Figure 36-5. The metabolic flow during prolonged starvation. The brain
adapts to use ketone bodies as a source of energy, thus decreasing its utiliza-
tion of glucose.
concentration and a dramatic increase in the concentration of ketone bodies,
reflecting altered metabolic poise. In the liver the tricarboxylic acid (TCA)
cycle is slowed by the drainage of four-carbon intermediates to gluconeogen-
esis, fatty acid breakdown continues apace, and body proteins continue to
be broken down to replenish the tricarboxylic acid cycle intermediates. In
muscle the fuels used for energy generation are fatty acids and ketone bodies.
Muscle activity decreases as result of the mobilization of muscle protein,
which itself slows as the period of starvation increases. In adipose tissue the
breakdown of triglycerides to fatty acids is accelerated. In brain and the central
nervous system an adaptive change occurs allowing this tissue to use ketone
bodies as an energy source relieving both the total body demand for glucose and
the use of muscle protein as a carbon source for gluconeogenesis in the liver.
Ketone body concentration increases in the blood reflecting the accelerated
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