For years, we’ve heard that losing weight quickly is impossible and slow progress is inevitable. But at its core, weight loss is governed by the laws of thermodynamics: we lose fat by consuming fewer calories than we burn.
Hormonal fluctuations and metabolic regulation do play a role, but the balance between calorie intake and expenditure remains fundamental.
Theoretically, but also practically, it is possible to lose a lot of excess fat in a very short time. but, The human body has mechanisms that help ensure our survival; thus, the so-called yo-yo effect occurs.
As a result, everything we lose is regained very quickly. Why is this the case, and how can it be avoided? These questions will be the focus of this article.
What is fat?
Chemically, fat consists of triacylglycerols (TAG). These are fatty acids linked to a glycerol molecule and found in fat cells called adipocytes. These cells are found almost everywhere in the body. Fat is divided into subcutaneous and visceral depending on location.
Subcutaneous fat is the fat layer just under the skin. It is the type of fat that is visible and that people commonly try to reduce if it accumulates in excess.
Visceral fat is located around the organs, and its function is the mechanical protection of the organs, but also the regulation of metabolism and hormone levels. It is also a source of substances that are involved in the regulation of the immune system.
Adipose tissue is also divided by its ability to generate heat. The most common is white fat, which stores energy. White adipocytes have few mitochondria, so they are not sites where energy is burned. These cells are almost filled with large fat vacuoles.
By contrast, brown adipose tissue is specialised for energy expenditure and heat production. Brown adipocytes contain more mitochondria, making them more metabolically active than white fat cells. These cells store fat in several small vacuoles rather than one large one.
How does our body gain weight?
To understand weight gain, consider what happens when we consume more calories than we burn. This excess energy does not simply disappear and is, instead, stored as fat in our cells.
The number of fat cells increases mainly in childhood and adolescence. In adulthood, this number becomes fixed. With weight changes, the volume of fat cells changes until they can no longer grow, and new cells must be created.
The enlargement of adipocytes due to excessive fat deposition is called hypertrophy, as in the case of muscles.
The process of creating new fat cells, rather than enlarging existing ones, is known as hyperplasia.
Hypertrophy of adipocytes—when they grow larger—greatly affects their function. This process is typical for visceral adipose tissue. Hypertrophied adipocytes produce many pro-inflammatory substances.
Hyperplasia, which is the creation of new and smaller fat cells, usually happens in subcutaneous fat. It is generally less harmful than hypertrophy.
What happens when we lose weight, and why do we gain it back?
The human body has several mechanisms by which it controls the state of fat stores and food intake:
1. Number and size of fat cells
Overweight and obese people have more fat cells than those with a healthy weight. In severe obesity, there can be up to twice as many. After weight loss, the number of fat cells does not change; only their size shrinks.
These “empty” cells are ready to be filled again. The body remembers the number of fat cells from obesity. After weight loss, these cells try to refill themselves.
2. Epigenetic memory
Besides fat cell count, the body uses epigenetic memory to remember past states. Obesity changes the function of genes in fat cells. Cells overfilled by obesity “remember” that condition. Normal weight is unusual for them, so they strive to regain lost fat.
After weight loss, these cells absorb glucose more quickly. This can result in fat being regained faster.
3. Leptin and ghrelin
Weight gain does not turn food into fat by magic. It depends on how many calories we eat. Whether we lose or gain weight is determined by our food intake. The brain monitors our fat stores. The main factor 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 we have, the more leptin is formed in the fat cells, and thanks to this, the brain knows that we have enough fat reserves.
If we accidentally lose weight, the production of leptin decreases, and the brain receives a signal that we have begun to spend supplies. The entire leptin system is designed to keep us alive.
The brain cannot precisely measure fat stores. It can adapt to higher fat levels if we keep extra weight for a long time. This overweight state becomes the new normal. Anything below this is a threat to survival. If an overweight person loses weight, the brain drives them to eat more and exercise less.
In severe obesity, leptin resistance can occur. The brain no longer responds to signals from fat tissues and loses track of fat reserves. This leads to uncontrollable overeating.
Leptin levels also depend on sleep. If we sleep too little, leptin drops, and the brain tells us to eat more. Ghrelin, the hunger hormone, rises with lack of sleep, causing overeating.
Ghrelin, the hunger hormone, is naturally suppressed in overweight and obesity, where it serves another purpose. Even with low ghrelin, leptin resistance, and dopamine drive, overeating occurs. In weight loss, ghrelin levels rise, making dieting much harder.
How does our body use fat, and how does it get rid of it?
If dietary energy runs low, the body uses fat stores for fuel. Fat cell triglycerides break down into fatty acids during lipolysis. Mitochondria use these fatty acids to make energy through beta-oxidation. A piece of each fatty acid chain becomes acetyl-CoA, which enters the Krebs cycle.
This process allows the body to use fat, producing water and CO2 as end products. During weight loss, most fat leaves the body as water vapour and CO2, which we exhale. Some water is excreted by the kidneys and through sweat. Sweating during exercise only helps cool the body; most fat is lost by breathing.
Does it make sense to try to lose weight?
The answer is yes, losing weight makes sense. Overweight and obesity increase the risk for over 200 chronic diseases. Sustainable weight loss is hard, and the body resists it, but a healthy weight is important.
Weight loss should include physical activity and a varied diet. This approach, along with reduced adipose tissue, supports long-term physical and mental health.
Conclusion
Weight loss is a difficult process, during which the body must use the energy stored in adipose tissue. However, the body is trying to survive, so it strictly guards its fat reserves. The mechanisms by which it does this make it difficult for us to lose weight and force us to eat more.
To avoid the yo-yo effect and regain weight, it is important to adhere to the caloric deficit for a long time and not expect miracles in a very short time. The body remembers how much fat it had, and it will take him a long time to get used to the new standard.