The more muscle you have on your body the more calories you will burn.
Your metabolism is a measurement of how many calories you burn each day.
If your metabolism slows down, it will from a cardio program, then it will be much HARDER to lose fat. So you struggle. And you think, "OK, I'll just do MORE cardio." But that's the worst decision you could make. It will only slow you down more.
The result of dieting and cardio often leaves you "Skinny Fat" - a curse worse than chubby - because you now you have no muscle and a lower metabolic rate. Double YIKES!