Stages of Fasting by Hour and Fat Burning Stage

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When starting an intermittent fasting journey, many are curious about the specific stages of fasting and particularly when the fat-burning stage of fasting begins. Questions like “When does the intermittent fasting fat burning zone start” and “How long does it take for your body to start burning stored fat” are common among fasting enthusiasts.

This article sheds light on the stages of fasting by hour and explains when and how much fat is burned during fasting.

To take an even deeper dive into the fasting pool, take the quiz directly to create your personal fasting plan and start losing weight and/or adopting a healthier life.

What is intermittent fasting?

Intermittent fasting, a popular eating pattern, is lauded for its health benefits like weight loss, weight control, and efficient fat burning without muscle loss. It involves alternating cycles of eating and fasting, where the latter includes only water and zero-calorie beverages.

Popular plans include the 14/10, 16/8, 18/6, and 20/4 intermittent fasting schedules.

What are the stages of fasting by hour?

When you finish the last meal of your day, your fasting period begins. During the fasting period, your body goes through different stages hour by hour, depending on your fasting plan.

The five main stages of fasting are as follows:

Anabolic stage (0-4 hours after eating) 

This is your body’s growth phase. After consuming a meal, the body enters what is scientifically referred to as the anabolic phase, typically spanning the first four hours of digestion.

Enzymes are actively engaged in breaking down complex food particles into absorbable units, such as amino acids, fatty acids, and simple sugars, which are then assimilated into the body’s systems. This anabolic window can sometimes be affected by the composition of the meals and your metabolic rate.

For instance, a meal rich in complex carbohydrates and proteins might take longer to digest compared to a meal rich in simple sugars, which can be quickly absorbed.

Catabolic stage (4-12 hours after eating) 

Once the body has completed the task of digestion and absorption, which usually concludes around the four-hour mark, it transitions into the catabolic stage.

This stage of fasting is characterized by a shift in internal fuel sources, as the body must now turn to stored energy to power its functions. Glycogen, the stored form of glucose found in the liver and muscles, is the body’s go-to reserve.

During this 4 to 12-hour timeframe, the liver breaks down glycogen into glucose, releasing it into the bloodstream to maintain stable blood sugar levels and energy supply.

During this process, your body uses its internal reserves instead of external food sources. This phase highlights the body’s remarkable adaptability in its quest to sustain physiological operations and homeostasis in the absence of immediate food intake.

Fat-burning stage (12-18 hours after eating)

Around the 12th hour, as glycogen depletes, the body starts the fat burning stage of fasting. This is when signs your body is in fat-burning mode become evident, a critical period for those targeting weight loss.

The depletion of readily accessible glycogen stores prompts the body to look for alternative energy reserves. Adipose tissue, which stores fat, becomes the new target for energy extraction.

Hormones such as norepinephrine facilitate the breakdown of these fat stores into fatty acids, which can then be used as fuel. This stage is often the goal for those practicing intermittent fasting for weight loss, as the body begins to consume its fat reserves, potentially leading to reduced body fat percentages.

However, the efficiency and rate at which your body enters this phase can vary and is influenced by factors like your metabolic rate, physical activity level, and overall body composition.

Ketosis stage (18-72 hours after eating)

Having exhausted the more easily accessible glycogen stores, the body intensifies its reliance on fat as a primary energy source. It begins transforming fatty acids into small particles, called ketones or ketone bodies around the 18th hour of the fast. This is when your body enters the ketosis stage.

Fatty acids are transported to the liver, where they are metabolized into ketone bodies or ketones. These ketones then serve as an efficient energy source for many cells, particularly brain cells, which can utilize them as an alternative to glucose.

Achieving ketosis is the main goal in diets like low-carb diet, ketogenic diet, and fasting protocols, as it is associated with increased fat-burning and may provide mental clarity. The journey to this stage is a testament to the body’s incredible ability to adapt to different sources of energy depending on availability.

Deep ketosis stage (72+ hours after eating)

Once the fasting period surpasses 72 hours, which ventures into the territory of prolonged fasting, your body can enter a state of deep ketosis.

In this advanced stage, your body not only continues to utilize ketones for energy but may also experience enhanced cellular repair processes called autophagy, the body’s way of cleaning out damaged cells, to regenerate newer and healthier cells.

This period can also trigger the production of new stem cells and potentially offer a reset to your immune system.

It’s important to note that, this extended fast is not appropriate for everyone and should only be applied with professional medical guidance to ensure that you eliminate any potential health risks.

When does the body start burning fat during fasting?

The intermittent fasting fat burning zone typically begins around the 12th hour of fasting. This is crucial for those aiming to lose weight while preserving muscle mass. Beyond the 18th hour, the body continues fat burning at a cellular level through ketone utilization.

If you’re following a fasting plan that has a fasting window longer than 12 hours, such as the 14/10, 16/8, 18/6, or 20/4 plans, you burn fat and begin to lose weight.

Does intermittent fasting burn belly fat?

Let’s give you the good news: Intermittent fasting does burn belly fat! People who fast indicate that their waist circumference reduced by 4–7% by the time they had fully adopted this eating pattern in their lives.

However, targeted fat loss also demands a combination of exercise, healthy eating habits, and adequate hydration.

Here are the top things you can do to burn belly fat:

1. Exercise

Whether it’s at-home cardio, swimming, cycling, or bodyweight workouts, your muscles need to work to burn fat. You can get Fitness Kompanion to get hundreds of bodyweight workouts suitable for all levels.

2. Build healthy eating habits

Eat more protein and fiber-rich foods to feed your muscles with good nutrients and to support your digestion. Eat less refined carbs and avoid junk food as well.

3. Hydrate

You should drink about 2 liters of water a day. This equates to approximately 8 glasses of water. This amount should be increased still further on your workout days. Here are some tips to increase your water intake.

4. Sleep adequately

A healthy adult needs at least 7 hours of sleep per day. Even if you think you’re satisfied with less sleep, your brain needs at least 7 hours of sleep to perform all its functions. If you’re suffering from insomnia, you may be able to combat it with a few lifestyle changes.

What is the best intermittent fasting window to lose belly fat?

As the fat-burning stage of fasting starts after 12 hours of calorie absence, the best intermittent fasting plans for belly fat burn should have more than 12 hours of fasting windows. So, if you’d like to trim your waistline and lose those extra pounds, you can choose one of these intermittent fasting plans:

• 14/10 fasting plan
• 16/8 fasting plan
• 18/6 fasting plan
• The warrior diet (20/4 fasting plan)
• The OMAD diet

However, bear in mind that fasting plans with fasting windows longer than 18 hours are considered advanced and you should not try them if you don’t have prior intermittent fasting experience.

Plus, some people report having fat-burning effects with the eat-stop-eat approach of fasting, where you pick one or two non-consecutive days of the week to fast for 24 hours. Afterward, you eat freely for 5-6 days. But this one is also not an easy fasting plan.

To sum up

Fasting is a very powerful tool for weight loss and fat burn. But you also have to remember that it’s not a magic wand and that it won’t be as effective if you continue with bad life habits such as smoking, excessive alcohol consumption, a sedentary lifestyle, and eating mostly junk food. It’s most effective when combined with healthy lifestyle choices.

Fasting Kompanion helps track your progress through different stages of fasting by hour and offers guidance on how long it takes for your body to eat its own fat and reach the stages of fasting.