You should consume sufficient amounts of protein each day to build muscle, maintain lean muscle mass and prevent loss of muscle tissue. (healthyeating.sfgate.com)
Focusing too much on cardio in combination with calorie restriction leads to greater loss of muscle tissue. (cathe.com)
If those calories you consume don't contain the desired amount of macro nutrients, weight loss can occur in the form of loss of muscle tissue, water and possibly even bone mass! (fitnessandpower.com)