From the Guidelines
During fasting, the body's primary energy sources shift from glucose and glycogen to fat and ketones over time, with fat becoming the predominant energy source during prolonged fasting. The energy sources used by the body during fasting can be broken down into several stages:
- Initially, in the first few hours of fasting, the body primarily uses glucose from the bloodstream and glycogen stored in the liver and muscles 1.
- As fasting continues beyond 12-24 hours, glycogen stores become depleted, and the body shifts to breaking down fat stores, converting them into ketone bodies (beta-hydroxybutyrate, acetoacetate, and acetone) in the liver through a process called ketogenesis 1.
- These ketones become the primary fuel for the brain and many other tissues, while the body increases protein breakdown to provide amino acids for gluconeogenesis, which maintains minimal glucose levels for tissues that cannot use ketones efficiently, such as red blood cells.
- During extended fasting (beyond 2-3 days), the body becomes increasingly efficient at preserving protein by further increasing ketone production and decreasing glucose utilization, with fat remaining the predominant energy source during prolonged fasting, as supported by a recent study in 2022 1.
- The body is capable of mobilizing significant fat stores to provide energy for weeks, while protein breakdown decreases to spare muscle mass, allowing humans to survive extended periods without food by efficiently transitioning between different fuel sources.
From the Research
Energy Sources at Different Times of Fasting
The body's energy sources change at different times of fasting. The following are the energy sources at various stages of fasting:
- Initial Stage (0-12 hours): The body primarily uses stored glucose (sugar) for energy 2.
- Intermediate Stage (12-24 hours): As glucose levels decrease, the body starts to break down stored fat for energy, releasing free fatty acids (FFAs) into the bloodstream 2, 3.
- Prolonged Fasting (24-48 hours): The body adapts to using FFAs as the primary energy source, with the liver producing ketone bodies as an alternative energy source 4, 5.
- Extended Fasting: The body continues to use ketone bodies and FFAs for energy, with the brain and other organs adapting to use these alternative energy sources 5.
Factors Influencing Energy Source Utilization
Several factors influence the body's energy source utilization during fasting, including:
- Free Fatty Acids (FFAs): FFAs play a crucial role in determining the body's energy source, with higher FFA levels promoting fat oxidation 3, 5.
- Glucose and Insulin: Glucose and insulin levels also impact energy source utilization, with lower glucose levels and insulin sensitivity promoting fat oxidation 3, 6.
- Hormonal Responses: Hormones such as glicentin and glucose-dependent insulinotropic polypeptide (GIP) also influence energy source utilization during fasting 5.