Our bodies have many different processes running at the same time, all of which require energy to function. This energy is gained from caloric consumption, and is called the Basal Metabolic Rate or BMR.
Everything we eat that’s over and above the BMR is likely to contribute to body fat.
Having understood, after reading the above, that weight gain and loss depends on the number of calories over or under consumed, relative to your body’s caloric requirements, I’m sure you’ll guess what my response will be. Yup – portion control is your way out.
Portion control refers to eating set quantities of a given food, the quantity being relative to the caloric value of the food in question, the body’s caloric requirements, the nutritional attributes of the food and their value to the body. For instance, I suggest loading up on fresh vegetables, legumes, lentils, dairy such as yogurt, protein and grains/carbs, in that order. This way, you can eat whatever you want, not exercise much and yet not (probably) gain weight.