Final answer:
During a fasting state, the primary source of energy is glucose stored as glycogen in the liver and muscles. Once glycogen stores are depleted, alternative sources like fatty acids and proteins are used.
Step-by-step explanation:
In a fasting state, the primary source of energy is glucose, which is stored as glycogen in the liver and muscles. Initially, the body relies on glycogen stores to maintain blood glucose levels. However, once glycogen stores are depleted, alternative sources of energy, such as fatty acids and proteins, are metabolized and used as fuel.