What Happens to the Body When we fast

Fasting is associated with many health benefits, this article will break down what happens to the body when we fast and why a lot of these changes are good for us.

The human body is actually really good at adapting to short and even long periods of time with very little or no food. If we take a look back through the history of human existence, we will see that we are actually living in a very short period of abundance with easy access to food.

During the earlier stages of human development, fasting was much more common. It was a consequence of seasonal changes, climate changes, droughts, inability to preserve food successfully, and many other factors.

Our body’s ability to store fat as a reserve for tough times is a consequence of our evolution. That is why, during fasting time, the body is able to reach for the stored fat and use it as an energy source.

What do we use for energy when we do not fast? The answer is glucose. Glucose is our primary energy source. The body prefers glucose because it is easily accessible and simple to metabolize. We get glucose from carbohydrates, normally the most abundant constituent of most foods.  The problem appears when we consume too many carbs. When that happens, the body stores the excess glucose as glycogen in the liver and, ultimately, as fat.

Most of the time we are utilizing glucose (sugar) from our diets and glucose stores in the body for energy. It is not until sugar is both removed from the diet and the stored sugar is fully burned off that the body reaches for fat for energy.

Step by Step Effect of Fasting

Step 1. The Beginning

Physiologically, the fasting period starts when all the food from the previous meal is digested and absorbed. We wake up every morning in the fasting state.

Step 2. Spending the Glycogen

After all the glucose from the last meal we ate has been used up, the body will reach for the glucose reserve that has been stored in the sugar cells. In other words, before the body reaches for fat, it will exhaust the stored glucose also. This usually takes 6 to 24 hours after all the glucose is gone.

Step 3. Gluconeogenesis

Once the body is out of glycogen (stored glucose), the liver starts to produce new glucose. This process is known as gluconeogenesis. New glucose is made from amino acids. This requires the use of excess protein, which is sourced out from various tissues including the muscles. However, the process does not last long enough to cause a significant loss of muscle mass.

Step 4. Ketosis

Approximately, after two or three days the use of fat for energy begins. At this point the levels of insulin, a hormone necessary for the metabolism of glucose, are very low. These low insulin levels trigger lipolysis, the process of breaking down lipids (fat) for energy.

Fatty acids, the products of lipolysis, are a good source of energy for all tissues except the brain. In order to fuel brain activity, fatty acids are broken down further into ketone bodies in a process called ketosis. These ketone bodies are successfully delivered by the blood to the brain tissue, maintaining its normal function.

From this point on, fasting makes a full effect and normal functioning of the body is maintained until all fat reserves are spent.

Fasting and Hormone Levels

Fasting also influences the levels of certain hormones in our body. However, our knowledge about the effects of fasting shows that these hormonal changes are mostly positive and health beneficial.


As it was explained before, insulin has an important role in glucose metabolism. Our body needs higher levels of insulin in order to use glucose as an energy source. Since most foods contain carbs (a source of glucose) insulin spikes are common after every meal in healthy, non-diabetic, individuals.

Fasting, on the other hand, maintains insulin at a low but healthy level. Some studies even show that fasting increases insulin sensitivity. Low insulin levels also help the body to excrete the excess water and salt. This is the reason why all low-carb diets result in a great average weight loss during the first week.

Somatotropin (Human Growth Hormone)

One of the main causes of age-related loss of muscle mass and bone density is the decreased secretion of somatotropin. This is a normal process, however, fasting seems to be able to stimulate the production of this hormone.

The reason why humans tend to have more muscles and less fat at a younger age is that human growth hormone has the ability to increase the metabolic use of fats as an energy source. During fasting periods the use of fat for fuel is promoted and the secretion of somatotropin is increased which helps the preservation of muscle mass.


Human body recognizes fasting as a period of low food availability. That is why before scientists feared that fasting can cause the metabolism to slow down significantly. Nowadays, it is known that this is not true.

In reality, the lack of food causes the body to increase the production of noradrenaline, a hormone known for its energy-boosting properties. Higher levels of noradrenaline actually speed-up the metabolism. One theory is that this happens because extra energy is needed to search for food more efficiently.

Electrolytes and Fasting

Fasting equals low food intake. Very often it also means limited food diversity. Both of these factors contribute to a common concern related to fasting – malnutrition.

For a long time, the prevailing opinion in the scientific community was that fasting inevitably leads to a deficiency of essential micronutrients such as:

·         Vitamins

·         Calcium

·         Magnesium

·         Potassium

·         Phosphorus

Recent studies have shown that this is not the case and that the levels of all micronutrients, except vitamins, remain stable even after long periods of fasting. However, high levels of uric acid as a consequence of a protein-based diet are still a concern. Taking a multivitamin while fasting would be a good idea as you will be losing many of the fat soluble vitamins you had stored.

Health Benefits of Fasting

Fasting cycles, such as intermittent fasting, have many scientifically proven health benefits. These benefits are a consequence of a change in the way our body metabolizes food. The metabolic effects of intermittent fasting are best observed through weight loss and reduced insulin resistance which can result in the improvement of certain cardiovascular conditions and diabetes.

Reduced Insulin Resistance

Insulin regulates blood sugar levels in healthy individuals. On a common diet, insulin levels increase in order to regulate blood sugar spikes after eating. Fasting helps blood sugar levels to remain more stable and therefore decreases insulin resistance.

Decreased insulin resistance makes the body more sensitive to the effects of this hormone. This is a positive thing because it enables glucose to be turned into energy and transported to the tissues without big changes in blood sugar levels.

A decreased amplitude of blood sugar levels (fewer spikes and crashes) greatly lowers the risk of diabetes.

Weight Loss

Decreased food intake equals a decrease in calorie intake. This is the main logic behind the fasting-induced weight loss. However, fasting also boosts the metabolism and promotes the use of deposited fat for energy while preserving muscle mass.

Increased Human Growth Hormone Secretion

Fasting stimulates the secretion of the human growth hormone (HGH). This hormone is important for the proper development of the body and the overall health. It has an important role in cell reproduction and regeneration. The increased levels of HGH help muscle tissue maintenance which is very important for the success of every weight loss program.

Improved Cholesterol Levels

The use of fat for energy helps to lower the levels of triglycerides and LDL cholesterol (“bad cholesterol”) in the body. In this way, fasting helps to lower the risk of diabetes, hypertension, and heart-related diseases.

Brain Function Boost

Intermittent fasting helps the process of nerve cells generation. This results in an improved cognitive ability (brain function boost). It also slows down neurodegeneration (aging of the brain). Because of its benefits for brain health, fasting is often used as a part of the treatment for certain types of epileptic seizures.

The potentially positive effects of fasting on the damage caused by Alzheimer’s and Parkinson’s disease are a subject of scientific research.

Types of Fasting

Fasting is a common name for many different kinds of eating-related restrictions. The rules of various types of fasting can also differ significantly.

Religious Fasting

The oldest fasting practices are derived from various religious rules and teachings. Almost every major religion has some kind of a fasting practice related to it.

Muslims, for example, are not allowed to eat or drink anything from sunrise to sunset during the month of Ramadan. Eastern Orthodox churches impose a forty-day fasting period, before Christmas and Easter, during which no foods of animal origin are allowed. There are many more similar examples.

Medical Fasting

Fasting for medical reasons is very common. It usually involves no food at all and only minimal amounts of water. Fasting may be required before certain medical tests, diagnostic procedures, or surgery.

·         Blood tests require fasting the night and morning before testing. The purpose is to obtain the most accurate result. Testing blood sugar levels after eating, for example, will surely result in higher values.

·         Colonoscopy is a diagnostic procedure which involves the insertion of a small camera inside the colon, through the anus. It is easy to understand why such a procedure requires a period of fasting. Aside from fasting, colonoscopy usually requires a laxative to completely empty the bowels before the procedure.

·         Gastroscopy requires an empty stomach to enable for a better view and lower the risk of vomiting.

·         Surgery (general anesthetic). Fasting is always practiced before a planned surgery under general anesthesia. Empty bowel reduces the risk of involuntary contamination of a surgery room by feces as well as the risk of vomiting during oral intubation.

Fasting for medical reasons usually lasts a short period of time. Religious fasting can last longer but it is almost never aimed at solely providing health benefits. For these reasons, this article will focus more on the modern trend of, lifestyle-changing and weight loss related, fasting. Surely, the most popular of these nowadays is intermittent fasting.

Intermittent Fasting

Intermittent fasting is experiencing a real surge in popularity lately. It is recommended by many as not only a method to lose weight, but as a good way to permanently change your lifestyle and improve your health.

Intermittent fasting is an eating pattern that allows for time periods of eating but also imposes strict periods of not eating at all. These time periods are either divided as hours during one day or days during a week.

There are several popular styles of intermittent fasting:

16/8 – is a fasting style that divides the day into two cycles. A shorter cycle last 8 hours. During this time you are allowed to consume all your daily calories. For example, this cycle can last from the time you wake up until eight hours later, let’s say 7 am to 3 pm. After this comes the longer cycle that lasts 16 hours and involves sleeping as well. During this period nothing except water is allowed. 16/8 intermittent fasting is usually practiced every day.

5/2 – these numbers relate to days of the week. Basically, this fasting method allows normal eating five days a week and very low caloric intake (no more than 600 calories) during the remaining two days.

Eat-Stop-Eat – is the third type of intermittent fasting. It is possibly the most extreme one. The method involves the so-called “eating days” and “non-eating days”. Every day (24 hours) of normal eating is followed by a day (24 hours) of not eating at all (except water).

All of the methods are mostly designed with weight loss in mind. Needless to say, none of them will work if the caloric intake during eating periods compensates for, or exceeds the energy needs during the non-eating cycles. 16/8 method is the least restrictive, simplest, and the most popular type of intermittent fasting.

Risks Associated With Fasting

Fasting is also associated with certain and more so if some underlying conditions are present or the person that is fasting belongs to any of the high-risk groups, such as:

·         Children

·         Pregnant and breastfeeding women

·         Diabetics and people with blood sugar regulating problems

·         People with hypotension (low blood pressure)

·         Underweight people

Some risks of fasting that can affect healthy individuals:

Hunger is the most obvious and the most common risk related to fasting. Luckily, for most people hunger only last for the first week or so. After this period the body gets accustomed to the reduced quantity of food and switches the primary energy source from glucose to fat.

Binge eating is another risk closely related to hunger. This urge requires mental strength to resist or it can easily turn your attempt at fasting into weight gain.

Weakness, headaches, and fatigue along with the feeling of reduced brain function are common but temporary. These symptoms usually last only for the first 2 or 3 days.

Dehydration is one of the more severe risks related to fasting, but also the one that can easily be avoided since the lack of liquids from foods can be replaced with drinking more water.

Low blood sugarBlood sugar levels should be monitored regularly during the period of fasting. Although, fasting normally has a positive effect on insulin resistance in cases where some underlying problem exists fasting can cause blood sugar to drop to dangerous levels.

The Final Word

Fasting is a well-known method for losing weight. Even from the mathematical point of view, it is easy to understand why a low-calorie intake leads to weight loss. However, fasting seems to have a deeper and important influence on metabolic processes that take place inside the human body. Currently, this influence seems to bring more benefits than risks and because of that potential fasting should remain a subject of medical research in the future.

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