At the most basic level, weight loss occurs when your body uses more calories than you consume. Your body burns calories at rest, just to keep your various physiological systems functioning correctly. You burn calories through activity and using your muscles. You also burn calories by digesting and metabolizing the food you eat. Even thinking, which requires cellular communication between the neurons in your brain, burns calories. You take calories in, of course, through food. Therefore, the process of losing weight almost always begins with reducing the number of calories you eat and increasing the number of calories you burn, thus creating a "caloric deficit." The most readily variable method of burning calories is physical activity, so exercise goes hand-in-hand with healthy nutrition in losing weight.

When your body faces a caloric deficit, it must turn to stored sources of energy to meet its caloric needs. Most of the body's excess calories are stored as fat, and the goal of most people in losing weight is to lose fat. As the body needs more energy than it takes in through food, it turns to these reserves (in addition to glycogen/sugar and sometimes protein reserves), and fat stores begin to deplete. As a rule of thumb, the body must have a caloric deficit of about 3,500 calories to lose one pound of stored fat. This translates to one pound fat loss per week if your daily caloric deficit through decreased intake and increased output is 500 calories.