Lets imagine that you are overweight and you would like to lose several excess pounds. To lose 1 pound of fat, what you have to do is burn off 3,500 calories. That is, over a period of time, you have to consume 3,500 calories less than your body needs. There are several ways you can create that deficit. If you assume that you weigh 150 pounds and that your body at rest needs 1,800 calories per day (150 * 12 = 1,800) to live, here are several examples (some realistic, some not):

You could lie in bed and starve yourself. Since you are lying in bed, you are consuming 1,800 calories per day. Since you are starving yourself, you are taking in no calories. That means that, every day, you create a deficit of 1,800 calories and, approximately every two days, you will lose 1 pound of body weight.

You could consume fewer calories than your body needs. For example, you might choose to consume 1,500 calories per day rather than the required 1,800 by controlling what you eat. That creates a 300-calorie deficit every day. That means that approximately every 12 days, you will lose 1 pound of weight (12 days x 300 calories = 3,600 calories).

You could consume 1,800 calories per day and then choose to jog 2 miles (3.2 km) every day. The jogging would burn about 200 calories per day, and over the course of 18 days you would burn about 1 pound of body weight (18 days x 200 calories = 3,600 calories).

You could consume 2,500 calories per day and run 10 miles per day. You will burn 1,800 calories per day at rest and then 1,000 calories per day running, for a total of 2,800 calories. You are consuming 300 calories fewer than you need, so you would lose a pound every 12 days or so (300 calories x 12 days = 3,600 calories).

As you can see from these examples, the only way to lose fat is to consume fewer calories per day than your body needs. For every 3,500 calories that your body takes from its fat reserves, you lose 1 pound (0.45 kg) of body fat. You can create the deficit either by monitoring and restricting your intake of calories, or by exercising, or both.

The idea behind most diets everything from Weight Watchers to the grapefruit diet is simply to help you somehow lower the number of calories that you consume each day. Thats all they do.