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It is said that the extra kilos gained over decades cannot be lost in a few weeks, so there is no need to be demotivated by progressing very slowly. However, the truth is that stored fat is governed by thermodynamics and, in principle, if you begin to consume fewer calories than you expend, you will lose weight. Despite the constant hormonal changes and regulation of metabolism, in the end what matters most is how many calories you take in and how many you burn.
Thus, theoretically, but also practically, it is possible to lose a lot of excess fat in a very short time. However, the human body has mechanisms to keep you alive, and that is why the so-called yo-yo effect occurs. All the weight you lose, you gain back very quickly. Why is this and how to avoid it? That is the topic of this article.
What is fat?
From a chemical point of view, fatty acids are triacylglycerols (TAGs), i.e. fatty acids linked to the glycerol molecule, which are found in fat cells – adipocytes. These cells are distributed virtually everywhere in the body and in terms of their location, fat can be divided into subcutaneous and visceral. [1]
- Subcutaneous is the one you see at first glance and try to get rid of first if.
- Visceral fat is found around the organs and its function is to provide mechanical protection to the organs, regulate metabolism and hormone levels. It is also a source of the substances involved in the regulation of the immune system.
Adipose tissue can also be classified according to its ability to generate heat (thermogenesis). The most common adipose tissue is white and functions as an energy store. The adipocytes of white adipose tissue do not contain a large number of mitochondria and thus are not where energy is burned. They are virtually filled entirely with a large reservoir of fat – the vacuole.
On the other side of the coin is brown adipose tissue, which expends energy and is responsible for heat production in all mammals. The cells of this tissue are more energetically active than those of white adipose tissue and contain a large number of mitochondria, whose metabolism consumes a large amount of energy to produce heat. Unlike white adipose tissue, they have more smaller fat-containing vacuoles. [2]
How does the body gain weight?
When you begin to consume more calories than you can expend, something must happen to the excess energy. Energy doesn’t just disappear, so it is stored as fat in the fat cells.
The number of fat cells increases especially in childhood and adolescence. Later in adulthood, the number of fat cells is fixed and changes with weight gain and weight loss. This happens to the point where they can no longer increase in size and new cells need to be created.
- The enlargement of adipocytes due to excessive fat storage is called, as in the case of muscle, hypertrophy.
- The formation of new adipocytes is called hyperplasia.
- Hypertrophy of adipocytes greatly affects their function and is typical of visceral adipose tissue. Hypertrophied adipocytes are the source of a large number of pro-inflammatory substances.
- Hyperplasia, i.e. the formation of new smaller adipocytes, typically takes place in subcutaneous adipose tissue and, unlike hypertrophy, is not as dangerous to the body. [1]
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What happens when you lose weight and why do you gain it back?
The human body has several mechanisms in place to control fat stores and food intake:
1. Number and size of fat cells
The number of fat cells in people who are overweight and obese is therefore higher than in people of a healthy weight. In more extreme cases, obese people can have up to twice as many fat cells. If they lose weight, this number does not change. What changes is the size of the fat cells, which “deflate” like balloons, waiting to be “inflated” again. Thus, the body remembers how many fat cells it had while obese. After weight loss, these cells naturally try to replenish their fat stores. [3]
2. Epigenetic memory
In addition to the number of fat cells, the body also has an epigenetic memory. In other words, being overweight and obese will change the regulation of genes in fat cells. Adipocytes, which were once full of fat and their function impaired due to obesity, remember this state. Normal body weight is not normal for epigenetically modified adipocytes and they make great efforts to regain the lost fat. After weight loss, fat cells exhibit a higher rate of glucose uptake, leading to faster regain. [4]
3. Leptin and Ghrelin
Of course, gaining weight does not take place by some miraculous conversion of food into fat, but is influenced by how many calories you ingest. Whether you will lose or gain weight depends on how much food you put on your plate. This affects the brain, which has a very good overview of your fat stores. The main offender is the hormone leptin.
Leptin, also known as the satiety hormone, tells the brain how much fat is in the body. The system works very simply – the more fat you have, the more leptin is produced in the fat cells, and this lets the brain know that you have enough fat stores. If by some chance you lose weight, leptin production drops and the brain gets the signal that you’ve begun to deplete your fat stores. The whole leptin system is supposed to keep you alive. [5]
However, the brain does not have an exact value for fat stores and can get used to more fat. If someone maintains 10 kg of excess weight for a long time, this state becomes normal for the brain. Anything below this excess weight will be evaluated by the brain as life threatening. In other words, if an overweight person loses weight, the brain will start forcing them to eat more and move less. [5]
In extreme cases of obesity, the phenomenon of leptin resistance occurs. The brain can no longer respond properly to signals from the adipose tissue and thus has no insight into the state of the stores. The result is out-of-control overeating. [6] Leptin levels are also affected by sleep. If there is a lack of it, leptin levels are lower, which tells the brain that you should eat more. At the same time, the level of ghrelin, which is the hormone that signals the feeling of hunger, is higher. Lack of sleep leads to overeating. [7]
Ghrelin (the hunger hormone), which signals that you are hungry, is naturally suppressed in those who are overweight or obese (it has a different regulatory function). Despite its lower levels, it is the partial leptin resistance and the power of dopamine that makes overweight and obese people eat. Conversely, in the process of weight loss, ghrelin levels increase, which can greatly complicate the whole dieting process. [8]
How does the body use fat and how does it get rid of it?
If the body doesn’t get enough energy from food, it starts reaching into its fat stores. The triacylglycerols in fat cells are broken down into individual fatty acids by the process of lipolysis, which are used by mitochondria to produce energy. This energy production is due to beta-oxidation of the fatty acids, during which a portion of the long chain fatty acid is “chopped off” to form acetyl-CoA. This then enters the Krebs cycle.
Thanks to this process, the fats are used and their final by-products are water and CO2. Theoretically, during weight loss, fat is exhaled in the form of water vapour and CO2. Some of this water can be excreted by the kidneys and through perspiration. It is therefore obvious that sweating it out on the treadmill does not mean that you are losing weight. It just means that the body needs to effectively cool itself. Most of the fat disappears from the body through respiration.
Is there any point in trying to lose weight?
The simple answer is that it does. Being overweight and obese brings with it increased risks of more than 200 chronic diseases. Although sustainable weight loss is a challenging process that the body actively struggles with, it is important to maintain a healthy body weight. The weight loss process should be accompanied by sufficient physical activity and a varied diet, which, together with the loss of fat tissue, bring the benefits of good physical and mental health in the long term.
Conclusion
Weight loss is a difficult process during which the body has to use the energy stored in fat tissue. However, the body is trying to survive and therefore closely guards its fat stores. The mechanisms by which it does this make weight loss difficult and force us to eat more. To avoid the yo-yo effect and regaining weight, it is important to maintain a calorie deficit for the long term and not expect miracles in a very short time. The body remembers how much fat it had and it will take a long time for it to get used to the new standard.
[1] Horwitz A, Birk R. Adipose Tissue Hyperplasia and Hypertrophy in Common and Syndromic Obesity-The Case of BBS Obesity - doi: 10.3390/nu15153445
[2] Harms, M., Seale, P. Brown and beige fat: development, function and therapeutic potential- https://doi.org/10.1038/nm.3361
[3] Spalding KL, Arner E, Westermark PO, Bernard S, Buchholz BA, Bergmann O, Blomqvist L, Hoffstedt J, Näslund E, Britton T, Concha H, Hassan M, Rydén M, Frisén J, Arner P. Dynamics of fat cell turnover in humans. Nature. - 10.1038/nature06902
[4] Hinte, L.C., Castellano-Castillo, D., Ghosh, A. et al. Adipose tissue retains an epigenetic memory of obesity after weight loss - https://doi.org/10.1038/s41586-024-08165-7
[5] Mendoza-Herrera K, Florio AA, Moore M, Marrero A, Tamez M, Bhupathiraju SN, Mattei J. The Leptin System and Diet: A Mini Review of the Current Evidence - doi: 10.3389/fendo.2021.749050
[6] Martin SS, Qasim A, Reilly MP. Leptin resistance: a possible interface of inflammation and metabolism in obesity-related cardiovascular disease - doi: 10.1016/j.jacc.2008.05.060
[7] Taheri, Shahrad; Lin, Ling; Austin, Diane; Young, Terry; Mignot, Emmanuel . (2004). Short Sleep Duration Is Associated with Reduced Leptin, Elevated Ghrelin, and Increased Body Mass Index - doi:10.1371/journal.pmed.0010062
[8] Makris MC, Alexandrou A, Papatsoutsos EG, Malietzis G, Tsilimigras DI, Guerron AD, Moris D. Ghrelin and Obesity: Identifying Gaps and Dispelling Myths-doi: 10.21873/invivo.11168
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