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The influence of fasting on the physiology and hormonal balance of the organism

To fully understand the meaning of fasting, it is necessary to understand the physiological processes occurring in the body when we are not eating.


Glucose and fat are the body’s primary energy sources. Glucose is the most readily available, but when it is lacking, the body can switch to fat metabolism without any harm.

It is a completely natural condition. Throughout human history, there have been times when food was less readily available. The possibility of adapting to this situation appeared as a result of evolution in the Paleolithic period. The transition from satiated to fasting occurs in several stages:

  1. Eating stage – while eating, glucose levels rise, which is transferred to the muscles and the brain as a direct source of energy. Excess glucose is stored in the liver as glycogen.

  2. Post-absorption stage – 6-24 hours after starting the fast. Insulin levels begin to drop. Glucose begins to be produced from the breakdown of glycogen, which stores are only available for 24 hours.

  3. Gluconeogenesis – 24 hours to 2 days. The liver makes glucose from amino acids by gluconeogenesis, which literally means “making new glucose.” In non-diabetics, glucose levels drop but remain within the normal range.

  4. Ketosis – 2-3 days after starting fasting. Low insulin levels achieved during fasting stimulate lipolysis, the breakdown of fat for energy. Triglycerides, the form in which fat is stored, are broken down into glycerol and 3 chains of fatty acids. Glycerol is used in gluconeogenesis, while fatty acids can be used directly for energy by most tissues in the body. However, the brain cannot use them. It is powered by ketone bodies produced from fatty acids that can cross the blood-brain barrier. After 4 days of fasting, ketones make up about 75% of the energy used by the brain. The two main types of ketones produced are acetoacetic and β-hydroxybutyric acids.

  5. Protein preservation stage – 5 days. The high level of growth hormone allows the muscle and fat tissue to be preserved. Energy for metabolism is provided by fatty acids and ketones. A higher level of norepinephrine prevents the metabolic rate from slowing down.

It can be seen that in gluconeogenesis (step 3), protein is used to produce glucose. Some people misinterpret this and believe that the body “burns muscle”. During this period, excess protein from connective tissue and other old cells that can be removed from the body is burned. This is a process called autophagy.

The human body has developed mechanisms to cope with times when food is not available. The process of switching from burning glucose to burning fat then takes place. Fat is simply a stored energy reserve. So the body does not need to “burn muscle” to meet its needs until it has used up all its fat reserves.

If this were true, one could say that the human body is not very well designed since it stores energy in the form of glucose and fat, but starts to burn protein when glucose from food is not available. It’s like stockpiling coal for the winter and then chopping up the sofa for fuel. You could imagine our ancestors from thousands of years ago as living fat balls, because their fasting periods were particularly frequent and they would have to burn muscle, right? Of course, this was not the case.


Intermittent fasting is the best method to lower your insulin levels. This was noticed decades ago. This is understandable because food raises this level, so the most effective method of lowering it is to avoid eating. This effect already occurs with fasting periods ranging from 24 to 36 hours. Intermittent fasting has recently been recognized in research as an acceptable method of lowering insulin levels.

Regular fasting, in addition to lowering insulin levels, also improves the sensitivity of tissues to its effects. Most diets reduce the foods that cause increased insulin production but do not address the issue of insulin resistance. This initially leads to a temporary weight loss, but insulin levels remain high, preventing long-term weight loss.

Lowering insulin levels causes your body to lose water and salt. Insulin keeps them in the kidneys. The Atkins diet and similar diets increase diuresis, expel excess water, which may lead to the conclusion that this is the cause of weight loss. Increased diuresis helps reduce gas and gives a feeling of lightness. Some people also experience a slight reduction in blood pressure. Fasting initially leads to rapid weight loss, with an average of 0.9 kg per day for the first 5 days. This is a decrease, the scale of which does not allow this effect to be attributed to the reduction of calories or the depletion of water from the body.

Growth hormone

Growth hormone increases the availability of fat for energy. It also helps to maintain muscle mass and bone density at a proper level. Its production decreases with age. Fasting is one of the best methods of stimulating the production of growth hormone – five-day fasting causes more than a twofold increase in its production rate.

It is the increase in the production of growth hormone that closes the cell cycle. Research on intermittent fasting shows that using it gives four times better results in maintaining muscle mass compared to reducing calories.


Fears of malnutrition during fasting are unfounded. Not having the right amount of calories provided by the food does not harm you, because the stored fat is completely sufficient. The greatest doubts concern the shortage of micronutrients. However, research on fasting has shown that even without supplements, potassium levels have not dropped below 3.0 mEq/L, and levels of magnesium, calcium, and phosphorus remain stable. This is probably due to the abundance of these minerals in the bones. 99% of the calcium and phosphorus in the human body is stored there.


The level of norepinephrine increases during fasting to give the body the energy to obtain food. 2 fasting days increases the metabolic rate by 3.6%. 4 days of fasting increased resting energy consumption by 14%.

Interestingly, it’s fasting, and not low-calorie diets, that results in hormonal changes that appear to be very beneficial for the body. Fasting causes you to switch from burning sugar to burning fat. Metabolism does not slow down, it speeds up. During fasting, we feed on our own fat. Fat, basically, is processed and stored food by the body. Research shows that the fat burning caused by an increase in epinephrine levels is not dependent on a reduction in sugar levels.

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