QUOTE from Dr Fung (emphasis mine):
“THE BODY’S RESPONSE TO FASTING:
GLUCOSE AND FAT are the body’s main sources of energy. When glucose is not available, then the body adjusts by using fat, without any health detriment. This compensation is a natural part of life. Periodic food scarcity has always been part of human history, and our bodies have evolved processes to deal with this fact of Paleolithic life. The transition from the fed state to the fasted state occurs in several stages:
Feeding: During meals, insulin levels are raised. This allows glucose uptake by tissues such as the muscle or brain for direct use as energy. Excess glucose is stored as glycogen in the liver.
The post-absorptive phase (six to twenty-four hours after fasting starts): Insulin levels begin to fall. The breakdown of glycogen releases glucose for energy. Glycogen stores last for roughly twenty-four hours.
Gluconeogenesis (twenty-four hours to two days): The liver manufactures new glucose from amino acids and glycerol. In non-diabetic persons, glucose levels fall but stay within the normal range.
Ketosis (one to three days after fasting starts): The storage form of fat, triglycerides, is broken into the glycerol backbone and three fatty acid chains. Glycerol is used for gluconeogenesis. Fatty acids may be used directly for energy by many tissues in the body, but not the brain. Ketone bodies, capable of crossing the blood-brain barrier, are produced from fatty acids for use by the brain. Ketones can supply up to 75 percent of the energy used by the brain.4 The two major types of ketones produced are beta hydroxybutyrate and acetoacetate, which can increase more than seventy-fold during fasting.5
Protein conservation phase (after five days): High levels of growth hormone maintain muscle mass and lean tissues. The energy for maintenance of basal metabolism is almost entirely met by the use of free fatty acids and ketones. Increased norepinephrine (adrenalin) levels prevent the decrease in metabolic rate.” 
NOTE (my commentary):
If fasting happens, and no sugar, carbs or protein flood in to increase insulin, how long does it take for the body to convert from burning carbohydrates to burning body fat for fuel?
GLUCOSE AND FAT are the body’s main sources of energy.
How long does it take for the carbohydrates and glucose yielding foods to be used up so that insulin in the body decreases?
When glucose is not available, then the body adjusts by using fat, without any health detriment.
STRATEGY: What is the best blend of alternating eating and fasting so as to permanently lose weight?
What gets your attention?
What does it say about Reality? (Any misperceptions to correct?)
What does it say that I need to obey? (What should be added to my strategy?)
How can I incorporate what is true into my Disciplines? (Specific methods?)
What questions do I have for the experts? What might be the answers?
SOURCE – Footnotes:
 “The Obesity Code: Unlocking the Secrets of Weight Loss” by Dr. Jason Fung, Timothy Noakes, p. 238-239.
Image by U.S. Army Corps of Engineers Nashville District Cumberland River Flood 1937 – Eddyville, Kentucky, via Flickr.
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Key: Fung-10-30.63 Last Revision: 07/21/2020