When adrenaline and metabolic shifts kick in during a fast.
What the evidence says
- Glycogen depletion and transition begins 6-24 hours[1]
- Switch to predominantly fat oxidation occurs around ~15h of fasting, when endogenous carbohydrate stores begin to deplete[2]
- Metabolic rate increases by 36 hours; heart rate elevated — and `caf02` qualifies its own result: RMR is up ~6% at 36 h, no longer significantly above baseline at 72 h (P = 0.06), and not significantly changed in men at all[3]
- Ketosis begins 1-3 days as body switches to fat-burning[1]
- Norepinephrine more than doubles by day 3-4[4]
- Adrenaline and noradrenaline significantly elevated by 72 hours[3]
- Forearm blood flow nearly doubled by 72 hours (3.32 → 6.21 ml/100ml per min), indicating increased peripheral circulation[3]
- After 5 days: protein conservation phase with sustained norepinephrine[1]