Why short-term fasting doesn’t slow your metabolism like chronic dieting does.
What the evidence says
- Contrary to popular belief, short-term fasting does not decrease metabolic rate[1],[2]
- Resting energy expenditure rose ~14% over 84 hours in `caf01` — 3.97 to 4.53 kJ/min, day 1 to day 3, 11 lean subjects[1]
- `caf02` is the larger study and found a smaller, shorter-lived rise: RMR up ~6% at 36 h (4.60 to 4.88 kJ/min, P < 0.001) and no longer significantly above baseline at 72 h (4.72 kJ/min, P = 0.06), in 29 subjects[2]
- And it did not hold for men: analysed by sex, RMR did not change significantly in men across 72 h; the increase-then-decline pattern was the women’s[2]
- This differs from chronic calorie restriction, which can reduce metabolic rate[3]
- Norepinephrine more than doubles during short-term fasting (1716 → 3728 pmol/L over 84 hours)[1]
- Norepinephrine signals fat cells to release fatty acids (lipolysis)[2]
- The hormonal environment during short-term fasting includes high norepinephrine and high growth hormone — but `caf02`’s authors decline the causal step: 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”[1],[4],[2]
- Extended fasting (60+ hours) does reduce metabolic rate — 24h energy expenditure dropped ~11% in 60-hour fast[5]
- Neither `caf01` nor `caf02` measured alertness, focus, energy or any subjective state. `caf01` measured REE, 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 this file establishes is metabolic, not experiential[1],[2]
- And nothing here supports a purpose. The catecholamine rise is real; ”to increase alertness”, ”to maintain energy” and ”to help find food” are teleology no source supplies — `caf01`’s authors call the rise a signal following a glucose decline, and `caf02`’s conclude sympathetic activity ”does not correlate with elevations in RMR, HR, or FBF”[1],[2]
- `cfs01` classifies fasting types — intermittent, periodic, time-restricted feeding — as biologically distinct regimes[6]
- No source held here compares intermittent fasting to chronic restriction on metabolic adaptation. The comparison is plausible from the parts — short fasts raise REE in `caf01`, sustained restriction lowers it in `cls03` — but it is an inference across two designs, not a finding, and neither paper tested it