Certain tissues, such as the brain and red blood cells, are absolutely dependent on glucose as their primary fuel. Ketone bodies aren't sufficient to provide the energy needs of these tissues. In the fasted state glycogen stores are sufficient to supply these tissue's requirement for glucose for approximately one day, so when the body is in the fasted state glucose must be formed from noncarbohydrate precursors, primarily lactate, certain amino acids, and glycerol. Gluconeogenesis is the pathway for converting noncarbohydrate precursors into glucose. The main routes of entry into gluconeogenesis are through pyruvate, oxaloacetate, or dihydroxyacetone phosphate. Most tissues do not have the ability to carry out gluconeogesis. Gluconeogenesis takes place mainly in the liver and to a lesser degree in the cortex of the kidney.
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Conceptual Vocabulary for Gluconeogenesis