Simply stated, if you take in more calories than you need on a daily basis, the excess calories turn into extra weight. Think of food as fuel and your body as a car. Food is the energy your body "runs" on. Overeating is like over-fueling; any excess fuel you don't "run on" turns into fat. This is how weight gain occurs. Because of this basic principle (calories consumed versus calories spent), the cause of weight loss is just as straightforward as the explanation of weight gain. If you consume fewer calories than you burn, you will lose weight. The basis of every weight loss diet is based on this. Whether you follow a low-fat or low-carbohydrate/high protein diet or you reduce portion sizes, all varied means to the same end: fewer calories consumed.