It isnt much of a document on Fasting per se, and indeed, I think the food in the study was all starch and not much else, but you could read a lot about already lean humans fasting in the Minnesota starvation experiment.
Now, I don’t think I would equate someone with adequate body fat doing a long fast to some of these people who clearly had very little left, but its a morbid read if nothing else. They basically did a very low kcal high carb diet, and it wrecked these people. I’d honestly suspect that from a hormonal and mental standpoint, if you have the body fat, its probably far healthier to just fast than to eat low cal high carb. (Not a doctor, not medical advice, just my opinion etc).
I find it a little ironic, and wonder if Ancel Keys was just so up his own butt about his nutrition theories that he didn’t even want to test anything else, or… well, the guy is long dead so we’ll never know why he decided to lie to the world. On a tangentially related note, when I learned that he designed the K-ration, I suddenly realized why the K-rat was so horrible compared to the not that great but so much better C ration.
Couple links there, although there are hundreds of them. The full research paper was around 1100 pages each, 2 parts.
I think I’ve totally drifted from my original thought. TL:DR:
Probably not a great idea to do long fasts if you don’t have enough body fat to provide enough calories to get by on. I’ve read in a few places that each pound can only give you at most, 20-30 kcals a day depending on the health of those cells and your endocrine system. That means the body fat percentage means less than simply, do you have enough pounds of non essential fat to live on?