Today’s prompt, and the companion piece behind it.
The Body’s Response
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, and a review argues it spares protein indirectly rather than acting on muscle. A later experiment by that review’s own author tested the argument: 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 most people expect, 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.
Learn More
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.[1]
On one review’s account growth hormone spares protein indirectly, by driving lipolysis, with the freed fatty acids as the protein-sparing agents: that chain is the review’s synthesis at two removes, not a measurement.[3]
That is a hormonal signal, not an outcome. Trepanowski’s 2018 trial measured the outcome and found fat-free mass falling in both fasting 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 Increases.
This is the most counterintuitive finding. Conventional wisdom says skipping meals slows metabolism. The research shows the opposite during short-to-medium fasts.
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.
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; a review argues it spares protein indirectly
- Norepinephrine rises
- Body shifts to burning fat stores
- Metabolic rate maintains or increases
These are fundamentally different metabolic states. The body isn’t just responding to “fewer calories”; it’s responding to the hormonal environment, which differs completely.
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. The body handles it well because it evolved handling it.
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
Written by AI, directed and approved by me. How this is made →
You already know what to eat
You already know enough.
The information isn’t the problem. You have plenty of information.
The problem is that by the time you’re standing in front of the fridge — tired, negotiating with yourself about whether you’ve earned a snack — it’s already too late. Your habits have voted. Willpower shows up late, outnumbered.
So what if the right thought arrived before all of that?
One thought, every morning, before your day fills in
365 Changes sends you a single prompt about eating. It arrives early — before hunger, before decisions, before habits kick in.
Not a meal plan. Not a rule. Just a question or idea to sit with while you make coffee. Each is simple, but they accumulate.
You need the same core ideas to arrive from hundreds of different angles until they stop being things you know and start being who you are.
Get the daily prompt free:
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Each morning you’ll get a question or scenario like:
You’re in a bad mood and craving comfort food. What do you do first?
Click any of these and read the response. Each one is a complete thought worth reading on its own. The daily email makes sure one arrives before you need it.
- What happens to growth hormone, adrenaline, and metabolic rate during a 24-36 hour fast?
- Your expectations are unrealistic and you’re constantly disappointed. What expectations need adjusting?
- Do you drink flavored or enhanced waters? Check if they contain sweeteners.
Why daily? Why morning? Why small?
Daily, because change doesn’t happen in one big moment of clarity. It happens through accumulation — the same ideas arriving from slightly different angles until one day you notice you’re thinking differently about something you used to not think about at all.
Morning, because evenings are when you negotiate. Mornings are when you can still hear yourself think. There’s a small window before the momentum builds, before the habits wake up. That’s when a thought can land.
Small, because you’ve already heard it all. You don’t need another article, another tip, another list. You need the right ideas to keep showing up until they stick.
This is not a diet program
There’s no tracking. No streaks. No badges. No notifications begging for attention.
If you open the email, good. If you don’t, it doesn’t guilt you. There’s no streak to break. Tomorrow another one arrives.
One email. It asks almost nothing. It just shows up.
Free. No tracking. No streaks. Just one email, every morning.
Does it work?
Honestly? We don’t know if it will work for you.
I built this for myself. I was tired of feeling like I know exactly what I should eat and never quite eating that way. Tired of the mental math — what I ate, what I’ll eat, what I’m going to eat tonight.
I didn’t need more information. I needed the right thought to arrive at the right time. So I built something that does that, every morning, and I started using it.
It’s working. Slowly. I can’t point to a moment when things changed. I just notice that they have.
—Craig
How this gets made is written down too.
The writing here is done by AI. I direct it, and I approve everything before it publishes. Methods is a journal about the work behind the writing: what gets checked, what gets corrected, and what goes wrong. Methods is free, public, and nothing you have to read.
One way to find out:
Want more depth?
Some mornings the prompt will make you curious. The prompt is complete on its own, but each one also has a companion piece — 3 to 5 minutes of depth, grounded in research, for when you want to understand why.
All 500 companions are free to read. Here are four to start with:
- The Future Self — The person you’ll be in five years is being built by today’s choices
- The Food Mood — You’re in a bad mood and craving comfort food. What do you do first?
- The Tolerant One — You can sit with hunger and discomfort without reacting
- The Self-Deception — In what ways do you deceive yourself about what or how much you eat?
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