No. Enough protein intake is a necessary but not sufficient condition. Your body is constantly breaking down and rebuilding muscle tissue (and all other tissue

No. Enough protein intake is a necessary but not sufficient condition. Your body is constantly breaking down and rebuilding muscle tissue (and all other tissue). The amount that it rebuilds is effected by many things like protein availability, and (basically) how much you have been using the muscle. This is why staying active is important. In addition, since your body has a limit of how quickly it can metabolize fat, even if you had sufficient protein intake and sufficient activity to signal to your body to retain muscle mass, if your caloric deficit is large enough, your body may still be forced to burn protein (ie directly or indirectly reduce muscle mass) to stay alive. These are factors that you can usually manage around with some careful, but relatively straight forward planning. EDIT: Note that the 'careful but straight forward planning' is basically exactly the same planning for "a diet". Traditionally, you'd see diets aim for no more than a 300-500kcal deficit for this exact reason. It's not just a question of keeping the food reduction minimal to help your brain stick to the plan, but also as your calorie deficit becomes larger, it becomes much more difficult to fit in all of your proteins (amongst other things).