1. The body quickly begins to decrease its basic calorie expenditure (the basal metabolic rate) to conserve energy. Thus, fewer calories are burned, not more.
2. The body does not function properly without enough carbohydrate, and it begins to convert muscle protein to carbohydrate so that it can continue to make energy. Losing muscle tissue is not desirable. Of a ten-pound weight loss in two weeks, only approximately two to three pounds are fat because approximately five pounds are water and two to three pounds are lost from muscle.
3. While all this is going on, the body is increasing its ability to store fat when it gets enough calories sometime in the future. This is a basic survival mechanism for times of inadequate food intake, but it is not desirable when the goal is weight loss or weight maintenance.
It certainly is possible to lose weight on a low-calorie diet. However, the period of rapid weight loss typically lasts only about a week or two, and then it stops. During that period the body "read justs" its metabolism to survive on fewer calories. When a more normal caloric intake resumes, the body takes advantage of its increased "fat-storing" ability to increase its energy reserves in stored body fat. The weight lost as water and muscle is not replaced as water and muscle; it is replaced as fat. Over time, even though total weight may not actually change significantly, the percent of body fat may increase dramatically. This clearly is not the desired goal.
Because of the associated risks and poor long-term success rate, low-calorie diets should only be attempted under the supervision of a physician. Keep in mind that in the long run success most likely will be achieved by eating enough of the right kinds of foods and by being as active as possible within the limits of your individual situation.
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