The Transition from Fed to Fasted
Summarized from The Complete Guide to Fasting by Jason Fung and Jimmy Moore, page 44 with additional information from Ace, of the Ace of Spades blog
You eat normally. Blood sugar rises. When there is too much glucose in the blood, the pancreas releases insulin. Insulin is a hormone that makes cells (like muscle cells) permeable to glucose. Excess glucose is then stored as glycogen in the liver or stored as fat.
- The Postabsorptive phase (6 to 24 hours into fasting)
Blood sugar and insulin levels fall. For energy, the liver breaks down glycogen, releasing glucose. Glycogen stores last about 24-36 hours. Your body might start producing grehlin, a hormone to encourage you to eat.
Glycogen stores have run out. The liver now manufactures new glucose from amino acids in a process called Gluconeogenesis
- Ketosis (3 days or so into fasting)
Your low insulin stimulates lipolysis, and your body starts breaking down fat (tryglycerides) for energy, making glycerol, fatty acids and then ketones. After 4 days of fasting, your brain is getting 75 percent of its energy from ketones.
- Protein Conservation (5 days or so into fasting)
High levels of HGH maintain muscle mass and lean tissues. Almost all the energy for basic metabolism is supplied by the fatty acids and ketones. Adrenaline levels rise to prevent decreases in metabolic rate.