AAMC FL Practice Exam 2025 - Free Practice Questions and Study Resources.

During fasting, the body primarily relies on glycogen stores for energy. Glycogen breakdown starts to occur in order to release glucose, which can be used for energy production, particularly for the brain and red blood cells that predominantly rely on glucose. As fasting continues, the body will gradually shift towards using fatty acids and ketone bodies for energy, but the initial response to fasting is to utilize the readily available glycogen in the liver and muscles. This breakdown of glycogen into glucose provides a quick source of energy until other metabolic processes can take over.

In contrast, protein synthesis typically occurs during periods of nutrient abundance, whereas fat accumulation and lipid synthesis are processes favored during excess food intake, not fasting. The metabolic state of fasting leads to the mobilization of energy stores rather than their accumulation or synthesis, making glycogen breakdown the primary process initiated during this state.