What happens in your body from the first hour to days of fasting.
What the evidence says
- Insulin raised, glucose uptake by tissues for energy[1]
- Excess glucose stored as glycogen in liver[1]
- Insulin begins to fall[1]
- Glycogen breakdown releases glucose for energy[1]
- Glycogen stores last approximately 24 hours[1]
- Liver manufactures new glucose from amino acids and glycerol[1]
- Blood glucose falls but stays within normal range in non-diabetics[1]
- Triglycerides broken into glycerol and fatty acids[1]
- Ketone bodies produced from fatty acids for brain fuel[1]
- Ketones can supply a large share of the brain’s energy during extended fasting: Fung’s book says up to 75%, while Owen’s 1967 study, the primary on the finding, states no percentage at all; the circulating 60–70% is a secondary derivation from its arteriovenous figures[1],[2]
- Upon >24 hours fasting, humans enter alternative metabolic phases relying on ketone body-like carbon sources[3]
- On Fung’s account, high growth hormone maintains muscle mass and lean tissues[1]
- Energy for basal metabolism almost entirely from free fatty acids and ketones[1]
- Increased norepinephrine prevents decrease in metabolic rate[1]