The title is quite banal, but many people and even "pseudo-specialists" still do not understand the fundamental rules of our metabolism, thereby harming themselves with a special diet, or, for example, stupid workouts.
And so, we have all heard that if you eat a lot of high-calorie food, you can gain weight. Let's first find out what the calorie content of food is. This is the amount of energy that our body receives during the biochemical processing of this food. To understand the essence of calorie content, non-physicists need only imagine how two substances of the same mass / volume burn, for example, alcohol and diesel fuel (diesel fuel), which of them will give more heat? Anyone who has ever seen such combustion will immediately answer - diesel fuel. That is, its calorie content is higher (~ 10300 kcal / l., versus ethyl alcohol ~ 7000 kcal / l). So with food, some products give a lot of heat / energy, others less. We found out.
Now let's imagine the situation of a hypothetical person who constantly eats a lot of high-calorie foods, will he increase his body weight? To this question, fitness trainers unanimously answer - "yes, in such conditions he will always gain weight"
I will answer - not a fact. 
A person will increase his weight only by the weight of food that entered his gastrointestinal tract. But whether the weight of his fat tissue, muscle tissue and other tissues will increase depends very much on the processes of his individual metabolism. You all probably have a friend who eats everything and does not gain weight. Now I will tell you why this happens.
The fact is that the "fuel" that enters the body in the form of food is "processed" to produce energy differently in each person. Moreover, there can be both safe genetic configurations and pathological ones. Here are the most common pathological cases:
A person has low stomach acidity, as a result, protein breakdown reactions are slowed (these reactions require an acidic environment with a certain pH). These people usually have a poor appetite and an aversion to meat. Some individuals, influenced by internet idiots, begin to think this is a sign to become a vegan. 
A person's gastrointestinal tract is not functioning properly, for example, the number of enzymes/bacteria needed to break down proteins into peptides, fats into fatty acids, and carbohydrates into glucose is reduced. 
A person has problems with anabolic hormones. For example, loss of growth hormone secretion, low insulin levels, low androgen or estrogen levels.
With all these scenarios, no matter how much high-calorie food you eat, you won’t gain weight! Because the food won’t go through all the necessary stages of biochemical processing to obtain energy.
Moreover, if you add physical activity (like going to the gym to a trainer who has completed a trainer course:) ) then you will harm your body even more! The body will experience a deep deficit of energy that you spent on exercises but cannot fully restore with meals.
Therefore, if you've been struggling to gain weight for years, specifically muscle weight (as gaining fat is not beneficial ), first have your body examined for the issues described above by qualified specialists, or figure it out yourself. Only after addressing these issues should you begin to challenge your body with food and exercise.
