What short-term fasting does to metabolic rate and catecholamines — and why the energy story is not established.
What the evidence says
- Neither cited study measured alertness, energy or any subjective state. `caf01` measured resting energy expenditure, norepinephrine and glucose; `caf02` measured RMR, heart rate, blood pressure, forearm blood flow, catecholamines and substrates. What is established is metabolic, not experiential[1],[2]
- Catecholamines do rise: norepinephrine 1716 → 3728 pmol/L by day 4 in `caf01`, and in `caf02` noradrenaline and adrenaline are unchanged at 36 h but significantly raised at 72 h[1],[2]
- But `caf02`’s authors decline the mechanism this file used to assert: RMR peaks at 36 h, when noradrenaline has not moved, and they conclude sympathetic activity ”does not correlate with elevations in RMR, HR, or FBF”[2]
- Resting metabolic rate rises rather than falls in short-term fasting — ~14% in `caf01` (3.97 → 4.53 kJ/min, day 1 to day 3)[1]
- `caf02` is the larger study and qualifies that: RMR up ~6% at 36 h, no longer significantly above baseline at 72 h (P = 0.06), and no significant change in men across 72 h[2]
- `caf03` argues an evolutionary rationale — alertness while foraging as adaptive. A review’s framing, not a measured result, and the only place this file speaks of alertness at all[3]