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"NO MORE SHINES BILLY" Joe Pesci in Goodfellas

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>your excreting ketones, thus you are using ketones as fuel.

Shouldn't this have been,"...you're using FAT as fuel"?

Aren't ketones a by-product of fat metabolism? (That is, ketones themselves are not a source of fuel, energy?)

Fatty acids can be used as the major fuel for tissues such as muscle, but they cannot cross the blood-brain barrier,
and thus cannot be used by the central nervous system (CNS). This becomes a major problem during starvation
(fasting), particularly for organisms such as ourselves in which CNS metabolism constitute a major portion of the
resting basal metabolic rate. These organism must provide glucose to the CNS to provide for metabolic needs, and
thus during the initial fasting period must break down substantial amounts of muscle tissue (protein) to provide the
amino acid precursors of gluconeogenesis. Obviously the organism could not survive long under such a regime. What
is needed is an alternate fuel source based on fat rather than muscle. The so-called ketone bodies serve this function:

Note that only two of the ketone bodies are in fact ketones, and that acetone is an "unintentional" breakdown product
resulting from the instability of acetoacetate at body temperature. Acetone is not available as fuel to any significant
extent, and is thus a waste product.

Ketone Body Metabolism

CNS tissues can use ketone bodies any time, the problem is the normally very low concentrations (< 0.3 mM)
compared to glucose (about 4 mM). Since the KM's for both are similar, the CNS doesn't begin to use ketone bodies in
preference to glucose until its concentration exceed's the concentration of glucose in the serum.

The system becomes saturated at about 7 mM. The limiting factor in using ketone bodies then becomes the ability of
the liver to synthesis them, which requires the induction of the enzymes required for acetoacetate biosynthesis.
Normal glucose concentrations inhibit ketone body synthesis, thus the ketone bodies will only begin to be synthesized
in high concentrations as serum glucose concentrations fall. As an example, ketone bodies might start at about 0.1
mM after an overnight fast, rise to 3 mM after a 3 day fast, and go to 7-8 mM with prolonged fasting (>24 days).

Ketogenesis

Ketogenesis occurs in the mitochondrial matrix in liver mitochondria. Fatty acids are first broken down to acetyl CoA
via beta-oxidation (providing energy for liver metabolism from the reducing equivalents generated). The acetyl CoA is
then used in ketogenesis:

Ketone Bodies as Fuel

The ketone bodies are water soluble and are transported across the inner mitochondrial membrane as well as across
the blood-brain barrier and cell membranes. Thus they can be used as a fuel source by a variety of tissues including
the CNS. They are preferred substrates for aerobic muscle and heart, thus sparing glucose when they are available.