Only one book that I have read has an answer to this question. I’ve read many, many research papers on autophagy and none of them go there either. I suspect they just don’t know. Well, once again, apparently, the body knows and does tell us when this threashold is reached. This subject was discussed in one of Dr. Herbert Shelton’s books titled Fasting and Sunbathing.
First, he clearly defines in chapter 6 the difference between fasting and starving and I quote “The fasting period is, therefore, determined by the amount of reserves the body has on hand. Starvation sets in after the reserve stores have been sufficiently exhausted that they are no longer adequate to maintain functional and structural integrity.”
He defines fasting in this way: “Fasting is a rest — a physiological vacation. It is not an ordeal nor a penance. It is a house-cleaning measure which deserves to be better known and more widely used.”
This doesn’t yet tell us where the line is that we should not cross where benefits stop and damage begins. Shelton defines that point as “ Fasting begins with the omission of the first meal and ends with the return of natural hunger. Starvation begins with the return of hunger and terminates in death.” “The return of natural hunger” How is natural hunger different from the hunger pangs we all know and hate?
In chapter 13 Shelton begins to describe this phenomenon, but does so in pieces so let me put them together for you. Natural Hunger is “unmistakeable” from normal hunger pangs. Unlike hunger pangs, natural hunger does not come and go, but is “constant”. Natural hunger does not come from the stomach, but the “chest and throat”. One very important benefit of the signal of natural hunger is that it comes well before starvation begins, before the body’s safe reserves are depleted.
This book is the best of the twelve I’ve read and is deserving of your time and it’s FREE! So far I’ve lost 70 of a planned 100 pounds and refer to this book more than any other. The depth of knowledge from Shelton’s over 30,000 supervised fasts is obvious in his prose.