I have read Dr. Fung extensively, listened to the Podcasts by the 2Keto and have done some 5 day fasts. However, I was concerned when I saw this study mentioned by someone on Reddit who I think was quoting one of Atkin's books. https://www.reddit.com/r/keto/comments/kksaq/one_of_the_strongest_arguments_for_keto_fat_lost/
"Now let's look at some other research that supports the fat-burning theory, this time from the Oakland Naval Hospital. Impressed with the Kekwick and Pawan success, Frederick Benoit and his associates decided to compare a 1,000calorie, 10-grams-of-carbohydrate, high-fat diet with fasting (the same principle that Kekwick and Pawan found most effective), using seven men weighing between 230 and 290 pounds. They used state-of-the-art body composition technology. After ten days, the fasting subjects lost 21 pounds on average, but most of that was lean body weight; only 7.5 pounds was body fat. However, on the controlled carbohydrate regimen over the same period of time, 14 of the 14.5 pounds lost was body fat. Think of it. By eating foods low in carbohydrate and high in dietary fat, subjects burned their fat stores almost twice as fast as when they ate nothing at all!
Benoit's other exciting discovery was that on a fat-burning regimen, subjects maintained their potassium levels, while subjects who fasted experienced major potassium losses. (Potassium depletion can cause heart arrhythmia, which, in severe cases, can be fatal."
I would imagine some of the 21 lbs was water weight but I am still concerned about lean muscle preservation. Could it be autophagy? That they are eliminating old protein, excess skin? My theories and I am not a scientist.
Here is the study if you are a member of the American College of Physicians. I would love to see the entire study but am not willing to pay. It seems it uses potassium loss as a proxy for muscle loss but that is only a guess since I can only access a tiny portion.