If you vigorously exercise before you eat, you will eat less. Most people are not overweight because they eat too much. They're overweight because they're inactive. Weight loss depends on creating a demand for the energy stored in fat tissue. Dieting just won't do the trick, because no matter how few calories you take in, they must be used somehow, or they will be stored.
Exercise promotes weight loss by helping to regulate appetite.
Appetite refers to the food we crave, while hunger refers to the nutrition the body needs. You must learn not to want what you should not have. Muscles cannot turn to fat, contrary to popular conception, that if you stop exercising regularly, muscles will turn to fat. They simply do not have the physical capability of changing from one type of tissue to another. No diet aid can firm you up-only exercise can.