What happens to your body during the stages of intermittent fasting?

Learn what happens to your body as it shifts fuel sources during the stages of intermittent fasting. Check out the insightful tips for success with your plan.

Intermittent fasting: Here's what happens to your body during various stages

Intermittent fasting (IF) is a popular diet approach that alternates between periods of eating and fasting. You may choose to try IF for weight loss or potential health benefits. The different intermittent fasting “protocols” range from short to extended fasts.

But what exactly happens to your body during intermittent fasting? Let’s explore the stages of intermittent fasting and the physiological processes behind them.

Note: Everyone’s body is different, and these phases may start at slightly different times depending on your health and activity.

Stage 1: The fed state (0-4 hours after eating)

Main fuel source: Glucose from food

The first stage of intermittent fasting occurs right after you eat. In this stage, your body digests food and absorbs nutrients. Your pancreas releases insulin to help your cells absorb glucose.

The primary fuel source during this time comes from glucose, mainly from carbohydrates. Your liver and muscles store some glucose as glycogen, while the rest fuels immediate energy needs.

Tip: Remember to set your fasting timer after eating based on your fasting protocol.

Stage 2: The post-absorptive state (4-12 hours after eating)

Main fuel sources: Glucose and glycogen (stored glucose)

After about 4 to 12 hours, depending on how much you’ve eaten and your activity levels, your body enters the post-absorptive state. At this point, insulin levels drop to baseline, and food digestion and nutrient absorption are complete. Your body starts to shift from using glucose to using glycogen stored in the liver and muscles for energy.

In this phase, relatively little fat is burned, though the body is ready to begin the transition from glucose to fat burning.

Tips: Depending on when you start your fast, you may be sleeping for much of this time.

Find alternatives to your night-time snacking routine, such as doing a puzzle, bathing, or reading a book.

Stage 3: The early fasting state (12-24 hours)

Main fuel sources: Glycogen, then fat (ketones)

Once you’ve been fasting for about 12 hours, your body enters the early fasting state. This is when a major fuel shift happens and fat burning kicks in. Insulin levels drop, and glycogen stores deplete, especially if you’ve had an active day. Fat cells release fatty acids, from which the liver makes ketones (fats burned without carbohydrates). The ketones serve as an alternate fuel source for your brain and muscles.

Tips: You may start to feel hungry as the body adjusts to the new fuel source, but some people also report an increase in mental clarity as ketones begin to provide energy for the brain.

Plan for how you will break your fast if you will be ending your fast during this stage.

Stage 4: The fasting state (24-48 hours)

Main fuel source: Fat (ketones)

Between 24 and 48 hours into fasting, your body has run out of glycogen stores and adapts to burning fat as its primary fuel. Ketosis, a metabolic state where ketones are the main energy source, is in full swing. Fat cells continue to break down triglycerides into fatty acids used for energy.

Growth hormone levels rise at this point, helping burn fat and protect muscle. The body also starts autophagy, a process that cleans out damaged cells and regenerates new ones.

Tips: This stage is not for beginners. Staying hydrated and maintaining electrolyte balance are essential, as prolonged fasting can cause dehydration. Talk to your doctor if you are considering an extended fasting period.

Stage 5: The starvation state (beyond 48 hours)

Main fuel source: fat (ketones)

Fasting beyond 48 hours is not typically recommended, as it is difficult to achieve and increases health risks. In this state, the body slows down metabolism to conserve energy and breaks down muscle tissue as an alternate energy source.

Tip: Talk to your doctor if you are considering an extended fasting period.

Tracking fasting with MyNetDiary

No matter your fasting protocol, MyNetDiary makes it easy to set goals and track your progress. Premium users have access to a convenient fasting timer and tracker. You can view your fasting status on your dashboard and review it on the fasting calendar. See this video for tips on setting up your fasting plan.

Finding a plan that works for you

Intermittent fasting taps into your body’s natural ability to shift between fuel sources. From burning glucose in the early stages to utilizing fat and ketones, your body undergoes significant changes that may positively impact your health. As with any dietary approach, listening to your body and finding a fasting window that works for you is important in succeeding with intermittent fasting.

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Weight Loss->Diets