Last Updated on November 3, 2023
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Did you know that there are 5 stages of fasting that your body goes through, even though it looks very quiet from the outside? The fat-burning stage is by far the most curious stage for intermittent fasting enthusiasts. That’s why “Does intermittent fasting burn fat? How does it work? Is it good for burning belly fat? How long does it take for your body to start burning stored fat?” are just some of the many frequently asked questions about intermittent fasting.
Long story short, yes, intermittent fasting does burn fat. If you’re curious about how it happens, you’ll love our deep dive into the details of the fasting stages hour by hour, along with the long-waited fat-burning stage. 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.
- 1 What is intermittent fasting?
- 2 What are the stages of fasting by hour?
- 3 When does the body start burning fat during fasting?
- 4 Does intermittent fasting burn belly fat?
- 5 What is the best intermittent fasting window to lose belly fat?
- 6 To sum up
- 7 Lose weight with fasting
What is intermittent fasting?
There are two different periods of intermittent fasting:
- The eating period, during which you eat your meals.
- The fasting period during which you don’t eat anything but can only consume water, and zero-calorie beverages.
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. During this period, the human body is primed for growth and repair, effectively making use of the ingested nutrients to rebuild and rejuvenate tissues.
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, as it’s sometimes known, is critical and its duration can be influenced by factors including the composition of the meal and your metabolic rate.
For instance, a meal rich in complex carbohydrates and proteins might extend the digestive process slightly longer than one composed predominantly of simple sugars that are 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 phase 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.
This process is a subtle yet sophisticated recalibration of the body’s energy management, pivoting from external food sources to its internal reserves. 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)
At some point around the 12th hour of not eating anything, the glycogen stores deplete and the body starts looking for a new energy source. This is when the stored fat gets caught on your body’s radar. Burnt fat becomes its fuel.
Transitioning into the fat-burning stage, usually occurring around the 12-hour mark post-meal, marks a significant metabolic shift. By this time, the readily accessible glycogen stores are nearing depletion, prompting 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)
As your body runs out of stored excess body fat, it keeps looking for more sources of energy again and begins transforming fatty acids into small particles, called ketones or ketone bodies around the 18th hour of the fast. This is when you enter the ketosis stage.
When the fast extends beyond 18 hours, your body’s metabolic state continues to evolve, entering a phase known as ketosis. Having exhausted the more easily accessible glycogen stores, the body intensifies its reliance on fat as a primary energy source.
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 many low-carbohydrate diets 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. It’s important to note that such an extended fast is not universally appropriate and should only be undertaken with professional medical guidance.
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, in order to regenerate newer, healthier cells.
This period can also trigger the production of new stem cells and rejuvenate the immune system, potentially offering a reset to one’s immune system. Due to the intensity of this fasting stage, it is imperative to consult with your healthcare provider to ensure that you are appropriately supported in your fasting journey and that you mitigate any potential health risks.
When does the body start burning fat during fasting?
Burning excess body fat and losing weight for good are very common fitness and wellness goals. To achieve these goals, intermittent fasting is an easy tool. Here’s how and when it happens:
Your body enters the long-awaited fat-burning phase when it runs out of its glycogen stores. This starts approximately the 12th hour of your fast. The fat-burning stage continues until the 18th hour. This is especially important for people who are trying to lose weight without losing muscle mass.
After the 18th hour of the fast, the fat-burning phase continues on a more cellular level as the body consumes ketone bodies for energy.
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.
The most important thing to be cautious about is that although fasting can help you to a certain extent, burning the fat in specific areas of your body requires a bit more effort.
Doing cardio exercises regularly and building healthy eating habits are essential for losing belly fat.
Here are the top things you can do to burn belly fat:
Whether it’s at-home cardio, swimming, cycling, or bodyweight workouts, your muscles need to work to burn fat. You can get the 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 too.
You should consume 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.
However, when combined with healthy lifestyle choices, intermittent fasting can create big changes in your body.
Fasting Kompanion enables you to visualize your fasting stages, track your fasting progress, and keep you informed with notifications and blog articles.