Losing muscle is a bad thing. Muscle tissue is very active--it burns a lot of calories. Every pound of muscle burns about 35 calories per day, just staying alive. Using the above example, if you lost 12 pounds, then, you lost three pounds of muscle. You would be burning 105 less calories per day or 3150 per month. Since there are 3500 calories in a pound of fat, you would gain almost one pound of fat per month just from the muscle mass that you lost!
If you go "on a diet", what does that mean? That you will eventually go "off the diet". If you haven't changed your eating and physical activity behaviors, what's going to happen? Rationally, you know the answer, but emotionally, you engage in wishful thinking, hoping that this time you will lose the weight and keep it off. What weight are we talking about? When people tell me they lost 12 pounds in two weeks, I ask "12 pounds of what?" On low-calorie diets, about 25 percent of the weight loss may be muscle (Ballor & Polehlman 1994).