You can however, loose fat, and gain muscle, if you lift regularly, and do cardio/bikie you may notice a plataue, where you dont loose anything on the scale, but everything fits better, more defenition, its because you are loosing fat and putting on muscle at the same time. Yet as it was said, it is almost imposible to gain muscle and loose weight, this only happens if you have faster then safe weightloss, or have reached a brief muscle platau.