Final answer:
During the fasting state, glucagon is the primary hormone that regulates blood glucose levels by stimulating the liver to break down glycogen into glucose to maintain energy balance.
Step-by-step explanation:
The primary hormone in control during the fasting state, also known as the postabsorptive state, is glucagon. When the body is in this state, glucose levels in the blood begin to drop as it is absorbed and used by cells. As a result, insulin levels also diminish. To maintain essential blood glucose levels, glucagon is released by the alpha cells of the pancreas. Glucagon acts on the liver to inhibit the synthesis of glycogen and stimulate its breakdown into glucose, which then is released into the bloodstream to maintain adequate glucose levels for cell function and survival.