What Really Happens to Our Bodies During Intermittent Fasting?


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Choosing healthy foods, properly chewing, and maintaining good bowel function, minimize the need for prolonged fasting. However, when the balance of the body is lost, we can resort to the oldest method which is an instinctive treatment for many diseases and a tool for the prevention of chronic diseases – periodic fasting.

Periodic fasting – also known as “intermittent fasting” – can have many benefits for your body. From weight management and cellular cleansing to supporting digestion and mental clarity, intermittent fasting can offer a number of benefits.

But have you ever wondered, what happens to your body, every hour that passes, during fasting?

Let’s take a look at the different stages our body is entering so that we can understand the benefits and risks we may face in each of them.

0 to 4 hours

The first four hours after eating is known as the phase of anabolic development. Your body uses the energy it just ate to fuel current activity and tissue cell growth. The pancreas produces the hormone insulin. This allows your body to use the glucose released into the blood, and store any excess energy in your cells for future use.

4 to 16 hours

The second phase starts after four hours and lasts up to about 16 hours from the last meal. This is the catabolic phase when all these stored nutrients begin to release to be used for energy.

Once the energy stored in your cells is finished, your body begins to rely on stored fat. The process of releasing fat and burning it up for energy is called ketosis. It releases chemicals known as ketones which are energy carriers. This usually occurs around the 16-hour mark.

The rate at which you reach this stage really depends on what you have consumed in the last two meals. For example, if you have taken on a lot of carbohydrates and starch, it will take a little longer than it would if the fat and protein content were higher.

One of the most powerful features of fasting, called autophagy (self-eating), also begins during this phase. Autophagy is a form of purification of useless substances within cells, which are responsible for premature aging, the appearance of chronic diseases, and cancer.

16 to 24 hours

Once you exceed the 16-hour limit, the glucose in the cells and glycogen in the liver and muscles begin to decrease rapidly, which causes you to burn stored fat in order to meet your energy needs. Your energy requirements at this stage probably haven’t changed much, so the amount of energy needed is quite significant.

In addition, the production of another chemical, energy from AMP-activated Protein Kinase, is activated at this point, which further increases the process of autophagy.

plate with a flower

24 to 72 hours

When you complete 24 hours of fasting, your body will be put in a state called ketosis, where energy production depends on burning stored fat. As fat cells break down for energy, ketone bodies are created and released into the bloodstream.

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Ketone bodies, in the absence of glucose, act as fuel for the brain. Glucose is our brain’s main fuel source, but ketone bodies provide the brain with some additional benefits.

Therefore, it is possible to feel an increase in your mental clarity and energy in a multi-day fast.

Around 24 hours also increases, the production of the brain hormone BDNF (Brain-derived neurotrophic factor) that supports the development of neurons in the brain. Brain hormone is associated with improving long-term memory, coordination, and learning. In addition, it is believed to be the key to reducing the likelihood of developing Alzheimer’s disease in later life.

72+ hours

When you complete three days of fasting or more, your body enters a deep state of ketosis. All previous benefits: autophagy, fat loss, and mental clarity continue to grow. However, thyroid hormones will probably start to get affected at this point, as your body thinks you’re starving.

Research shows that prolonged fasting can increase your resistance to stress and exposure to toxins and that certain hormones produced at this stage may have beneficial anti-cancer and anti-aging effects.

When You Should Not Fast

You are not allowed to enter the fasting process if you are pregnant or breast-feeding, have heart failure, or a weak immune system. In addition, it is recommended in people in preoperative or postoperative stages, cancer patients, or those suffering from eating disorders.

Probably, for those who have higher energy during the evening hours and during the day feel falling, the process of prolonged fasting is not tolerated.

In any case, prolonged fasting should only be carried out under the supervision of a specialist.


Intermittent fasting can be the tool to improve your health and contribute to longevity when it is done under the right guidance. By understanding these stages, you can choose the right type and duration of fasting for your body and your goals.

Featured image by ededchechine – www.freepik.com

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