Final answer:
Depletion of liver glycogen during starvation leads to reduced insulin secretion and increased glucagon activity, decreasing glucose uptake and increasing lipolysis and fatty acid oxidation as alternative energy sources. Gluconeogenesis, supported by fatty acid oxidation, becomes crucial to provide glucose to glucose-dependent organs, thereby favoring fatty acid use for energy.
Step-by-step explanation:
The depletion of liver glycogen stores during starvation leads to a shift from glucose utilization to fatty acid oxidation as an energy source for most tissues because of several metabolic changes. Firstly, as liver glycogen stores decrease, blood glucose levels drop which reduces insulin secretion and leads to decreased GLUT4 translocation (a. lack of insulin signaling reduces GLUT4 translocation). Without sufficient insulin, the uptake of glucose by cells diminishes, and they begin to use alternative energy sources such as fatty acids.
Secondly, in response to low blood glucose, the pancreas secretes glucagon. Glucagon activates hormone-sensitive lipase (b. glucagon activates hormone-sensitive lipase), which facilitates lipolysis, breaking down triglycerides into glycerol and free fatty acids that are released into the bloodstream. These free fatty acids are then oxidized for energy.
Thirdly, due to low glucose, the body starts gluconeogenesis, a process that synthesizes glucose from non-carbohydrate sources such as lactate, pyruvate, and glycerol. For gluconeogenesis to continue effectively, fatty acid oxidation provides the necessary ATP and acetyl CoA, thus linking the requirement of gluconeogenesis with the oxidative breakdown of fatty acids (c. gluconeogenesis requires fatty acid oxidation as a fuel source). When glucose is limited, the liver can also convert glycerol into glucose, which adds to gluconeogenesis activities (d. lack of liver glycogen stores results in a drop in serum glucose concentration). Overall, these metabolic responses are essential to maintain energy production during periods of extended fasting or starvation and ultimately favor the utilization of fatty acids as an alternative energy source.