Calories are what food consists of. There are three types of calories: Protein, Fats and Carbohydrates, which are called macro nutrients. All foods that contain calories are made up of different ratios of these 3 types of macro nutrients. 1 gram of both protein and carbs provides 4 calories each, while 1 gram of fats provides 9 calories. Caloric intake is directly related to body weight and body weight changes. Each person burns a certain amount of calories per day, which we refer to as maintenance calories. In other words, if a person were to eat their maintenance calories every single day, they would stay the same weight. If a person consistently eats MORE calories than this number, they will gain weight, and if they consistently eat LESS than this number they will lose weight. This is how gaining and losing weight works. It is NOT about the foods you choose to eat, but it is directly related to HOW MUCH you eat. Remember, HOW MUCH you eat, not WHAT you eat, is what determines body weight changes. One pound of fat equals approximately 3500 calories. That being said, if a person wanted to lose one pound per WEEK, they should eat 500 calories BELOW their maintenance calories every day (500*7 = 3500) which will result in a caloric deficit of 3500 calories by the end of the week. On the other hand, if a person wanted to gain one pound a week, they should eat 500 calories ABOVE their maintenance calories.