I just took a practice MCAT today and there was a passage in it about starvation. What do you think? I'm typing the whole thing out, so don't mind the typos.
The human body responds to starvation by using different substances for its primary metabolic fuel. During the first day of starvation, low blood glucose inhibits insulin secretion and stimulates glucagon secretion. Consequently, glycolysis is inhibited and glycogen utilization and glucose synthesis are promoted. In adipose cells, triacylglycerols are hydrolyzed, leading to the release and oxidation of fatty acids. In muscle, protein is degraded to provide some of the precursors for glucose synthesis. The use of fatty acids by liver and muscle tissue allows glucose to be conserved for use by the brain.
After three days of starvation, the liver produces mass quantities of ketone bodies (acetoacetate and 3-hydroxybutyrate) from acetyl CoA generated from fatty acid oxidation. The brain begins to consume acetoacetate in place of glucose, thereby lowering the need for blood glucose even further. After several weeks of starvation, ketone bodies become the major fuel of the brain, markedly diminishing the need for glucose. Hence, less protein ( and therefore less muscle tissue) is degraded than in earlier stages of starvation.
Damn, our bodies are smart.