I always hear that if you not getting enough energy, calories, your body will burn muscle instead of fat, because fat does not cost any energy. So at how much of a calorie deficit must your body be running on for it to start burning muscle instead of fat.
You need to find out what your resting metabolic rate is, which is basically how many calories your body burns normally without any exercise. If you take only a little amount of protein and cut a couple hundred calories below your rmr you will lose muscle and get tired.