Minsc,
You are right in the fact that when you run your body does use energy. I think that you'll find that a lot of the energy you use is going to come from glucose which is in your muscles, in your blood stream, or it's actually going to come from fat which does break down and does get used for energy. (Usually in slow state cardio such as jogging.) Higher intensity does have the possibility of breaking down muscle, but not usually a significant amount.
If you are eating enough in the day moderate cardio should not be a problem in the least bit. Even when in a calorie deficit (Losing weight) people are still able to retain muscle mass even while doing lots of cardio. (Just look at bodybuilders, natural, or pro for proof.) They all have lots of muscle, and I am almost 100% positive they all do cardio to at least prepare for their competitions.
Personally I do 30 mins of running 3 times a week. I do it when I'm bulking or cutting. I think it's a healthy thing to do.
It's not necessary to do, but it can speed your results of losing weight up, or it can hinder your weight gain. But in the end this is primarily because cardio burns calories and calories are responsible for weight loss and gain.
If you want to get more complicated we can talk macronutrients to gain further efficiencies.
But these are the basics.
Bottom line:
If you eat enough and do moderate cardio, muscle loss should be non-existant and you should actually gain muscle instead of losing muscle.
Any muscle loss or breakdown will actually stimulate future muscle growth and your excess amount of calories will be the building blocks for that growth.
No building blocks = no growth.  |