I do about 2-3 cardio days during the week which generally consists of a fast paced 3-5 mile run. I try to keep a pace that I can barely hold for the duration of the run. I sometimes do little 1-1.5 minute semi-sprint intervals and then rest for a bit. My question is how do I make sure when im doing these cardio efforts that I am losing fat and not losing precious muscle. Should I drink a protein shake before i go?