Why Cardio Burns Muscle?

Hi All,

I just had a quick question and was hoping to get some feed back.  I was talking to my friend about exercise and how it is commonly agreed that by doing cardio you loose more weight but a higher percentage of that weight is muscle.  He seemed to be under the impression that doing cardio burns  the current calories you have from eating then goes to burning fat, and I really didn't have a good, scientific reason on why cardio burns more muscle.  So I was hoping that someone could explain what actually happens in the body that causes it to burn muscle while doing a lot of cardio.  Any input, suggestions, ideas would be much appreciated.

Thanks All!

Wow, thanks for all the replies, it's always great to hear both sides of something.  So it seems that everyone (almost) is in general agreement that muscle can be lost (albeit not necessarily 'burned') through high intensity Cardio work outs or through insufficiently suppling for the muscle in your body with what it needs.  So I guess the questions that are begging to be asked are what is 'high intensity' and what is 'insufficient' as far as your diet goes.  Is someone that is trying to loose weight, say in a 500-1000 calorie deficit (I maintain at 3400 and am trying to target my daily intake at 2400+, with 30-50 percent coming from protein usually), providing insufficient dietary fuels to their muscles?  And what is a 'high intensity' cardio work out?  Is it doing cardio for 45 mins or 60 mins or 90 mins a day?  Is it all about your heart rate and how long you keep it elevated for? 

Anyways thanks for all the great replies and info on the subject.  Like I said Im just trying to understand what exactly is going on in the body, and everyone here has so much knowledge that I might as well tap into that knowledge.  Thanks again everyone for the replies and taking the time to comment, and keep those answers coming!

Argh.

That first sentence was supposed to read " Eastside: So, this has been invalidated?"

It was supposed to be a question, not a flame, but without the link it turned into something quite different.

Recognizing that a) Biotest has no interest in publicising information that would undermine sales of their supplements, b) Berardi is sponsored by Biotest, and c) you know more about the subject than I do (as the leucine issue demonstrated), I meant to get your opinion on my current understanding of the issue.

 My field is chemistry and chemical engineering, not biochemistry and sports medicine, and while I can read a paper my understanding is less comprehensive than someone with a deeper background in the field - which is why I'm deferring to Bodyscience and (now) you on these questions.

 With that out of the way - has this been invalidated then?

Well, I am really not sure where all this leads.  melkor, are you suggesting that cardio workouts are a bad thing?

What are the symptoms of this loss of muscle?

This is really important because I have been a cardio guy all my life and I have probably lost every muscle in my body by now.  For instance I have ridden over 6000 miles in the last 16 months.  In August I rode 105 miles in a single day.  There just can't be much of me left.  The funny thing is that my weight and %BF have been stable for the past year.  I would think that my lean body mass should be declinging from all of this muscle burning.  What am I missing?

 Sports drink with sugar? The presence of insulin has an anti-catabolic effect, or so the linked article says anyway. I feel like crap when I drink that stuff though, and my workouts suffer. Which is probably why I'm not a big cardio person.

 Don't look at me though - that was my question more or less: I've looked over the references Berardi provides, and I buy his take on things, but there's still the issue of us humans being tougher than practically anything else on the planet when it comes to endurance.

 Only canines have the endurance and fortitude to keep up with us, we can literally chase to death any other animal. Well, you cardio guys can anyway. I wouldn't bet on myself doing that... (And while you could probably endurance-chase a cheetah or tiger to death if they'd run away when you came close, I wouldn't trust them to not stand their ground and wait for dinner to come up to them....)

 So how does this work, really? There's a hole in my understanding somewhere, which is why I'm asking the question.

(Edited to add:) Or would have asked, if I'd managed to link to Berardi's article the first time around. Or made myself clear this second time around. Does this third try make it abundantly clear that I'm confused, because on the one hand cardio should put you in a negative protein balance, on the other hand, humans as a class have an endurance physiology that outclasses any land-based animal that isn't a canine, and on the gripping hand there's how nutrition interacts with physiology?

Wow ! I did not know this. I always work out hard. My assumption was that the "fat burn cycle" was on the machines because they did not want fat people to have a heart attack .... But what you say makes sense. I am going to try slowing down a little for a week or two and see if it helps 

Ski Wolf