What you are speaking of is clinical "survival" terms where there is a demarcation point by where eminent damage has already occured whether we are talking dehydration or starvation. By holistic and optimal health standards the term is used differently and it is more loosely defined by a point where the body starts depleating other vital resources and affecting optimal healthy functions other than a demarcation between fat and muscle lossthough it also applies just not as academicaly ridgidas defined by the AMA.
I have no interest whatsoever to get into any kind of debate between what is defined by AMA academia and alternative holistic medicine. Since this website is an alternative healing site I assume or maybe that is just wishful thinking on my part, that as a medical student you have an interest in alternative practices so I would like to invite you to do some research and read some of the better known proponents of fasting like Dr Fuhrman and Shelton both of whom have done extensive documented studies on fasting and suprevised thousands of fasters.