Brain in Fasting

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Chapter: Biochemistry : The Feed-Fast Cycle

During the early days of fasting, the brain continues to use only glucose as a fuel.


BRAIN IN FASTING

During the early days of fasting, the brain continues to use only glucose as a fuel (Figure 24.16, 1). Blood glucose is maintained by hepatic gluconeogenesis from glucogenic precursors, such as amino acids from proteolysis and glycerol from lipolysis. In prolonged fasting (beyond 2–3 weeks), plasma ketone bodies (see Figure 24.12) reach significantly elevated levels and replace glucose as the primary fuel for the brain (see Figure 24.16, , a nd Figure 24.17). This reduces the need for protein catabolism for gluconeogenesis: ketone bodies spare glucose and, thus, muscle protein. [Note: As the duration of a fast extends from overnight to days to weeks, blood glucose levels initially drop and then are maintained at the lower level (65–70 mg/dl).] The metabolic changes that occur during fasting ensure that all tissues have an adequate supply of fuel molecules. The response of the major tissues involved in energy metabolism during fasting is summarized in Figure 24.19 (see below).


Figure 24.16 Major metabolic pathways in the brain during fasting. [Note: The numbers in the circles, which appear both in the figure and in the corresponding citation in the text, indicate important pathways for metabolism of fat or carbohydrates.] CoA = coenzyme A; TCA = tricarboxylic acid; P = phosphate.


Figure 24.17 Fuel sources used by the brain to meet energy needs in the well fed and starved states.

Figure 24.12 Major metabolic pathways in liver during fasting. [Note: The numbers in circles, which appear both in the figure and in the corresponding citation in the text, indicate important metabolic pathways for carbohydrate or fat.] P = phosphate; CoA = coenzyme A; TCA = tricarboxylic acid; NADH = nicotinamide adenine dinucleotide.


Figure 24.19 Intertissue relationships during starvation and the hormonal signals that promote them. P = phosphate; TCA = tricarboxylic acid; CoA = coenzyme A.

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