Then from 4-24 hours is the Postabsorptive State. Here dietary nutrients are no longer absorbed. The body's source of energy is supplied by the breakdown of store energy, primarily glycogen supplied by the liver (about 75% of the glucose is supplied this way). Also some fat is burned. "Some scientists believe that maintaining the human body in the postabsorptive state may have health benefits." This is based on studies of monkeys and rats that show that calorie restricted (not starved) animals live longer, and look younger and healthier at a given age than the normally fed animals.
From 24 hours to about 1 week is the Acute Starvation stage. Here I think body fat supplies most of the energy, but muscles (lean tissue) also contribute to the production of glucose (by supplying amino acids, lactate, and glycerol). Ketosis also starts up. This is where the muscles, brain and kidneys can convert fatty acids and some amino acids to glucose.
Then you go into Prolonged Starvation, where the body relies on fat and ketones for energy.
During my 48 hour fast, The first 36 hours were fine. There were 2 night's sleep in there so lots of rest. I felt much weaker after that. That must have been when the liver-stored glycogen ran out, and the Acute Starvation phase started. I didn't enjoy this phase and can't say I have any interest in repeating it, unless I have some health reason and am in a position to completely rest during the fast. My muscles felt tired for a day after I started eating. They must have been depleted of energy even 24 hours after I started eating. Also I wonder if I lost lean muscle mass. I don't want to do that. I guess I'll have to reread Dr. Fuhrman's fasting book. I don't recall what the benefits of fasting for more than 24 hours are if you are healthy. I can see where 12-24 hour fasts are probably beneficial, and doing a daily fast between dinner and breakfast of, say 15 hours, is probably beneficial. That sounds good enough for me.
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