What happens to adrenaline levels during fasting, and how does this affect energy and metabolism?
The Short Answer
Adrenaline (epinephrine) increases during fasting. This is your body’s way of mobilizing energy stores: it signals fat cells to release fatty acids and tells the liver to produce glucose. Many people report feeling sharper rather than sluggish while fasting, and the usual explanation offered is this hormonal shift. Treat the explanation as unproven: the studies measured metabolism, not how anyone felt, and the timing runs against the simple version; metabolic rate peaks before noradrenaline has moved.[1]
Something to Sit With
Have you experienced the energy and clarity some people describe during fasting?
Does knowing that adrenaline rises during fasting change your expectation of how fasting should feel?
What would happen if you approached your next fast without assuming it must feel depleting?
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The Counter-Intuitive Reality.
Common assumption: Not eating = low energy, fatigue, brain fog.
What many people report instead: mental clarity, stable energy, alertness. Those reports are not measurements (the studies measured metabolic rate and catecholamines, not alertness or energy), so this is an assumption set against an anecdote, and neither is established.[1]
This puzzles people. If you’re not putting fuel in, shouldn’t you feel depleted?
The usual answer offered is hormonal adaptation. Your body has sophisticated mechanisms for handling periods without food. It doesn’t just shut down; it shifts gears.
What Happens During Fasting.
When you stop eating, several hormonal changes occur:
Insulin drops. Without incoming food, insulin levels fall. Low insulin allows stored fat to be accessed for fuel.
Glucagon rises. This hormone signals the liver to release stored glucose (glycogen) and produce new glucose (gluconeogenesis).
Adrenaline increases. During fasting, adrenaline and noradrenaline levels rise, more than doubling by day 3-4.[1] This has multiple effects:
- Signals fat cells to release fatty acids: circulating fatty acids rise steeply during a fast[2]
- Slightly elevates heart rate and metabolic rate[1]
What was not measured, and it is the part people most want: neither study recorded alertness, mood, vigor, or any subjective state at all.[1] What is established is metabolic, not experiential. People do report feeling sharper on a fast; that is not what these measurements show.
Growth hormone increases. As companion #131 covers, Jason Fung describes growth hormone maintaining muscle in a protein-conservation phase after about five days of fasting. That is his account, not an established finding.
The Evolutionary Logic.
On the usual evolutionary account, this response makes sense.
Imagine an early human who hasn’t eaten in a day. Two possible responses:
Response A: Feel tired, sluggish, and unmotivated. Lie around conserving energy. (Continue not finding food. Starve.)
Response B: Feel alert, energetic, and motivated. Metabolic rate stays up. Hunting/foraging abilities enhanced. (More likely to find food. Survive.)
Natural selection would favor Response B, and the metabolic side of it is real: expenditure holds up rather than collapsing early in a fast.[1] Neither study measured motivation, alertness, or foraging in anyone.
The Experience of Fasting.
Many people report during extended fasting windows (16+ hours):
Mental clarity. Sharper thinking, better focus. Some of this may be from stable blood sugar and ketone production. The contribution of adrenaline is not established.
Stable energy. No post-meal slumps. Consistent energy throughout the fasting period.
Reduced hunger after initial period. Hunger often peaks early in a fast, then subsides. The adrenaline response may contribute to this.
Slight elevation in mood. Some people feel a mild euphoria during extended fasts, possibly related to the hormonal state.
This is why many people who try intermittent fasting are surprised: “I thought I’d be miserable and tired. Instead I feel great.”
The Metabolic Effect.
Whatever the adrenaline rise does to how you feel, which was not measured, it affects metabolism directly.
Lipolysis. Adrenaline activates hormone-sensitive lipase in fat cells, releasing fatty acids into the bloodstream for use as fuel.
Thermogenesis. Metabolic rate doesn’t decrease during short-term fasting; it may increase slightly: about 14% over three days in one small study[1], though the larger study puts it at 6% by 36 hours, gone by 72, and not significant in men.[2]
Glycogenolysis. Adrenaline helps mobilize liver glycogen, maintaining blood sugar without eating.
Short-term fasting doesn’t typically cause the metabolic slowdown that chronic calorie restriction can. Why the two differ is not settled: the study measuring both found norepinephrine and metabolic rate did not correlate.[2]
Comparing Fed vs. Fasted States.
Frequently fed state:
- Insulin high → fat storage favored
- Constant digestion → energy used for processing food
Fasting state:
- Insulin low → fat mobilization favored
- No digestion → energy available for other uses
What this comparison cannot tell you is how either state feels. The studies behind it measured resting metabolic rate, catecholamines, and substrates (not alertness, energy, or any subjective state), so steady energy and the post-meal slump are not what they found, whatever the arrows above might seem to promise.[1] Neither state is inherently “better”; both are normal.
Further reading:
- Zauner C et al., “Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine”[3] — American Journal of Clinical Nutrition, 2000
- Cahill GF Jr, “Fuel metabolism in starvation”[4] — Annual Review of Nutrition, 2006
- Mattson MP et al., “Intermittent metabolic switching, neuroplasticity and brain health”[5] — Nature Reviews Neuroscience, 2018
Written by AI, directed and approved by me. How this is made →