What are the five stages the body goes through during an extended fast?
The Short Answer
The fed-to-fasted transition is usually presented in five stages: fed (0-4 hours), early fasting on liver glycogen (6-24 hours), gluconeogenesis (24-48 hours), ketosis (one to three days), and protein conservation (five days and beyond).[1] That framing is Jason Fung’s presentation of mainstream physiology rather than a measurement, and the boundaries overlap and blur: the phases are real, the tidiness is editorial. Understanding them removes mystery from fasting; treating the hours as thresholds you cross on a schedule puts it back.
Something to Sit With
Which stage does your typical eating pattern reach?
Knowing these stages, would you consider occasionally extending your fasting window?
What stage would you need to reach for your specific goals?
Learn More
Stage 1: Fed State (0-4 Hours).
What’s happening: Your body is processing the food you just ate. Blood sugar rises. Insulin is released to shuttle glucose into cells and signal storage.
Energy source: Incoming nutrients from digestion.
Hormones: Insulin dominant. Fat storage is occurring.
How it feels: Satisfied, possibly sleepy if the meal was large or carb-heavy. Digestion is active.
What this means: You’re in accumulation mode. The body is dealing with incoming energy.
This is the “fed” state that many people in modern society spend much of their time in: eating, digesting, processing.
Stage 2: Early Fasting/Post-Absorptive (6-24 Hours).
What’s happening: Digestion is complete. Blood sugar begins to normalize. Insulin drops. The liver starts releasing stored glycogen to maintain blood sugar.
Energy source: Liver glycogen primarily. Some fat burning begins.
Hormones: Glucagon rising. Insulin dropping.
How it feels: Hunger may appear, typically in waves. Energy often stays steady.
What this means: You’re transitioning out of fed state. The metabolic switch is beginning.
This is where intermittent fasting (16:8, 18:6, and 20:4) operates. Many people reach this stage overnight.
Stage 3: Gluconeogenesis (24-48 Hours).
What’s happening: Liver glycogen is significantly depleted. Fat oxidation increases substantially. The body is fully relying on stored energy.
Energy source: Predominantly fat. Gluconeogenesis (making glucose from non-carb sources) increases.
Hormones: Growth hormone rising.[2] Insulin remains low. Adrenaline and noradrenaline do rise across a fast, but later than this: in the larger study they are unchanged at 36 hours and significantly raised only by 72.[3]
How it feels: Initial hunger often subsides. Beyond that, reports vary, and it is worth knowing that the studies that tracked this stage measured metabolism rather than how anyone felt.
What this means: The body is now fully accessing fat stores. The metabolic “switch” to fat burning is complete.
A one-meal-a-day pattern sits at the boundary between this stage and the one before it, and Stage 3 overlaps Stage 4 in the schema itself, which is the clearest sign that these are phases rather than thresholds.
Stage 4: Ketosis (One to Three Days).
What’s happening: Fat breakdown is significant. The liver converts fatty acids into ketone bodies. Ketones become a major fuel source, including for the brain.
Energy source: Fat and ketones. Minimal glucose produced through gluconeogenesis for organs that require it.
Hormones: Growth hormone stays high; the rise itself has already happened by the first fasted day rather than building to a peak here.[2] Cellular autophagy is the process usually named at this stage, and the finding offered for it is in yeast.[4] Insulin is very low.
How it feels: Experiences vary. Ghrelin declines slightly overall across a fast rather than climbing, though that is a hormone measure: the fasting studies took no hunger ratings.[5]
What this means: The body is fully adapted to running on stored fat. The brain is partially fueled by ketones.
This stage represents what our ancestors regularly experienced between successful hunts or foraging.
Stage 5: Protein Conservation (Five Days and Beyond).
What’s happening: Ketone production is high, and in prolonged fasting ketones become the brain’s predominant fuel, though the founding measurements were made in three obese patients at five to six weeks of therapeutic starvation, so the far end of that shift lies well beyond this stage.[6] Whether growth hormone spares protein here 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, in 10 healthy young men under partial blockade.[7]
Energy source: Predominantly fat and ketones. Gluconeogenesis is efficient and limited.
Hormones: Growth hormone remains elevated. On Fung’s account this is the phase where it maintains muscle and lean tissue: his description rather than a measured finding.[1]
How it feels: Variable. Some feel excellent; others experience fatigue. How hunger behaves this far into a fast is unclear.
What this means: The body is drawing heavily on stored fat. What that does for lean tissue at this duration is a separate question.
This stage requires medical supervision for extended periods and is not typical for most people’s practice.
Why This Matters.
Understanding these stages:
Normalizes the process. Each stage is a known physiological response, not something strange or dangerous.
Explains changing sensations. Hunger arriving in waves early, then subsiding as the fast extends: these make sense with context.
Informs practice. Different fasting lengths achieve different effects. A 16-hour fast operates differently than a 48-hour fast.
Reduces fear. Knowing the body has evolved mechanisms for each stage removes the anxiety of “what if something goes wrong?”
Practical Implications.
For daily intermittent fasting (16-18 hours): You’re operating in Stage 2. Fat burning is occurring; full ketosis is not.
For 24-hour fasts: You’re entering Stage 4. Ketone production begins.
For 48-72 hour fasts: You’re fully in Stage 4. Autophagy is described as increased, on a finding made in yeast[4], and growth hormone is elevated, having risen on the first day rather than accumulating.[2]
For longer fasts: Stage 5 territory begins around five days. Should be done with medical guidance.
Many people practicing intermittent fasting never reach Stage 4, and for many goals Stages 2-3 may be enough.
Further reading:
- Cahill GF Jr, “Fuel metabolism in starvation”[8] — Annual Review of Nutrition, 2006
- Fung J & Moore J, The Complete Guide to Fasting[9] (2016)
- Longo VD & Panda S, “Fasting, circadian rhythms, and time-restricted feeding in healthy lifespan”[10] — Cell Metabolism, 2016
Written by AI, directed and approved by me. How this is made →