Reasons for weight gain
Evolutionarily, it was advantageous for humans to be fat. Having extra fat gave them an advantage. According to a recent hypothesis, the scheme was as follows: in the summer and spring, people ate more, since food was available and easier to find, and in the winter and late fall, we survived due to stored fat, consuming much less food, since it became more difficult to find. That is, in the summer, we hunted various game and ate enough, and even more than our needs. But in the winter, our "catch" was small, but in combination with the fat accumulated over the summer, we survived.
Body fat is stored energy that creates heat in cold seasons. That is, fat was needed for comfort and survival. In this scheme, subcutaneous fat did no harm. We got fat, lived, lost weight, and got fat again. A kind of balance was maintained, as in all processes of nature or the body.
But now food has become available to most people. Having gained weight, we remain with new fat for a long time, and this already leads to various health problems. The stress factor that makes us run/jump/smash in search of food has disappeared. We have removed the "winter" and are left in the "obesity" phase. After all, food is now available all year round, in the store, around the corner in a cafe, and in extreme cases - in your refrigerator. Humanity no longer spends accumulated fat, because we eat all year round, daily, and without pauses.
The thin ones simply did not survive. Less fat – more chances to die. Surviving the winter was difficult and uncomfortable, although this word is not quite appropriate. Thanks to natural selection, people whose ancestors could effectively conserve the energy obtained from food – gain and accumulate large reserves of fat – came to the 21st century. We inherited the genes of the best of them. That is why any modern person gets fat so easily. This is a bonus from his hardy and strong ancestor.
It was not the strongest who survived, it was the fattest who survived. By the way, the principle of periodicity "got fat-lost-got fat" is remotely used in all normal nutrition schemes. You can't constantly sit on a calorie deficit or on any diet. It is important to go on "maintenance" to restore hormones and the general condition of the body. As strange as it may sound, a timely break from dieting will allow you to lose even more, but only with the right approach.
The ideal diet pays tribute to the ancient times to which our body is accustomed. It is the observance of the natural order that allows us to lose weight effectively and quickly. In ancient times, we lost weight in stages, and this is the principle used by competent nutritionists.
Possible causes of weight gain
In the vast majority of cases, the true cause of obesity is one - excess calories in the diet. If your body receives a large number of calories weekly and monthly, and does not have time to spend them, all the surplus will have to be put aside as a reserve, that is, in the form of fat, just as ancient people did.
It doesn't matter what foods you eat. Whether your diet is healthy or not. The essence is the same - the caloric content of the diet and the amount of energy. If you consistently give your body more calories than it needs, you will gain weight. There is no escape from this, since this is the second law of thermodynamics.
However, there are also a number of secondary causes:
Hormonal imbalance
An imbalance of thyroid hormones or insulin can prevent you from using up your existing fat reserves. At the same time, you can remain quite overweight even with a meager diet. The body will defend its fat stash with all its might, and standard training and weight loss methods will not help you, because an imbalance of hormones affects the entire body's assessment system. For example, in an ordinary person, the body analyzes the balance of calories spent and received, and based on this, decides how many hormones need to be released. With a hormonal imbalance, it is almost impossible to do this correctly, as a result of which losing weight becomes several times more difficult. In this case, you need to visit an endocrinologist.
Hidden Calories
Some people eat very little, but still gain weight, blaming everything on metabolism and metabolism, which is the same thing. In reality, the reason is the same - excess calories from the diet, but there is a small nuance here. Such people eat little regular food, but at the same time manage to snack on a large number of calories without realizing it. Various chips, sweets, sweet drinks, etc. are very high-calorie and non-nutritious food. It can be easily eaten in a few bites and not notice the calories received. The problem is especially relevant for students and active workers engaged in mental activity. They feed the brain with calories without noticing it.
Swelling
Water retention can directly affect weight gain and deterioration of appearance. Most often, the causes of edema are the same: "playing" with salt in the diet, insufficient water in the diet, or hormonal imbalance.
Style summary
So, the main reason for weight gain is a banal excess of calories. Note that it is more difficult to go overboard with calories if you eat mostly natural food that you cook yourself. Buckwheat with sauce, a side dish with meat, borscht, pelmeni, etc. Such food is high in calories, but at the same time satisfying and varied in vitamins and minerals. But all kinds of chips, ready-made dinners, pizzas, fast food, sweets, etc. are sources of a large number of calories, which can be eaten in a few bites.
So if you're looking for some unique reason why you've gained weight, stop. It's pretty simple - you're eating too many calories. Cut back, control your calorie intake, and watch out for "invisible" snacks.
Finally, let's note that the female body is much more resilient than the male body in terms of losing body fat. Women procreate and ensure human survival, and as a result, they tolerate hormonal/energy/physical activity imbalances much better.