(Part 1 of 3 in Ned Kock's "Health Correlator")
There are a few health-related blogs which I recommend very highly. Health Correlator is one of the better ones I’ve found. I’ve posted some excerpts here of his posts which I’ve found most interesting.
(Part 3 of 3 in Ned Kock's "Health Correlator")
Growth hormone stays flat for about 40 minutes, after which it goes up steeply. At around the 90-minute mark, it reaches a level that is quite high; 300 percent higher than it was prior to the exercise session. Natural elevation of circulating growth hormone through intense exercise, intermittent fasting, and restful sleep, leads to a number of health benefits. It helps burn abdominal fat, often hours after the exercise session, and helps builds muscle (in conjunction with other hormones, such as testosterone). It appears to increase insulin sensitivity in the long run.
(Part 2 of 3 in Ned Kock's "Health Correlator")
When the body is running short on glycogen, it becomes increasingly reliant on fat as a source of energy, sparing muscle tissue. That is, it burns fat, often in the form of ketone bodies, which are byproducts of fat metabolism. This state is known as ketosis. There is evidence that ketosis is a more efficient state from a metabolic perspective (Taubes, 2007, provides a good summary), which may be why many people feel an increase in energy when they fast.
~ Ned Kock, from Muscle loss during short-term fasting