How your hormones shift as a fast progresses through stages.
What the evidence says
- *Insulin:* Drops progressively through stages[1]
- *Glucagon:* Rises to mobilize stored energy[1]
- *Growth hormone:* 5-fold increase in 24-hour production rate by day 2; integrated concentration 3-fold increase over 5 days[2],[3]
- *Adrenaline/noradrenaline: Norepinephrine more than doubles by day 3–4, increasing 117%. Not "to maintain energy" — that is teleology no source supplies: `caf01`'s authors conclude the rise "seems to be due to a decline in serum glucose" and may be the primary initial signal, and `caf02`'s authors find sympathetic activity "does not correlate with elevations in RMR, HR, or FBF"*[4],[1],[5]
- *Thyroid (T3):* Free triiodothyronine fell progressively during 72h fast (P<0.001); free thyroxine unchanged[5]
- *Thyroid (rT3):* Reverse T3 (inactive metabolite) increases markedly during fasting as T3 drops — an energy-conservation adaptation that rapidly normalizes after refeeding[6]
- *Cortisol:* Minor, gradual elevation during fasting[6]
- *The pattern as a whole (10-day total fast, 12 healthy males):* growth hormone rose pronouncedly, nocturnal adrenaline excretion increased, and T3 fell early and markedly while rT3 rose — mobilization and targeted energy conservation running together. The authors read the coordinated shift as an adaptive mechanism to secure energy supply to vital organs, distinguishing fasting from chronic caloric restriction, which instead triggers metabolic suppression[6]