i think it can be looked at both ways. my wife was sick for years and it made me learn what i could about what could be done to prevent muscle burn, and this is basically how i know it. it has been a few years, so i may be a little off, but i'm fairly confident this is how it goes:
the human body burns glycogen (glycolysis) and fat (ketosis), before muscle atrophy. a person with muscle degradation is malnourished, or has a deadly disease, and should not be fasting. common with very high protein and little to no carb or fat diets.
muscle growth requires simply protein and exercise, on top of what is needed to maintain growth. for this, the body needs just enough carbs and fat to prevent the amino acids within muscle from being converted into glucose, enough protein, converted into essential and non-essential amino acids, depending on the organ, and to replenish those that have been used (we pee/sweat out excess).