What the catecholamines are measured to do during fasting — and why the energy-and-alertness framing is not among it.
What the evidence says
- Signal fat cells to release fatty acids (lipolysis) — fatty acids increase over 370% during fasting[1],[2]
- Increase heart rate slightly[2]
- Metabolic rate rises rather than falls: in `caf01`, REE 3.97 → 4.53 kJ/min day 1 to day 3 (P < 0.05) while norepinephrine rose 1716 → 3728 pmol/L, the two correlated (Spearman 0.49, P < 0.05)[1],[3]
- *`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*[2]
- *`caf02`'s authors decline the causal step the older framing asserted: 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]
- *Neither cited study measured alertness, focus, energy or any subjective state. `caf01` measured resting energy expenditure, norepinephrine, epinephrine, glucose, insulin, fatty acids, β-hydroxybutyrate and respiratory quotient; `caf02` measured RMR, heart rate, blood pressure, forearm blood flow, catecholamines and substrates. What is established here is metabolic, not experiential*[3],[2]