We know the effect calories have on your body. For each pound of weight that your body carries, it takes about 12 calories per pound per day to keep it alive. If you weigh 150 pounds, you therefore need about (150 * 12) 1,800 calories per day to keep yourself alive and maintain that same weight. 

If you were to eat more than 1,800 calories per day, the surplus turns into fat. It takes about 3,600 excess calories to make a pound of fat. 

Let's say that you were to eat, on average, 2,000 calories per day. That extra 200 calories per day is going to turn into fat. However, you will eventually hit a point of equilibrium (平衡，均衡）because as you gain weight, you need more calories to maintain that weight. So, a person eating an average of 2,000 calories per day will hit equilibrium at (2,000 / 12) 166.67 pounds. 

So let's say that you eat, on average, 2,000 calories per day and reach equilibrium at 166 pounds. Then you go on a crash diet where you eat only 1,000 calories per day for 60 days. You lose 16 pounds and reach your target weight of 150 pounds. But then you go right back to eating 2,000 calories per day again. The graph below shows your eating pattern before, during and after the diet: 

Note that the graph assumes that you, like most people, eat a random number of calories per day. Some days you are "good" and eat less, and some days you are "bad" and eat more, but the assumption is that it averages out to 2,000 calories per day over the long run. 

This graph shows what happens to your weight before, during and after the diet: 

The diet really takes the weight off. But the weight comes right back when you return to your "normal" eating pattern. The weight appears to come back so quickly because when you are at your lightest, you tend to gain more weight each day. The lighter you are, the fewer calories you need. If you only need 1,500 calories per day and you're eating 2,000, you're going to gain weight faster than if you need 1,800 and you're eating 2,000. 

There are only two ways to keep the weight off: 

• Change your overall eating pattern so that you take in, on average, fewer calories than before you went on the diet.

• Start exercising so that you "burn" the extra calories you take in. 
  The best course of action is a combination of these two options.