Weight gain/loss is very simple but people make millions of dollars pretending it isn’t.
Your body adapts to compensate for the number of calories consumed by eating/drinking vs the number of calories utilized by the body every day/night. This includes both the number of calories used to maintain itself at rest (BMR) and the number of calories burned during exercise.
Food/drink is broken down into macronutrients (Carbohydrates, Proteins and Fats) they contain different numbers of calories. Carbohydrates and Proteins contain 4 calories per gram while Fats contain 9 calories per gram.
If the number of calories consumed exceeds the number of calories the body needs to maintain itself and complete exercise, it stores the excess calories as body fat (Calorie Surplus).
If the number of calories the body uses to maintain itself and to perform exercise exceeds the number of calories consumed, the body sources the energy stored in the form of fat (Calorie Deficit).
1kg of body fat = roughly 6700 calories. So over the course of a fortnight if you had a Calorie Deficit of 480 calories each day, you would lose 1kg of body fat (this does not include water weight which may make it seem like you are losing a lot more).
A quick Google search will tell you a rough estimate of your BMR based on your age weight and activity levels.
That’s basically it, there is no magic fat burning pill, no secret diet to cheat the system, eating carbs doesn’t make you fatter, eating at night doesn’t magically make you obese and regardless of what rubbish you read in Men’s Health or Women’s Day, YOU DON’T HAVE TO KILL YOURSELF WITH EXERCISE TO LOSE WEIGHT.
Just know what is in the food you put in your body.