Ask Mary

You can gain muscle while losing fat if you weight-train two to three days a week and eat enough calories to spare protein for building muscle.  If you don't eat enough, you will burn your protein to meet your calorie requirements.  You, personally, need about 2000 calories and 80 grams of protein a day to lose approximately one pound per week.  Note that the scale cannot measure changes in your fat to muscle ratio, and so you should measure your body composition periodically (at the gym) if you have the opportunity to do.  Also measure your chest, waist, hips, thighs, and upper arms to track inches lost.  Read more about building muscle while losing weight from the Guide to Exercise at About.com.