Final answer:
During early fasting (first 24 hours), the body primarily uses stored glycogen for energy. As fasting continues, fatty acids and proteins are metabolized into ketone bodies as alternative energy sources, particularly when glycogen stores are depleted.
Step-by-step explanation:
The process your body undergoes when it's not receiving food is commonly categorized into different states: the absorptive state (or fed state), the postabsorptive state (or fasting state), and eventually starvation. In the early stages of fasting, within the first 24 hours, your body primarily relies on stored glycogen for energy. As the glycogen stores in the liver and muscle cells are depleted, the hormone glucagon promotes the breakdown of these stores into glucose, maintaining blood sugar levels. If the fast continues and the glycogen stores are exhausted, the body begins to mobilize fatty acids and later proteins, which can be metabolized into ketone bodies serving as a new energy source, especially for the brain and heart.
During the initial hours of fasting, glucose derived from glycogenolysis (breakdown of glycogen) and later gluconeogenesis (synthesis of glucose from non-carbohydrate sources) acts as the primary fuel for the brain and body tissues. As fasting progresses into the state of starvation, the body conserves protein by increasingly utilizing ketone bodies for energy. When energy demands cannot be met by these mechanisms, the body resorts to breaking down muscle tissue, further transitioning into starvation and protein catabolism.