The fear is that while fasting, the metabolism will slow down and afterward there will be more weight gained than before the fast. Initially, any calorie reduction will cause a lowering in the BMR or base metabolic rate. Cleansing will bring health that will increase metabolic performance and energy output. In other words, you will eat less and feel more energetic.

After fasting, the body is more efficient. Digestion is better due to a cleaner colon. The colon walls have been cleansed of impacted feces, allowing improved absorption of nutrients. The good news is that greater efficiency equals enhanced energy. Healthy people burn more calories. They also eat less because they are more active and feel better about themselves. They don’t need food as a security blanket.

The greatest impact you can have on increasing your resting metabolism is exercise. After a workout, you can be sitting on the couch reading while burning calories, as the muscles are grabbing glucose from the blood to replace glycogen supplies. The key to weight loss is fewer calories and more exercise. A diet high in raw food provides fewer, low-density calories, and higher-fiber, more-filling foods.

Oh, by the way, a slower metabolism diminishes aging. A tortoise can live to 400 hundred years of age.

source: http://www.healthrecipes.com/fast10.htm