What happens to growth hormone, adrenaline, and metabolic rate during a 24-36 hour fast?
The Short Answer
All three increase. Growth hormone rises steeply, though whether it protects muscle is not shown: in a direct test, blocking the hormone’s receptor across a 36-hour fast suppressed fat mobilization while protein metabolism was unaffected.[1] Adrenaline (norepinephrine) rises, and free fatty acids become available as fuel. Metabolic rate increases rather than decreases, though what drives that rise is not settled. This is the opposite of what is often expected, and the opposite of what happens during chronic calorie restriction.
Something to Sit With
Your body doesn’t panic when you don’t eat for a day. It shifts into a mode it knows well: hormones move, fuel comes from fat, and resting expenditure holds or rises rather than falling. What that mode does for muscle, and whether it feels like anything, are the two questions this page takes care over below.
The panic is in your mind. Your body is fine.
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Growth Hormone Surges.
Growth hormone (GH) is primarily known for building muscle and tissue, and it is often credited with a second job during a fast: protecting existing muscle.
Research by Palmblad and colleagues, in twelve healthy men through a ten-day total fast, showed a pronounced rise in growth hormone.[2] Measured directly elsewhere, growth-hormone production rose five-fold over a two-day fast in normal men, and integrated concentration rose from 2.82 to 8.75 µg·min/ml in six men by day five of a fast, most of that rise already there by the first fasted day.[3]
Whether that rise protects muscle is not shown. The one direct test was partial by design, over 36 hours in 10 healthy young men, and it does not demonstrate protein sparing at that duration, which is not the same as showing there is none.[1]
That rise is a hormonal signal, not an outcome. Trepanowski’s 2018 trial measured the outcome and found fat-free mass falling in both intervention arms rather than being preserved: 1.2 kg on alternate-day fasting and 1.8 on daily restriction, with the difference between them not significant.[4] The signal is worth knowing and does not settle what happens to muscle.
Adrenaline (Norepinephrine) Rises.
Norepinephrine rises during fasting.[5] Zauner’s authors suggest noradrenaline may be the primary initial signal, but the larger study finds metabolic rate peaking at 36 hours, when noradrenaline has not yet moved, and concludes sympathetic activity does not correlate with the rise.[5]
It is tempting to reason that a body which grew sluggish whenever food ran short would have starved. That is a story about why alertness would be useful, not evidence that fasting produces it.
Subjectively, many people report feeling clearer and more energetic during fasts, especially after the first 12-16 hours. What produces that report is unclear from these studies: the larger study finds sympathetic activity does not correlate with the metabolic rise, so putting the feeling down to norepinephrine goes past the record.[5]
Metabolic Rate Holds, and Can Rise.
This is the most counterintuitive finding. Conventional wisdom says skipping meals slows metabolism. The research finds otherwise during short-to-medium fasts: no slowdown, and in some studies a rise.
Zauner’s study, in eleven lean subjects, found resting energy expenditure rising up to ~14%. A larger trial found about 6% at 36 hours, nothing significant by 72, and no significant change in men.[6] What both support: short fasting does not slow you down. The body is burning more energy, not less, though that reverses if you keep going: past about 60 hours, 24-hour energy expenditure drops by around 11%.[6]
What rises with it is mobilization of energy stores: stored fat converted to available fuel, a process with metabolic costs of its own. What drives the rise is not settled, and norepinephrine is not established as the mechanism: the larger trial found sympathetic activity did not correlate with it.[5]
This contrasts sharply with chronic calorie restriction, which does reduce metabolic rate over time. The Minnesota experiment is the standing example: 32 men eating about 1,570 calories a day, roughly half of normal, for 24 weeks, losing 24.24% of body weight, with basal metabolism falling by about 40%.[7] But that was six months of continuous restriction, a very different signal to the body than a 24-36 hour fast.
Why Fasting Differs from Restriction.
On Fung’s account, the key difference is hormonal:
Chronic restriction:
- Insulin remains elevated (you’re still eating, just less)
- Body receives “famine” signal: conserve resources
- Metabolic rate drops to match reduced intake
- Hunger intensifies as body fights to restore weight
- Muscle may be broken down for energy
Short-term fasting:
- Insulin drops to very low levels
- Growth hormone surges; whether it spares protein is not shown
- Norepinephrine rises
- Body shifts to burning fat stores
- Metabolic rate maintains or increases
On that account these are fundamentally different metabolic states, the body responding to the hormonal environment rather than just to fewer calories. What is measured is narrower: a short fast does not slow resting metabolism, while months of severe restriction do. Whether ordinary fasting and ordinary dieting differ in outcome is another question, and in six randomized trials they matched for weight loss.[7]
What You Experience.
During a 24-36 hour fast, you might notice:
Hours 0-12: Not much different from normal. You’re using recent food energy and glycogen stores.
Hours 12-18: Hunger may increase as you enter unfamiliar territory. This is largely psychological and habitual.
Hours 18-24: Hunger often decreases as ketone production begins. Mental clarity may improve. Energy often stabilizes or increases.
Hours 24-36: Many people report feeling good: clear-headed, energetic, not particularly hungry. What drives that report is not established.
The fear of 24-hour fasting is usually worse than the experience. Healthy bodies generally handle it well.
Further reading:
- Zauner C et al., “Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine”[8] — American Journal of Clinical Nutrition, 2000
- Palmblad J et al., “Effects of total energy withdrawal (fasting) on the levels of growth hormone, thyrotropin, cortisol, adrenaline, noradrenaline, T4, T3, and rT3 in healthy males”[9] — Acta Medica Scandinavica, 1977
- Ho KY et al., “Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man”[10] — Journal of Clinical Investigation, 1988
- Fung J, The Obesity Code[11] (2016) — Chapter on fasting hormones
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