The human body has developed and adapted over thousands of years for survival and part of that evolution is it will burn our own muscle in order to stay alive if there is no more fat available. If you have low body fat and cut your calories back your body will naturally move on to burning muscle mass if it runs out of fat to burn. As a male if your body-fat is between 3-5% you are considered to be in the low category and should pay particular attention to the following. So how do you balance the amount of protein to build muscle with burning fat in a healthy way?
Whether you are doing high calorie burning activities like cardio or not, your fat vs. muscle burn has just as much to do with the food that you eat. One factor in particular as mentioned before is if your protein intake is insufficient your body will resort to burning muscle, it's as simple as that. This commonly happens when someone is on a dangerous low calorie diet like a “crash diet”. The “Essential" Amino Acids that you get from protein are considered to be just that "essential," due to the fact that you need them to produce and retain muscle or your body will naturally burn off all of that muscle you have worked so hard to obtain.
Unless you have a lot off excess fat, reducing your daily calorie intake below 20% of your daily calories expended makes it extremely difficult to get the required essential amino acids your body needs by just eating protein or taking supplements. This is true even if you do not have any high calorie burn activities in your workout plan since your body will be at a deficit to replace expired muscle cells that help restore and build muscle.
The bottom line is if you don’t get enough protein in your diet the cells in your muscles do not get replaced. Therefore when the body goes to burn and has no fat reserves its only option is to burn muscle to survive. If you add increased cardio into the mix when you’re already not getting enough protein then you will burn muscle even faster.
If your specific objective is muscle gain you will need to calculate your daily calories and factor that into what you are going to burn. Not only will it help you achieve your goals but it is just as important for optimal health. A mantra in the bodybuilding community is that muscles are destroyed in the weight room and built in the kitchen eating the correct balance of carbs, protein and fats. Remember this when you are planning your muscle gain goals for maximum success.