sugar level and using the reserves of glucose found in liver glycogen, the basal metabolic rate (BMR) is reduced in order to conserve as much energy within the body as can be provided. Growth hormones are also released during a fast, due to the greater efficiency in hormone production.
An individual's basal metabolic rate (BMR) is typically measured after a full night's sleep in a laboratory under optimal fasting conditions of quiet, rest and relaxation.
In order to calculate an individual's approximate basal metabolic rate, the following questions may be answered: what is the gender of the individual; what is the weight in pounds of the individual; what is the height of the individual in inches; and what is the age of the individual?
Men: BMR = 66 (6.22 x weight in pounds) (1.96 x height in inches) - (6.8 x age in years)
Women: BMR = 655 (4.36 x weight in pounds) (0.71 x height in inches) - (4.7 x age in years)
Note: Keep in mind that the BMR calculator is an estimation of the calories needed for the body to perform basic functions. An individual's total daily caloric requirements are affected by muscle mass, illness, stress, food digestion and most importantly, exercise.
Only an overnight laboratory test can accurately measure an individual's actual BMR.
This is basic biochemistry FastingGuru where calorific expenditure is far less on a fast than while eating and why the body will lose less and less weight as the fast progresses: a 1/4 pound per day and even less than this.
We will just have to disagree FastingGuru, but I believe you are making some very fundamental mistakes regarding the purpose of the fast and how it has been designed to be used in Nature and in particularly in the human body.