VERIFIED CONTENTAuthor: Aleksandra Żyłowska-Mharrab, dietitian, food technologist, educator

Intermittent fasting is not a typical diet, but a diet in which you alternate between fasting and eating normally. Unlike most popular diets, intermittent fasting is well researched by science. Does intermittent fasting really help you lose weight? Can anyone use it? What are the types of intermittent fasting?

What is intermittent fasting?

Some people equate intermittent fasting with a diet - short-term, seasonal, leading to the achievement of the weight goal. However, it is more of a lifestyle and approach to nutrition that has not only weight but also he alth benefits. Intermittent fasting is certainly one of the strongest nutritional trends in recent years. But what exactly is intermittent fasting?

Intermittent fasting (IF) is an eating method that involves alternating periods of fasting and eating. This definition is very general as there are several different ways of fasting under the term "intermittent fasting". The duration of fasting periods, how often they occur, and the amount of calories allowed during fasting can vary. Intermittent fasting is not a diet in the common sense, as it does not indicate what you can and cannot eat, but it focuses on when to eat.

Fasting and periods of hunger are completely natural for humans. Something our organisms are evolutionarily adapted to. Humanity has never had permanent access to food. It is a boon only in the last centuries (and in some regions of the world access to it is still limited).

From the time of the nomadic lifestyle, before agriculture, when the only source of food was hunted meat and plants collected in the vicinity, people's ability to cope well with hunger remained in their genes. Or even to reap the benefits of times of hunger. Occasional fasting is more natural for humans than eating 5 times a day.

Fasting accompanies a person also for spiritual and religious reasons. They are part of the world's greatest religions - Christianity, Islam, Judaism and Buddhism.

Intermittent fasting, unlike many fashionable diets, is finedocumented by science. The popularity of this method began in animal studies in 1935, and extreme interest in it has been observed since 2012 - the date of the documentary program "Eat, Fast and Live Longer", and then the publication of the book "The Fast Diet" in 2013.

Does intermittent fasting help you lose weight?

Most people become interested in intermittent fasting as a weight loss method. According to scientific research, intermittent fasting is an effective slimming method. But no more effective than a low calorie diet. While the effectiveness of both nutrition systems for weight loss is similar, intermittent fasting has several advantages over a low calorie diet.

First, intermittent fasting doesn't involve counting calories. During eating times, you can eat to your heart's content. In research studies, participants covered their energy requirements in the range of 100 to 125% on non-Lent days. This shows that in some intermittent fasting systems, you can eat even more than your body needs. However, you should remember not to overeat if the goal of fasting is to lose weight. Intermittent fasting therefore requires a bit of listening skills to know when to stop eating.

Secondly, intermittent fasting reduces the feeling of hunger and causes a spontaneous reduction in calories in the diet. Even eating to your heart's content, but for a limited period of time, makes many people eat less and don't feel it as lacking.

Third, intermittent fasting allows you to maintain a better body composition. When losing weight with this method, fat loss is greater and muscle mass is reduced compared to classic low-calorie diets. Most often, fat loss is 75% of the weight lost, and muscle weight loss is 25%. In the case of intermittent fasting, these proportions are 90% to 10%.

Is it possible to lose weight despite the lack of caloric restrictions?

How is it possible to lose weight despite the lack of caloric restrictions while using intermittent fasting? Experts explain that incomplete compensation is responsible for this. At the time when eating is allowed, those who adhere to intermittent fasting do not fully cover the amount of energy used by the body at times when eating is prohibited. Additionally, fasting causes an acceleration of the metabolic rate.

There are also studies that show that consuming the same amount of calories only within a certain time frame allowed you to lose more weight than eating the same amount of energy in a typical 5-meal diet. Thishowever, the issue requires more scientific evidence.

It is certainly known that intermittent fasting is as effective in losing weight as classic low-calorie diets and much more effective than no changes in the diet.

How to use intermittent fasting?

Intermittent fasting comes in several variants. It is always a question of alternating periods of eating with periods of non-eating, but the number of fasting hours and the frequency of fasting may vary. Several different systems of intermittent fasting allow you to choose the most suitable for your lifestyle and the easiest to follow.

What are the most popular methods for intermittent fasting?

  • 16/8

16/8 intermittent fasting was popularized by fitness expert Martin Berkhan. The 16/8 method assumes no food for 16 hours and eating within the 8-hour window. During fasting, it is allowed to drink only calorie-free drinks: water, tea, black coffee.

During 8 hours of interruption of fasting, you can eat anything without caloric restrictions. The number of meals during this time is optional. Of course, it is not about filling yourself with food during the permitted 8 hours, but about eating to your heart's content and choosing mainly he althy products. The choice of the time frame in which meals are eaten is free and should be adapted to the lifestyle. People who do not feel like eating in the morning and often skip breakfast will find their way into the eating system from 12 to 20. Many people choose the window from 10 to 6 - eat breakfast at work and a big last meal before 6 p.m.

16/8 intermittent fasting is chosen by many people because it is not very difficult to apply. Most of the fasting time is overnight, and dinner until 8pm and skipping breakfast does not seem scary.

  • 20/4, 18/6, 14/10 or more generally - time-limited nutrition

These methods of intermittent fasting follow exactly the same principles as 16/8, but they shorten or extend the time window in which eating is allowed. Tests on websites and in applications are popular, which suggest that based on the answers provided, they will choose the perfect time window for you (your metabolism, body type and overweight) and the number of hours of fasting. However, the quality of these tests is questionable. And most of all, there is no scientific research that would show a relationship between the number of hours of fasting and its effectiveness for a specific body type.

  • 5: 2

British journalist Michael Mosley is responsible for popularizing the 5: 2 diet. Method 5: 2 fastingintermittent means 5 days a week of eating normally and 2 days of fasting. Fasting days should be separated by at least one day of habitual eating. They should not be consecutive day after day. During the 5th day of eating, there are normally no caloric and product restrictions, but you should use common sense and eat rationally, and not rely on fast food and sweets.

During the 2 days of fasting, you can eat 500-600 kcal per day. Calories can be delivered in one meal or eaten in two meals. It is allowed to drink water and other non-calorie drinks. Fasting two days in a row increases your risk of extreme hunger, weakness and irritability. People who play sports should not exercise on fasting days. Lack of enough calories, carbohydrates and protein makes training with normal intensity very difficult, and additionally, the breakdown of muscle proteins is increased.

  • Alternating post

In this system of intermittent fasting, one day is fast and the next one can eat normally. The cycle is repeated alternately - fasting day and normal day. The amount of calories consumed during a fasting day can vary. The most popular variant is that you should eat 500 calories on a fasting day, 200 of which come from protein, and drink calorie-free drinks.

On the day of normal eating, you can eat anything without caloric or restricted calories, but more than your body needs. One research study found that people who used alternating fasting (25% of their calories on a fasting day and 125% of their calories on a non-fasting day) for 6 months lost 6% of their starting body weight - the same amount as people on a low-calorie diet.

Alternating fasting seems to be the most difficult of all intermittent fasting methods due to severe dietary restrictions every other day.

  • Eat-break-eat

Intermittent fasting according to the "eat-break-eat" method assumes 24-hour fasting periods 1 or 2 times a week. You can fast from any time - from breakfast to breakfast the next day, but just as well from lunch or dinner, as long as the 24-hour fasting windows are kept. Two fasting days should not be consecutive.

"Eat-break-eat" seems to be a very similar method of intermittent fasting to 5: 2. The difference is that in 5: 2 the fasting period is longer - it begins after dinner on the first day, the entire second day is fast until breakfast on the third day. Fasting therefore takes about 36 hours, not 24 hours. DuringDuring a fasting day in the "eat-break-eat" method, you should not eat any solid food, but only drink calorie-free drinks. If you are using this method of intermittent fasting as a slimming diet, follow the eating plan on the remaining days and do not eat more than usual.

The difficulty of this method is the period of fasting without any solid food, but above all the fact that, unlike other methods of intermittent fasting, the emphasis is on limiting calories also on the other days.

Such a large number of different systems of intermittent fasting allows you to adjust the best fasting method to your abilities and expectations - working hours, resistance to hunger or physical activity.

What happens in the body during intermittent fasting?

Intermittent fasting affects not only body weight, but also he alth, and its effect is very beneficial. Intermittent fasting affects the body at the cellular and molecular level. It is known that intermittent fasting:

  • is beneficial in the treatment of insulin resistance and type II diabetes,
  • reduces fasting glucose and insulin levels,
  • decreases the concentration of total cholesterol and triglycerides in the blood,
  • reduces the level of inflammatory markers in the body,
  • can protect against dementia and neurodegenerative diseases,
  • can extend your life.

Intermittent fasting and hormone secretion

Growth hormone secretion is highly stimulated during intermittent fasting, which benefits you faster fat loss and muscle gain during resistance training. Another important hormone whose secretion is altered by intermittent fasting is insulin. As shown by a review of studies from 1990 to 2022 published inClinical Diabetes and Endocrinologyin 2022, its secretion very strongly decreases, and the sensitivity of cells to insulin increases during the use of intermittent fasting.

One analysis showed that fasting insulin levels drop in patients by up to 57% after 22 days of alternating fasting. Low levels of insulin in the blood are very important when losing weight, as it definitely facilitates the use of body fat as a source of energy. It also enables better use of glucose from food and hinders its storage.

It has been proven in numerous studies that intermittent fasting reduces the level of leptin and increases the level of adiponectin in the body. Both of these hormones play a very important role in the control of hunger and satiety as well as chronic inflammation and inflammation.low intensity.

Leptin itself is beneficial for humans - it inhibits the feeling of hunger and the accumulation of adipose tissue. However, the impaired secretion of leptin and the leptin resistance typical of obese people are very harmful. They cause unsatisfied appetite and the development of inflammation due to the secretion of IL-6, which induces the synthesis of C-reactive protein in the liver and an increase in the level of the pro-inflammatory cytokine TNF-alpha.

In contrast, adiponectin has anti-diabetic and anti-inflammatory properties. Adiponectin acts on various receptors, which results in an increase in fatty acid oxidation in the skeletal muscles and liver (better fat burning), a decrease in hepatic gluconeogenesis, and an increase in glucose uptake. It also exhibits anti-inflammatory activity by acting directly on inflammatory cells, NF-kB and interacting with TNF-alpha.

The level of adiponectin in the body decreases with the accumulation of visceral fat. Intermittent fasting reduces the feeling of hunger, allows for better use of energy from fat and glucose, and reduces inflammation, which results in a lower risk of numerous inflammatory diseases, including type 2 diabetes and atherosclerosis.

Intermittent fasting slows down the aging process

Energy and nutrient deficiencies from intermittent fasting have been shown to promote he althier aging and reduce chronic disease through increased activation of AMP-activated protein kinase (AMPK). AMPK, on ​​the other hand, initiates physiological responses that promote he althy aging.

During periods of hunger, body cells naturally activate repair processes, incl. autophagy, in which old, damaged proteins are broken down and removed, and in their place new and wholesome proteins are created. As a result of intermittent fasting, the expression of the so-called genes of youth or sirtuin. The SIRT1 and SIRT3 genes seem to be the most important. The main role of sirtuins is to slow down the shortening of telomere on the chromosomes. Telomeres can be imagined as "caps" protecting the arms of the chromosomes that carry genetic information.

Each DNA replication causes the loss of a part of the telomere, its shortening, and eventually the loss of genetic material. This results in the formation of abnormal proteins and cellular structures. It can be simplified to say that many diseases appear with age precisely as a result of the loss of telomeres that protect DNA. Sirtuins extend the time when genetic material is protected, reduce the risk of many diseases (e.g. cancer, neurodegenerative diseases) and are a factor contributing to the prolongation oflife.

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Intermittent fasting and metabolism

As a result of intermittent fasting, there are several important changes in the body that significantly affect metabolism. It is commonly believed that not eating slows down metabolism. It turns out, however, that when using intermittent fasting, the resting metabolic rate increases from 3.6 to even 14%. This is due to decreased insulin secretion and increased growth hormone and norepinephrine secretion during fasting.

It has also been noticed that intermittent fasting allows the body to "switch" from using glucose as the main source of energy for cells to an increased share of ketone bodies in providing energy for life processes. Thanks to this, the acquisition of energy from fat is promoted, i.e. fat burning.

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Who shouldn't use intermittent fasting?

Post is not for everyone. Certain physiological conditions preclude its use, or at least require consultation with a doctor. Intermittent fasting can be very beneficial and even curative for people with type II diabetes, but when using metformin and insulin, it is important to carefully monitor whether the combination of drugs and intermittent fasting does not lead to hypoglycemia. Intermittent fasting is discouraged during pregnancy and breastfeeding. It should not be used by people who are malnourished, who have a low body weight, or have an eating disorder or have a history of eating disorders.

What about sports? During alternating fasting, it is better to exercise on normal eating days, when you can get the energy and nutrients necessary for effective training. Using the 16/8 method, it's best to set the time of training so that you can eat a post-training meal after it.

What are the side effects of intermittent fasting?

For most people, the first few days of intermittent fasting are a difficult time to adapt. During the 3-5 days you can expect a lot of irritation, headache, weakness or a drop in energy. However, all these symptoms are temporary and of short duration. However, one must remember to listen to the body's signals and not to do anything by force. If we are unable to cope with the fast, stop it.

ExpertAleksandra Żyłowska-Mharrab, dietician Food technologist, dietitian, educator. A graduate of Biotechnology at the Gdańsk University of Technology and Nutritional Services at the Maritime University. A supporter of simple, he althy cuisine and conscious choices in everyday nutrition. To my main onesinterests include building permanent changes in eating habits and individual composition of the diet in accordance with the needs of the body. Because the same thing is not he althy for everyone! I believe that nutritional education is very important, both for children and adults. I focus my activities on spreading knowledge about nutrition, analyze new research results, and make my own conclusions. I adhere to the principle that a diet is a lifestyle, not strict adherence to meals on a sheet of paper. There is always room for delicious pleasures in he althy and conscious eating.

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