I don't think that this is true, I really hope it's not. I know if one doesn't eat much, or fuel their body and muscles they will lose strength and muscle mass. Anorexia causes the body to break down muscle, bone density, and even the body's organs. I'm pretty sure that if your eating healthfully, and enough, you aren't losing muscle after 30 minutes of cardio. I'm not completly positive on this, but this is what I've read in magazines and such. Maybe someone else knows more about this.