|
Possible Benefits |
Cautions |
|---|---|
|
✅ Better Blood Sugar Control |
❌ Mood swings, sleep issues |
|
✅ Cellular 'Clean-Up' |
❌ Not suitable for many people* |
|
✅ Mental & Cognitive Performance |
❌ Headaches, dizziness, fatigue |
|
✅ Cardiovascular Health |
❌ Cardiovascular event risk |
|
✅ Weight Loss and Fat Loss |
❌ Loss of Muscle Mass |
|
✅ Gut Microbiome |
❌ Digestive issues & Disordered Eating |
What Happens to Your Body When You Fast? (Hour by Hour)
By R.Davies, PhD・Fasting
Updated May 15, 2026 | 5 min read
What Is Fasting? (Types and Definitions)
Fasting is defined as the voluntary 'abstention' from food for a period of time. When you do it, it goes on to trigger a load of changes in your body. This article explains how the body responds during fasting based on scientific research.
When you skip a meal, whether by choice, for religious reasons, or as part of a purposeful intermittent fasting routine, your body starts a fascinating journey of change throughout your entire body.
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Hours 0–4: Digestion and Blood Sugar (The Postprandial Phase)
In the first few hours after eating, your body is still processing your last meal. Your pancreas releases insulin to help move carbohydrates (your blood sugar or glucose) into cells to use for energy. Any excess carbohydrate gets stored in your liver and muscles as a substance called 'glycogen'.
During this “postprandial” period, your digestive system is working flat out, and your blood sugar levels are elevated. Your gut, liver, and pancreas are all coordinating to extract nutrients from food and distribute them throughout your body as quietly and efficiently as possible.
Hours 4–18: Glycogen Depletion Begins (Early Fasting)
As you move past the 3-4 hour mark, things start to change. Your blood sugar and insulin levels begin to decline and your body starts converting glycogen back into glucose [1]. Your body wants to keep your blood sugar levels as steady as possible, with what it has available. So when your levels start to drop, your liver will start breaking down that stored glycogen and release it into your blood, which delivers it to the rest of your body.
Most of your glycogen is stored in your liver and in your muscles. You usually have enough to keep you going for about 24 hours of your normal daily activity. But once your carbohydrate stores start running low, your pancreas reduces the amount of insulin it releases and starts increasing production of another hormone: glucagon. Glucagon acts in the opposite way that insulin does. It tells your body to release stored energy rather than store it.
Your blood glucose levels remain relatively stable during the 'early fasting' period, though they're lower than in the 'postprandial' period. If you do not fast regularly, you may notice changes to your focus, mood, or energy levels during this time. However, there doesn't appear to be any drop off in your ‘cognitive performance’ during short-term fasts [2].
Hours 18–24: Fat Burning Begins (The Fuel Switch)
Around the 18-24-hour mark, your liver's glycogen stores are becoming depleted, which forces a major “metabolic” fuel switch [3]. At this point, your body fat ('adipose tissue') starts releasing stored fat (as 'fatty acids') into your bloodstream to be used for energy (“lipolysis”). Some of these fuels are converted by your liver into another type of fuel: 'ketone bodies', which are small molecules that can be used for energy instead of fat.
This ‘fuel shift’ from mainly burning carbohydrates (and fat) to burning ketones (and fat) is called ‘ketosis’. It is defined as an increase in blood ketone levels, which can take 24-72 hours to occur. However, in addition to fat, your body also starts breaking down protein (from your muscles) and converts it into carbohydrate for energy (a process called 'gluconeogenesis').
This can be as much as half a kilogram of muscle mass a day when you factor in water and your glycogen stores [4]. There are also some (relatively minor) changes in your cardiovascular system, where your heart rate increases, and your blood pressure decreases [5].
Hours 24-72: The Ketogenic Period
This is when fasting's effects on the brain become particularly interesting. Lower glucose levels are thought to trigger a ‘cleaning operation’ in your brain cells (via a process called 'autophagy'), which may be beneficial for brain health and cognitive function [6].
Many people also report better mental clarity during this phase, where your brain’s using ketones for fuel. During this time, your cells also become more sensitive to insulin, and inflammation and oxidative stress can also decrease in those who have high levels to start with. Your gut bacteria are also thought to respond positively by increasing the amount of beneficial “good” bacteria and decreasing the harmful “bad” bacteria [7].
Hours 72+: The Extended Fasting Period
These types of fasting are only done under medical supervision, as the changes that occur become more dramatic. These changes can be both positive and negative for health, depending on your baseline health status (where you’re starting from).
Autophagy ‘clean-up’ continues, which is thought to support healthy ageing and help protect brain health. You continue breaking down muscle for energy, and you also continue to burn fat and lose body weight.
Over a 7-day water fast, a group of young healthy adults lost 4.6 kg of muscle mass, 1.4 kg of fat and 5.8 kg of body weight [4]. This could have both positive and negative effects on your health, depending on your starting point and health goals.
Most of your major hormones start to change when you fast for this long, both the amount of them and how they interact with your body [8]. There are also some reported benefits to your immune system as well [9].
Frequently Asked Questions
What happens to your body after 16 hours of fasting?
Around 16 hours, glycogen stores are significantly depleted, and your body has measurably shifted towards fat burning. Insulin is low, glucagon is high, and fatty acids are being converted to ketones. Autophagy (the cellular clean-up process) begins to meaningfully increase around this point, though it peaks at 24–48+ hours.
When does fat burning start during a fast?
Fat burning begins to ramp up meaningfully around 12–18 hours into a fast, as liver glycogen depletes and insulin falls. By 18–24 hours, fat is the dominant fuel source (for most tissues), which is why protocols like 16:8 intermittent fasting are effective for fat loss over time.
How long does it take for autophagy to kick in during fasting?
Autophagy begins to increase after roughly 16–24 hours of fasting and accelerates significantly at 24–48 hours. Peak autophagy appears to occur around 48–72 hours in some studies. The exact timing varies with individual metabolism and prior food intake. Intense exercise also triggers autophagy independently of fasting.
What happens to your body after 24 hours without eating?
By 24 hours, glycogen stores are largely depleted, and fat burning has become the primary fuel source. Insulin is at its lowest, ketone production is increasing, and autophagy is meaningfully elevated. Appetite hormones like ghrelin, which typically peak around 12–16 hours, often begin to stabilise during extended fasting.
Does fasting put your body into ketosis?
Extended fasting can induce ketosis, but it typically takes 24–72 hours, depending on your metabolic rate and prior carbohydrate intake. Short intermittent fasting protocols like 16:8 may produce mild ketosis in some individuals, but full nutritional ketosis generally requires a sustained fast or a low-carbohydrate diet.
Key Takeaways
Fasting triggers a lot of changes throughout your body — some beneficial, others less so. Different people will respond differently to different fasts. Individual changes are difficult to predict and determined by your current health, age, sex, genetics, the type and duration of the fast you’re doing.
The science of fasting reveals a remarkably adaptive response in your body. Rather than shutting down, your systems coordinate an intricate response — switching fuel sources, spring-cleaning some of your cells, and making changes that may support long-term health.
But like any health change, fasting isn't one-size-fits-all. Understanding what happens during those hours without food can help you decide if (and how) fasting might fit into your own health priorities.
Below is just a summary of the general changes that have been reported.
* aged under 25, pregnant or breastfeeding, have diabetes, take insulin or medications that have to be taken with food, work night shifts, operate heavy machinery, have a seizure disorder, an eating disorder or other relevant medical condition, to name a few.
Sources
1. Sanvictores T et al. Physiology, Fasting. [Updated 2023 Jul 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan.
2. Bamberg C & Moreau D. Acute effects of fasting on cognitive performance: A systematic review and meta-analysis. Psychol Bull. 2025 Sep;151(9):1147-1169. PMID: 41182703.
3. Anton SD et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (Silver Spring). 2018 Feb;26(2):254-268. PMID: 29086496
4. Kolnes KJ et al. Effects of seven days' fasting on physical performance and metabolic adaptation during exercise in humans. Nat Commun. 2025 Jan 2;16(1):122. PMID: 39747857
5. Gonzalez JE & Cooke WH. Influence of an acute fast on ambulatory blood pressure and autonomic cardiovascular control. Am J Physiol Regul Integr Comp Physiol. 2022 Jun 1;322(6):R542-R550. PMID: 35380473
6. Gudden J et al. The Effects of Intermittent Fasting on Brain and Cognitive Function. Nutrients. 2021 Sep 10;13(9):3166. PMID: 34579042
7. Paukkonen I et al. The impact of intermittent fasting on gut microbiota: a systematic review of human studies. Front Nutr. 2024 Feb 12;11:1342787. PMID: 38410639
8. Fink J et al. Effects of Fasting on Metabolic Hormones and Functions: A Narrative Review. Juntendo Iji Zasshi. 2024 Oct 15;70(5):348-359. PMID: 39545228
9. Buono R & Longo VD. When Fasting Gets Tough, the Tough Immune Cells Get Going-or Die. Cell. 2019 Aug 22;178(5):1038-1040. PMID: 31442398.
Published: December 22, 2025
Updated: May 15, 2026
Lead Author: R.Davies, PhD | Author Bio
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