Final answer:
The liver uses gluconeogenesis, primarily from substances like pyruvate, lactate, glycerol, and amino acids, to synthesize glucose during the first few days of starvation, aided by hormones such as glucocorticoids and ACTH.
Step-by-step explanation:
During the first days of starvation, the liver utilizes primarily gluconeogenesis to maintain blood glucose levels, synthesizing new glucose molecules from non-carbohydrate sources such as pyruvate, lactate, and amino acids like alanine and glutamine. In the absence of dietary carbohydrates, and once the liver glycogen reserves are depleted, the liver starts to use glycerol, which is derived from the hydrolysis of triacylglycerols (TAG), as well as the aforementioned substrates for gluconeogenesis. Hormones such as glucocorticoids, GH, ACTH, and thyroid hormones are pivotal in facilitating this metabolic pathway, which is critical for providing glucose to the brain and other glucose-dependent organs during periods of fasting or starvation.