What Happens to Your Body When You Fast? (Hour by Hour)

Key Takeaways

• Fasting triggers a predictable sequence: blood sugar drops, fat stores are mobilised, and around 18–24 hours in, your body switches to burning ketones for fuel.
• Potential benefits include improved insulin sensitivity, autophagy, and better gut health — but muscle breakdown is a real trade-off beyond 24 hours.
• Fasting responses vary significantly by age, health status, and duration, and extended fasting requires medical supervision.

What Is Fasting?

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.

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 glucose¹. 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 fasts².

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 switch³. 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 stores⁴. There are also some (relatively minor) changes in your cardiovascular system, where your heart rate increases, and your blood pressure decreases⁵.

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 function⁶.

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” bacteria⁷.

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 weight⁴. 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 body⁸. There are also some reported benefits to your immune system as well⁹.

Frequently Asked Questions

Bottom Line

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.

Possible Benefits

• Better Blood Sugar Control
• Cellular 'Clean-Up'
• Mental & Cognitive Performance
• Cardiovascular Health
• Weight Loss and Fat Loss
• Gut Microbiome

Cautions

•  Mood swings and sleep issues
• Not suitable for many people*
• Headaches, dizziness, fatigue
• Cardiovascular event risk
• Loss of Muscle Mass
• Digestive issues & Disordered Eating

*people 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.