It is a generally agreed consensus that loss of appetite is actually a sign of illness and was recognised as such back in the 1800's......
"WANT OF APPETITE is not always a morbid symptom, nor even a sign of imperfect digestion. Nature may have found it necessary to muster all the energies of our system for some special purpose, momentarily of paramount importance. Organic changes and repairs, teething, pleuritic eruptions, and the external elimination of bad humors (boils, etc.), are often attended with a temporary suspension of the alimentary process. As a rule, it is always the safest plan to give Nature her own way.
--FELIX L. OSWALD MD.
Indeed: "health can be measured in how long a person can go without food while feeling great, and having plenty of energy, both mental and physical".
But this is due to Ketosis, FastingGuru rather than anything else.
"Fasting commences with the omission of the first meal and ends with the return of natural hunger, while starvation only begins with the return of natural hunger and terminates in death. Where the one ends the other begins. Whereas the latter process wastes the healthy tissues, emaciates the body, and depletes the vitality; the former process merely expels corrupt matter and useless fatty tissue, thereby elevating the energy, and eventually restoring the organism that just balance we term health." Hereward Carrington PhD.
Fasting is therefore a beneficial physiological process, whereas starvation is a pathological and destructive process.
Btw, Shelton did not advocate fasting to completion in all cases and in fact in a minority; he had this to say on the matter...........
"There are patients who do not need a complete fast, and those who should not have a complete fast, as well as those who do not get well without a complete fast. No hard and fast rule can be set down to guide us here. Each case will have to be handled according to its own needs and according to the general condition of the patient".
With a little more diligence and research you will find that no animal will fast voluntarily if food is available, but in the wild where food can be found sporadically, the body adapted to forced abstinence because of this. So indeed we did evolve with with frequent fasts because was this forced upon the body, but not by choice.
I mentioned that fasting can be superfluous because with a plentiful, abundant, and regular food supply, it is not essential to fast other than through sickness/injury and food scarcity.
The body is designed to eat and drink frequently in wellness and good health, because an abundance of life-energy or vitality, without enervation, maintains the nutritional and elimination cycle to pursue life to its fullest extent possible.
This is the experience of Natural Hygiene, as taught by this philosophy of living that meets all of our physiological needs in attaining and maintaining perfect health.................
"Writing in Physical Culture (May, 1915), Hereward Carrington PhD states: "If a well man starts going without food, he begins to starve (not fast). The nearer well you are the less you should fast." Mr. Bernarr Macfadden takes a similar view, saying that fasting by healthy persons "represents more nearly cases of starvation than of fasting. A man can only fast with benefit when he is ill. If he is well, and goes without his food, he commences to starve at once; and the two processes are very different. Hence the physiological experts observed only cases of starvation, and not fasting cases at all. The therapeutic side of the question seems to have been missed by them entirely!"--Encyclopedia of Physical Culture, Vol. III.