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this is some of the worst information i've heard...
Originally posted by cait987
Pretty much, when you run, your body cant keep up with the intensity and then naturally turns to the greatest energy source (carbs), however, what happens when carbs run out? Well naturally your wonderful body goes to the next best thing, Muscle! Muscle provides aprox 17x the energy that fat does so the body naturally relies on muscle due to cartigens(SP?) being released to bring more energy to the body part that is lacking.
1) unless you are on atkins, there's no chance of running out of carbs for energy when doing high intensity exercise. you'll fatigue due to lactic acid accumulation long before that ever happens.
2)muscle does not provide 17x the energy of fat. i don't know where you got that, but it's wrong. one gram of fat gives 9 calories, while 1 gram of protein gives 4. muscle is made up of protiens, and when it is broken down for energy, it is broken down to protein which is then converted into glucose.
3)the body turns to fat long before it turns to muscle because fat is more calorie dense.
4) cartigens? i have no idea what your talking about. do you mean cortisol? cortisol (sp?) is produced in your body by stress, and does promote muscle catabolism, but it is not a concern until you've been doing about an hour of cardio. if you're doing less than that, don't worry about it.
5) have you ever seen an olympic sprinter and marathoner? which is more musculer? which performs a more intense workout? point made?
However, by keeping your heart rate at a steady pace (65% aprox)
65% of what? max heart rate? heart rate reserve? vo2max? vo2max reserve?
your body can keep up with the intensity in a sense and instead of being forced to massively devour muscle/carbs it can rely entirely on fat once the carbs run out,
giving out the arbitrary 65% is meaningless. everyone is different and at different fitness levels. you may be able to work out at that level, while someone who is very out of shape has to start out working out at 40%, but it's relatively the same intensity for the two of you.
your body is never just burning carbs or just burning fat. there's something called the respitory exchange ratio (RER) that can be used to determine how much of each is being burned at one time. an RER of .7 means your buring only fat, while an RER of 1.0 means all carb. at above 1.0 your starting to hyperventilate. at .85 your burning equal amounts carb/fat. somewhere around 60-65% of your vo2max is where you hit .85 (might be a bit lower, i'd have to look it up). so when you're running at a slow easy pace, you're using mainly fat as energy, but when you go at a high intensity, you burn mainly suger. protein is not normally used for energy unless you have entered "starvation mode" or are running a marathon. in a marathon, towards the end, you will have burned up all the glycogen stores and then protein will start being burned, for about 15-20% of the energy used with the rest coming from fat.
pretty much, your body beleives its going to die and just like animals storing all their food as fat for when they get hungry, your body digests anything it can get ahold of for the most energy so you dont collapse dead which is carbs -> muscle/fat at high intensity
carbs -> fat -> muscle at medium intensity
i've already said what's wrong with this, i do believe.
Now dont get me wrong, high intensity workouts do work, why? Because you deplete your carb stores in your body so naturally all those extra carbs are then burned off and arent stored as fat like they would naturally be, however, go to long and fat isnt the only thing you are losing ...
that's part of it, along with EPOC (excess post-exercise oxygen consumption, although recent studies suggest that may be much less of a contributor than previously beleived.).
to answer the original question, you don't need to worry about burning muscle unless you have long cardio sessions. intensity won't burn muscle, duration will. higher intensity will fatigue you quicker before muscle is ever started being burned.
edited for format
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