What is a calorie? Well, in my own language if asked I say: A calorie is basically energy that you gain from eating, and drinking. Not all things have calories. Water for instance, has no calories. Water does cause you to burn calories, but not enough to change anything. Anyone who says otherwise is probably trying to promote a diet. Why are calories important? They are what make you who you are body wise. If you take in too many calories, you GAIN weight. If you take in too less, you LOSE weight. If you take in the right amount, you stay at a constant weight. That is called your Maintenance level. If you stay at that, you will neither gain weight or lose it. Now, how do you do this? Simple. A calorie calculator. I use this: Calorie Calculator - Daily Caloric Needs These cannot be exactly accurate, everyone has different genetics, but that is about as precise as you really need. So, if you want to lose weight, it is simple, calorie deficit. If you want to gain weight, calorie up. I do -1000 calories with my diet, then add about -400 calories due to my cardio/exercise. So, -1400 calories a day, x7 I lose 9800 calories, which is around 2+ pounds of fat a week, although that is a good chunk of muscle since I am not allowed to weight lift currently . It doesn't matter how many fat grams, carbs, or protein is in anything, if your taking in too much. Although, if you get in-depth: I advise this for anyone: Majority Protein, low carb/low fat. Why? Fat/Carb has more calories per then Protein.