As indicated in Figure...2.16A, lipolysis (breakdown of adipose triglyceride to free fatty acids and glycerol) plays a lesser role in postabsorptive energy supply, especially to the brain. However, glycogen stores are limited and become depleted in less than 24 hours. That point in time when liver glycogen stores are exhausted is, by definition, the beginning of the fasting state. Now the glucose needs of the brain must be met completely by gluconeogenesis, which means sacrificing amino acids from protein. Because protein is critical to body function, from enzyme activity to muscle function related to breathing and circulation, unrestrained use of amino acids for glucose production would rapidly deplete protein, causing death in a matter of days. Clearly, this does not occur, because people may survive without food and obese subjects may be fasted for weeks without protein intake (1). Adaptation occurs in starvation by the brain's switching from a glucose-based to a ketone-based fuel supply. Free fatty acids released from lipolysis are converted in the liver into ketone bodies that can then be used by the brain and other tissues for energy. That conversion begins in the fasting state and is complete during long-term fasting periods ( Fig 2...16..B.). In starvation, tissues such as muscle may use free fatty acids directly for energy, and the brain uses ketone bodies. The body's dependence upon glucose as a fuel has been greatly reduced, thereby conserving protein. This adaptation process is complete within a week of onset of starvation (124).
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