Quoted: Sure, it's possible for you to lose muscle from doing too much cardio, but it's highly unlikely. Shying away from cardio completely because you think you'll lose muscle is a huge mistake. Only excessive amounts of cardio would cause you to lose muscle because over-training tips the scale towards the catabolic side. It's difficult to generalize and pinpoint one specific amount as too much, but I think it's safe to assume that just about anyone could do up to 45 -60 minutes of cardio a day, 6 to 7 days a week without losing any muscle - as long as the proper nutritional support is provided.