How Does Intermittent Fasting Burn Fat

  • Post last modified:August 17, 2021
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You may be new to intermittent fasting and wondering when the fat burning starts.

The difference between intermittent fasting and any kind of diet is that intermittent fasting mainly restricts your eating time, rather than deliberately controlling calorie intake.

As your fasting time increases, your body will enter a state of nutritional ketosis and start autophagy. These are beneficial to the overall health of your body, including:

  • weight loss, reduction of inflammation, prevention of chronic diseases, improvement of brain function and may help to delay aging.

There are various fasting methods. The most common ones are the following:

  • The 5:2 method
  • The eat-stop-eat method
  • The 16/8 method
  • The 20/4 method
  • The one meal a day method

How Long Does It Take For Intermittent Fasting to Work

To answer this question, first, you need to understand what happens to your body during fasting.

Please take a look at the picture below:

How Does Intermittent Fasting Burn Fat

These are the changes in various physiological levels of the human body at different times during 24 hours of fasting.

Let’s analyze the information in detail.

1. Blood sugar (the orange line)

About an hour after a meal, blood sugar will start to drop and then it will remain at a low level.

Studies of obese people proved that during fasting, people with low blood sugar levels had greater weight loss results than those with high blood sugar levels.

Other studies have also proven that normal blood sugar levels can protect the health of the brain, especially being able to relieve brain atrophy in the elderly.

2. Glycogen (the solid black line)

Glycogen is the sugar stored in the liver and muscle cells. The carbohydrates you consume are converted into glucose to provide energy for your body.

As can be seen from the figure above, glycogen levels will rise rapidly after a meal, and then gradually decrease with energy consumption.

Finally, when the foreign glycogen is used up, your body begins to clear the inventory (consumption of glycogen stored in the body).

3. Glucagon (the green dotted line)

Glucagon and insulin are both hormones secreted by the pancreas and both play an important role in regulating blood sugar balance.

During fasting, when your blood sugar drops, the pancreas secretes glucagon and releases glycogen stored in the liver and muscle cells.

The longer the fasting time, the more glucagon is released and the more glycogen stored in the body is consumed.

As early as 1962, Christian de Duve, a cytologist, biochemist, and Nobel Prize winner, discovered in his research that an increase in glucagon may greatly stimulate autophagy.


Image source

Autophagy is a process by which cells clean up old parts and replace them.

In addition to resisting aging and prolonging lifespan, the benefits of autophagy include improving cell mitochondrial efficiency, reducing inflammation, improving immunity, fighting infection, preventing and alleviating degenerative diseases (such as Alzheimer’s disease) and inhibiting tumor cells.[*]

4. Insulin (the purple dotted line)

We know that the role of insulin is to control blood sugar. As you can see from the figure above, blood sugar and insulin are almost parallel.

As long as you don’t eat, this trend will continue until you eat again.

Maintaining a low level of insulin is good for health because:

  • First, it promotes the minimization of glycogen storage in the body and ultimately converts the body from sugar metabolism to fat metabolism, thereby achieving the goal of weight loss.
  • Secondly, low insulin levels can reduce inflammation in your body. Inflammation is the culprit leading to many chronic diseases including high blood pressure, heart disease and diabetes.

5. Free fatty acids (the black dotted line in the) 

From the figure above, when glycogen is consumed, the level of free fatty acids starts to climb rapidly. This is because the body begins to decompose and consume the fat stored in the body.

When lipolysis occurs, all parts of the body will be affected, such as the waist, stomach, face, thighs and internal organs. The places where fat is usually most likely to accumulate began to change quietly reducing fat.

6. Blood ketones (the blue dotted line)

The role of blood ketones is to help your body accelerate fat burning. Therefore, when the concentration of free fatty acids reaches its highest value, the level of ketone bodies in the body also begins to soar. After fasting for 16 hours, this effect is more obvious.

To sum up, when fasting for between 12-16 hours, your body will experience a series of physiological changes. By the 16th hour, most of the physiological indicators will stabilize. The real fat burning begins at exactly 16 hours.


How long it takes for intermittent fasting to work depends on the level of physiological indicators such as blood sugar, insulin, glucagon and blood ketones.

Studies have found that after 16 hours of fasting, the body begins to burn fat.

Now that you have an understanding of the changes in your body during fasting, could it be easier to schedule a fast?

If you want to try intermittent fasting, check out How To Do Intermittent Fasting Correctly

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