From the Guidelines
During the first 3 days of intermittent fasting, the body undergoes significant metabolic changes, including depletion of glycogen stores, increased gluconeogenesis, and initiation of ketosis, as the body shifts from using glucose to fat for energy. As fasting becomes prolonged for more than several hours, glycogen stores become depleted, and the low levels of circulating insulin allow increased fatty acid release from adipocytes, as noted in a study on management of diabetes during Ramadan 1. The key metabolic processes during this period include:
- Depletion of glycogen stores within 24 hours
- Increased gluconeogenesis, where the body creates glucose from non-carbohydrate sources
- Initiation of ketosis, where the liver converts fatty acids into ketone bodies, which serve as an alternative fuel source for the brain and body, as described in the study 1
- Decreased insulin levels, which improves insulin sensitivity and promotes fat burning
- Increased autophagy, a cellular cleaning process where damaged components are recycled
- Elevated growth hormone levels, which help preserve muscle mass and promote fat metabolism These metabolic shifts can lead to initial side effects such as hunger, fatigue, and irritability, but staying hydrated and maintaining electrolyte balance can help minimize these effects. It is essential to note that the study 1 highlights the importance of managing diabetes during prolonged fasting periods, and the metabolic changes that occur during intermittent fasting are similar to those that occur during Ramadan fasting. The body's adaptation to using fat for fuel during intermittent fasting can have various health benefits, including improved insulin sensitivity and weight loss, but it is crucial to prioritize hydration and electrolyte balance during the initial transition period.
From the Research
Metabolic Processes During Intermittent Fasting
The key metabolic processes during the first 3 days of intermittent fasting (IF) include:
- Alterations in substrate metabolism, with a shift from glucose to fat metabolism 2
- Increases in lipolysis, leading to the formation of ketone bodies 3, 4
- Changes in hormonal responses, including alterations in insulin and HOMA-IR index 3, 5
- Increases in autophagy, although the exact mechanisms and timing are still unknown 2
- Variations in protein metabolism, although these appear to be minimal within the first 24 hours of fasting 2
Energy Metabolism
During the first 3 days of IF, energy metabolism is affected in several ways:
- Energy expenditure is altered, with some studies suggesting increases in fat oxidation and decreases in glucose oxidation 5
- The body begins to adapt to using ketone bodies as a primary source of energy, leading to improvements in cardiometabolic health 3, 4
- Substrate oxidation is affected, with increases in protein and fat oxidation 5
Clinical Health Markers
IF has been shown to have positive effects on clinical health markers, including: