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 — `cif01` says up to 75%, while this library's primary for the finding, Owen's 1967 study, states no percentage at all; the circulating 60-70% is a secondary derivation from its arteriovenous figures (corrected 2026-09-10: this bullet said Owen was "recorded at 60-70%", which the record's own enumeration of the paper's percentages disproves; see `kbm01`)[1],[2]
- Upon >24 hours fasting, humans enter alternative metabolic phases relying on ketone body-like carbon sources[3]
- Fung's wording — high growth hormone maintains muscle mass and lean tissues. This is the one bullet in the file that names a mechanism rather than a timing, and the library holds a primary bearing on it — but not at this stage's duration. Read the note below before repeating it.[1]
- Energy for basal metabolism almost entirely from free fatty acids and ketones[1]
- Increased norepinephrine prevents decrease in metabolic rate[1]