Biochemistry - The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Biosynthesis: How New Molecules Are Formed
Regulation of Biosynthetic Processes
The Starvation State

During prolonged fasting, Glycogen reserves throughout the body are depleted, and fats become the primary fuel. Glucose and Pyruvate suffice only briefly. Although lipid Hydrolysis yields some glycerol (which is oxidized to dihydroxyacetone and phosphorylated), The amount of glucose precursors generated in this manner is limited. (It should be noted that the animal body cannot convert acetyl-CoA back into pyruvate.) Consequently, the demand for both glucose and pyruvate persists. The former is essential for biosynthetic processes, while the latter plays a vital role as a precursor to oxaloacetate, a substrate regenerated in The Tricarboxylic Acid Cycle. As a result, the Organism is forced to reorganize its METABOLISM during starvation. The Adrenal Glands secrete glucocorticoids (e.g., cortisol; ch. 12, sec. II, 3,6). Through enzyme induction mechanisms, these Hormones increase the levels of various Enzymes in the Cells of target Organs, such as the Liver. Furthermore, glucocorticoids enhance the sensitivity of cellular receptors to cyclic AMP and, consequently, to hormones such as Glucagon [57]. It has been suggested that this effect is due to corticoids ensuring the maintenance of a normal ionic environment, particularly normal concentrations of Ca²⁺, K⁺, and Na⁺ ions.

The action of glucocorticoids ultimately leads to an increase in the amount of glucose extracted from the liver (due to enhanced glucose-6-phosphatase activity), an elevation in Blood glucose and liver glycogen levels, and a reduction in the synthesis of mucopolysaccharides. The incorporation processes of Amino Acids resulting from protein breakdown are slowed down, whereas the synthesis of enzymes catalyzing protein degradation is intensified. Among these enzymes are Tyrosine and Alanine aminotransferases, which initiate Amino Acid Breakdown and ultimately ensure The formation of fumarate and pyruvate—precursors of glucose in Gluconeogenesis.

The metabolic reorganization carried out by the organism during prolonged fasting involves a transition from carbohydrate to Lipid Metabolism. For instance, 75% of the glucose consumed by the Brain can be gradually replaced by Ketone Bodies produced during fat breakdown (ch. 9, sec. A, 7) [44].