What Happens During A Fast: UK Timeline 2026 | Complete Nutrition

Understanding Fasting

What happens in the body during a fast

A predictable sequence of changes. Hours 0 to 12: digestion completing, insulin falling, glycogen breakdown starting. Hours 12 to 24: glycogen depleting, fat oxidation dominant, mild ketones starting. Hours 24 to 48: nutritional ketosis developing, growth hormone rising 5 to 10 fold. Hours 48 to 72: deep ketosis, sustained adaptations, autophagy upregulated in animal models. The body is doing real biology not magic. Understanding the timeline clarifies what different fast lengths actually achieve.

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Updated:
May 2026

Written by:
Dominic Walton, MD

Reading time:
7 min

The full answer

The hour by hour timeline

Fasting physiology unfolds in predictable stages. Four windows cover the main timeline.

1. Hours 0 to 12: digestion to early fasting

Hours 0 to 4: digestion of the last meal completes. Insulin peaks around 30 to 90 minutes after eating then returns toward baseline by 3 to 4 hours. Glucose and amino acids are absorbed and distributed. Hours 4 to 12: post-prandial state ending, insulin near baseline, liver glycogen breakdown beginning to supply glucose between meals. Fat oxidation rising modestly. This window is normal between-meal physiology and most people are in this state most of the day even without intentional fasting.

2. Hours 12 to 24: glycogen depletion and fat oxidation

Hours 12 to 16: liver glycogen substantially depleted (liver typically stores around 100 to 120 g of glycogen). Glucagon rises driving continued glycogen breakdown and starting gluconeogenesis. Fat oxidation rising substantially. Hours 16 to 24: liver glycogen approaching exhaustion. Gluconeogenesis from amino acids (glutamine, alanine) and glycerol becomes important for maintaining blood glucose. Fat oxidation dominant. Mild ketone production starting around hour 16 to 18. Blood ketones reaching 0.3 to 0.8 mmol/L by 24 hours in someone unaccustomed to fasting.

3. Hours 24 to 48: ketosis and growth hormone rise

By 24 hours fasting metabolism is well established. By 36 hours blood ketones reach 0.5 to 1.5 mmol/L (mild nutritional ketosis). Growth hormone rises 2 to 3 fold (per the 1992 Ho study). Brain shifts to using ketones for about 10 to 20 percent of energy. T3 thyroid hormone falls modestly. By 48 hours blood ketones reach 1 to 2.5 mmol/L (sustained nutritional ketosis). Growth hormone 5 to 10 fold elevated. T3 down 15 to 20 percent. Adrenaline elevated. The body is fully in fasting metabolism.

4. Hours 48 to 72+ : deep adaptations

By 72 hours blood ketones reach 2 to 5 mmol/L (deep nutritional ketosis). Brain uses ketones for 50 to 60 percent of energy. Growth hormone substantially elevated. Adaptive thermogenesis beginning to reduce energy expenditure. Significant autophagy upregulation in animal studies (less measurable in humans). Adrenaline plateauing. The body is now in sustained extended fasting territory which carries real medical risks. Beyond 72 hours risks of refeeding syndrome, electrolyte imbalance, gallstones and cardiac issues escalate.

What you experience

How the physiology translates to experience

Five subjective experiences and what is happening underneath.

Hunger waves at usual meal times

Ghrelin (hunger hormone) follows learned patterns peaking around your usual meal times per the 2004 Natalucci study. Even during a 24 or 48 hour fast hunger waves appear around the times you usually eat. The waves last 20 to 30 minutes then subside. This explains why hunger is not continuous during fasting but cyclical.

Energy stability between waves

Between hunger waves most people report stable energy through the first 24 to 36 hours of fasting. Adrenaline rises modestly maintaining alertness. Fat oxidation provides ample fuel. The fed-state energy crashes that people sometimes experience after meals do not occur during fasting. Stable energy is one of the more pleasant aspects of fasting.

The 36 to 48 hour transition

Many people experience the 36 to 48 hour window as the hardest moment. Glycogen is fully depleted, ketones are not yet at maximum, hormonal adjustments are still progressing. This is when people most often abandon longer fasts. Pushing through to 48 hours typically produces a sense of stable state as adaptations complete.

Possible cognitive changes

Some people report sharper thinking during the fasting state. Others report fog particularly in the first 1 to 2 weeks of new fasting protocols. The variability is genuine. Adrenaline elevation supports alertness for some. Ketones can support brain function efficiently. Adaptation typically smooths cognitive responses over 2 to 4 weeks of consistent practice.

Sleep effects vary

Some people sleep better during fasting periods (less digestive interference, more stable blood glucose). Others sleep worse (adrenaline elevation, hunger interruptions). Effects vary. Pay attention to your individual response. Persistent significant sleep disruption is a signal the protocol may not suit you.

Safety

When the timeline becomes problematic

Some points in the fasting timeline require specific caution.

Hours 0 to 12 in people on insulin. Hypoglycaemia risk significant. Medication adjustment required.
Hours 16 to 24 in those prone to gout. Uric acid rises as ketones develop. Possible flare risk.
Hours 24 to 48 first time. The harder transition. Hydration, electrolytes and gentle activity matter.
Hours 48 to 72 medical risks rising. Mild electrolyte imbalances possible. Anyone medication should have professional input.
Hours 72+ extended fasting territory. Multiple risks including refeeding syndrome on completion. Medical supervision recommended.

Standard contraindications apply: eating disorder history, pregnancy or breastfeeding, type 1 diabetes or insulin dependent type 2 diabetes, BMI under 18.5, children, adolescents and adults under 18. Anyone on medications or with significant medical conditions should discuss any fasting plan with their GP first.

For the wider picture on fasting from the gentlest protocols to extended fasts plus the science behind hunger, metabolism and refeeding, our Understanding Fasting hub brings every guide together in one place.

Part of the hub

Back to the Fasting Hub

This article sits inside our complete knowledge base on fasting covering protocols, physiology, safety and practical guidance. Head back to the hub for the full index.

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Keep reading

More on the underlying physiology

Several pages cover the timeline in more detail. Our piece on how the body responds to fasting covers the broader response pattern. Fat burning and ketone production during fasting covers the fuel shift specifically. And insulin levels and fasting covers the hormonal driver.

Frequently asked

During-a-fast questions

What happens in the body during a fast?

A predictable sequence of physiological changes. Hours 0 to 4: digestion completing, insulin still elevated. Hours 4 to 12: insulin falling, glycogen breakdown beginning. Hours 12 to 18: glycogen depleting, fat oxidation rising substantially. Hours 18 to 24: liver glycogen exhausted, gluconeogenesis maintaining glucose, mild ketones starting. Hours 24 to 48: nutritional ketosis developing, growth hormone rising 2 to 10 fold. Hours 48 to 72: deep ketosis, sustained adaptations, autophagy upregulated in animal models.

When does fat burning start during a fast?

Fat oxidation rises gradually from the moment insulin starts falling (around 4 hours after eating). By 12 hours fat oxidation is contributing meaningfully to fuel use. By 16 to 18 hours fat oxidation dominates after liver glycogen depletes. Fat is not switched on at a specific moment but rises along a continuum as insulin falls and glucagon rises. The misconception that fat burning starts at exactly 12 or 16 hours is wrong. The shift is gradual.

When does ketosis start during a fast?

Mild ketone production starts around 16 to 18 hours of fasting in someone unaccustomed to fasting. Blood ketones reach measurable levels (above 0.5 mmol/L) typically by 24 to 36 hours. Sustained nutritional ketosis (1 to 3 mmol/L) develops by 36 to 48 hours. Deep ketosis (above 3 mmol/L) develops by 3 to 5 days. People who follow ketogenic diets or fast regularly produce ketones faster than novices.

What happens to growth hormone during a fast?

Growth hormone rises substantially during fasting. The 1992 Ho study documented growth hormone increasing 2 to 3 fold by 24 hours and 5 to 10 fold by 48 hours of fasting. The rise serves to preserve lean mass and mobilise fat for fuel. The elevation is in physiological range not the dramatic levels of growth hormone administered medically. Growth hormone elevation is one of the genuine effects of fasting though the practical importance for typical health goals is debated.

What happens to insulin during a fast?

Insulin falls steadily. Post meal insulin returns to baseline by 3 to 4 hours. By 12 hours insulin is at fasting baseline (low single digit μU/mL). By 24 to 48 hours insulin can fall slightly below normal fasting baseline. The sustained low insulin allows hormone sensitive lipase to mobilise fat stores efficiently. Low insulin is one mechanism by which fasting promotes fat oxidation. Insulin sensitivity also tends to improve with repeated fasting practice.

Does autophagy happen during a fast?

Yes in animal studies clearly. In humans the evidence is more limited because measuring human autophagy directly is technically difficult. Animal studies show autophagy rising with fasting through 24 to 72 hours. Human inference from biomarkers and limited tissue studies suggests similar but slower upregulation. The magnitude and clinical significance of fasting-induced autophagy in humans is not well established despite popular claims. Some autophagy occurs continuously, fasting may enhance it.

What happens to muscle during a fast?

Less than people fear. Short fasts under 48 hours produce minimal muscle protein loss because growth hormone rises to preserve lean mass and gluconeogenesis uses other substrates preferentially. Extended fasts beyond 3 to 5 days can produce noticeable muscle loss particularly in those with lower starting muscle. Adequate protein on eating days (1.2 to 1.6 g per kg body weight) and resistance training help preserve muscle during fasting protocols. The Minnesota Starvation Experiment documented muscle loss with prolonged severe restriction.