(Part 2 of 3 in series, 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, http://healthcorrelator.blogspot.com/2010/01/muscle-loss-during-short-term-fasting.html
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