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Fasting is a concept we encounter on every corner. It is attributed with various health benefits, ranging from weight loss to “cleansing” the body. Whether it is intermittent fasting or total fasting, its impact on the body is not negligible. The second common term is caloric deficit, which is a prerequisite for effective weight loss. What is the difference between the two and what do they have in common? Is fasting better for weight loss? Does the body cleanse even during a calorie deficit? That’s what you will find out in this article.
What is fasting and why does it make sense biologically?
Fasting is known simply as fasting or intermittent fasting. It is a purposeful restriction of food intake for health, aesthetic or religious reasons. During fasting, processes are triggered in the body by which the body adapts to the reduced food intake. The very first adaptation of the body to prolonged fasting is the use of a secondary source of energy – fats.
The internet is full of different ways a person can incorporate fasting into everyday life. The most common and most convenient is to use the 16:8 system, which translates to 16 hours of fasting and 8 hours of eating. Regardless of the system chosen, the body’s response is virtually the same:
| System | Setting | Category | Characteristics |
|---|---|---|---|
| 12/12 | 12h fasting / 12h eating | time-limited eating | The mildest form, usually just an early dinner and breakfast only in the morning. |
| 16/8 | 16h fasting / 8h eating | time-limited eating | Most popular IF method, eating window e.g. 12 – 20 h. |
| Warrior diet | 20h fasting / 4h eating | extremely time-limited eating | Inspiration from ancient warriors, a big meal in the evening. |
| OMAD (One Meal A Day) | 1 meal per day | extremely time-limited eating | Full daily intake in one sitting, requires precise nutrient planning. |
| Alternate-day fasting | fasting every other day | alternation of days | A full-day fast every other day. |
| 5:2 diet | 5 days of eating, 2 days of fasting | weekly cycle | The days of fasting do not have to be consecutive. |
| Fasting for several days | 5 days and more | continuous | Risk of nutrient deficiency, preferably under the supervision of a physician. |
We are evolutionarily adapted to store excess energy in glycogen and fat stores, which we can use when needed. If we don’t use them and take in more and more energy, our stores will grow, which means we will start gaining weight. It follows that starvation is natural for the human body and we have all the tools to cope with temporary restriction of food intake.
Autophagy or self-eating
Another aspect of fasting is autophagy. It is a process in which the body “eats” itself (from the Greek autóphagos – self-eating). In fact, it is a set of processes by which the body recycles old and damaged cells and uses them to create new structures. Autophagy occurs naturally virtually all the time. However, restricting food intake forces the body to recycle old cells at a higher rate [1].
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Where does the body get its energy from during fasting?
The primary source of energy in cells is glucose. However, since the body is highly complex, it also uses fats that are in circulation at the same time. As long as we take in enough food, the body has no reason to reach into glycogen or fat stores unless we have a high energy expenditure, as in the case of some sports. However, when the available energy sources run out, the body starts reaching into stores. During intermittent fasting, which lasts only a few hours, the body doesn’t get to the point where it has used up all the carbohydrates and has to start burning only fats. So intermittent fasting does not lead to ketosis unless it is coupled with a ketodiet [2].
Prolonged fasting (3 days or more) leads to depletion of carbohydrate stores, which forces the body to use the second most efficient source of energy – fat. Without carbohydrates, ketosis is triggered in the liver, in which fats are converted to ketones/ketolates. These can be used immediately to create energy [2].
Ketosis acts on the body as a stressor. It triggers a stress response, but this response is adaptive – a moderate dose of oxidative stress will kick-start antioxidant mechanisms in the mitochondria. Thus, ketosis has the potential to improve the body’s response to oxidative stress, reduce inflammation caused by poor lifestyle choices and improve mitochondrial health [3 – 4].
The body can function quite well without glucose. However, one organ is selfish and forces the liver to make glucose specifically for it. That organ is the brain, which requires about 150g of glucose a day. The process by which glucose is produced from available substances such as lactate, pyruvate or amino acids is called gluconeogenesis [5 – 7].
Does calorie deficit have the same benefits as fasting?
What is the difference between fasting and calorie deficit? By definition, a calorie deficit means that we will take in fewer calories than we expend. It is therefore the only effective way of weight loss. With a calorie deficit, we don’t have to skip meals, the important thing is not to exceed the total amount of calories. Unlike calorie deficit, fasting does not automatically lead to weight loss. If we take in more total calories than our deficit when intermittent fasting, our weight will not change. Weight loss does not depend on whether we eat every day, every other day, or in timed windows. If we don’t stay in a caloric deficit as a result, we won’t lose weight.
In terms of health benefits, such as autophagy, there is no noticeable difference between caloric deficit and fasting. Autophagy, the body’s natural process of cell recycling and renewal, is triggered at a higher rate when the body is not receiving enough energy. The same effect as intermittent fasting has a continuous calorie deficit [8].
Just like fasting, a caloric deficit improves metabolic health. Unlike fasting, however, calorie deficit has one big advantage – if set up correctly, it has no risk of overeating. This is what happens with fasting, when we start eating again after a fast and eat too much, thus going over the calorie deficit.
Thus, from a health and practical perspective, caloric deficit appears to be a more sustainable approach to improving health and weight loss.
Fasting and its risks
- After prolonged starvation, there is a risk of compensatory overeating. It is a mechanism that tries to keep us alive after prolonged starvation, so we start eating more to replenish our stores.
- In the case of prolonged fasting, when we don’t consume protein, there is a risk of losing muscle mass. This risk is minimised in the case of a caloric deficit with sufficient protein intake.
- With extreme starvation and excessive cellular stress, prolonged activation of autophagy can progress to apoptosis – cell death. Instead of efficiently recycling old and damaged cells, we start to lose healthy ones as well. But this is a pathological condition, not a normal consequence of fasting.
- Prolonged starvation can lead to mineral and vitamin deficiencies, as well as hormonal imbalances.
Conclusion
Fasting, whether intermittent or long-term, has its justification. For some people, it’s a more convenient way to reduce the amount of calories they take in and lose weight effectively. It also finds its application in dealing with health problems that result from improper lifestyle choices and overeating. However, in terms of weight loss, even when fasting, a calorie deficit must be maintained.
Often praised health benefits, such as autophagy or the antioxidant potential of ketosis, are achievable even without extreme fasts of several days. A properly adjusted calorie deficit or ketodiet, in which we minimize the intake of carbohydrates and thus force the body to use primary fats, can also do it. What is important is long-term care of your body and not extreme short-term solutions. Feeling hungry is more natural than overeating, but it can be done without extremes.
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[5] Goyal MS, Raichle ME. Glucose Requirements of the Developing Human Brain.. doi: 10.1097/MPG.0000000000001875.
[6] Kuzawa CW, Chugani HT, Grossman LI, Lipovich L, Muzik O, Hof PR, Wildman DE, Sherwood CC, Leonard WR, Lange N. Metabolic costs and evolutionary implications of human brain development. doi: 10.1073/pnas.1323099111
[7] Chourpiliadis C, Mohiuddin SS. Biochemistry, Gluconeogenesis. – https://www.ncbi.nlm.nih.gov/books/NBK544346/
[8] Bagherniya M, Butler AE, Barreto GE, Sahebkar A. The effect of fasting or calorie restriction on autophagy induction: A review of the literature. doi: 10.1016/j.arr.2018.08.004.
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