The Fasting Timeline: What Happens to your Body Hour by Hour

This article is part of the Nutrition & Diet library.

When you skip a meal, whether by choice, for religious reasons, or as part of a purposeful intermittent fasting routine, your body starts on a fascinating ‘physiological’ journey of change throughout the entirety of your body.

Hours 0-3: “The Postprandial Period”

In the first few hours after eating, your body is still processing your last meal. Your pancreas releases insulin to help move carbohydrates (blood glucose) into cells to use for energy; any excess 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: “The Early Fasting Period”

As you move past the 3-4 hour mark, things start to change. Your blood sugar and insulin levels begin to decline, causing your body to start converting glycogen into glucose.

Your body wants to maintain your blood sugar levels as steady as possible (known as homeostasis); so your liver begins breaking down glycogen and releasing it to the rest of your body.

Most of your glycogen is stored here and in your muscles, and it has enough fuel to keep you going for about 24 hours of normal daily activity.

As a result of the carbohydrate supplies running low, your pancreas reduces the amount of insulin it releases and increases production of another hormone: glucagon. Glucagon is the opposite of insulin — it signals your body to release stored energy rather than store it.

Your blood glucose levels remain relatively stable during this 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.

Despite what the marketing suggests, there’s no drop off in your brain’s ‘cognitive performance’ during short-term fasts.

Hours 18-24: “Fuel Switch Period”

In around 18-24 hours, your liver's glycogen stores are becoming depleted, forcing a major “metabolic” fuel switch.

At this point, your body fat (adipose tissue) starts releasing stored fat (fatty acids) into your bloodstream to be used for energy (“lipolysis”). Some of these are converted by your liver into “ketone bodies” — a small molecule that can be used for energy instead of fat.

This ‘fuel shift’ from burning carbohydrate (and fat) to burning ketones (and mainly fat) is called “ketosis”. It is defined as an increase in blood ketone levels, which can take 24-72 hours.

However, in addition to fat, your body also starts breaking down protein and converts it into carbohydrate for energy (a process called gluconeogenesis). 

This can be as much as 80 g of protein a day, or around 370 grams of lean mass (when you add water and a few other things that aren’t just protein).

There are also some (relatively minor but measurable) improvements in your cardiovascular system, where your heart rate increases and your blood pressure decreases.

Hours 24-72: “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) that may be beneficial for brain health and cognitive function.

Many people also report better mental clarity during this phase, where your brain’s using ketones for fuel. 

During this time, your cells become more sensitive (or less resistant) to insulin, and inflammation and oxidative stress are also lowered 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.

Hours 72+: “Extended Fasting Period”

These types of fasting are only done under medical supervision, as the changes that occur become more dramatic, both positive and negative for health, depending on your health status and 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 protein and 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 2.6 kg of lean mass, 1.4 kg of fat and 5.8 kg of body weight. This can have both positive and/or negative effects on health, depending on your starting point and health goals

Most of your major hormones start to change, both the amount of them and how they interact with your body. And there are also thought to be some benefits to your immune system as well.

The Takeaway: Pros and Cons

Fasting triggers a load 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. 

Below is just a summary of the general changes that have been reported.

The science of fasting reveals a body that's remarkably adaptive. 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.

N.b. This article is for informational purposes only and is not medical advice. Please consult a qualified healthcare provider before making health decisions. No content on this site, regardless of date, should ever be used as a substitute for direct medical advice from your doctor or other qualified clinician.

* 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, eating disorder or other relevant medical condition to name a few.