I should make some graphs to explain this one of these days, but...
The reason why steady cardio = muscle eating is the fact that the enzyme Pyruvate Dehydrogenase catalyzes an irreversible step in glucolysis. Let me explain.
When burning sugar, the chain of reactions can be divided into two parts: One that happens regardless of whether there's oxygen or not, and a second part where the destination depends on the presence of oxygen.
1. Glucose (6 cabon atoms, 6C) <-> <-> <-> Pyruvate (2 x 3C). This chain of reactions is completely reversible, going left to right creates energy (ATP) and some reduced coenzymes (NADH). If you go the other way, you need to use energy and oxydize the coenzymes.
Now, NADH is limited, since it's a coenzyme. That means it's supposed to be used as intermediary of the reacions but not be altered itself. But that's okay, in the presence of oxygen, NADH + H+ + O2 -> NAD + H2O + Energy! That energy can be harnessed and create more usable ATP. Good stuff all around.
2a. Next step. IF THERE IS OXYGEN: Pyruvate (3C) -> Acetyl-Coenzyme A (2C). Acetyl-Coenzyme A has one destination, be turned into CO2 and lots of energy, with the use of Oxygen.
2b. If there isn't oxygen, the body has a problem. That problem is that it's using up all the NAD coenzyme to create NADH (see 1.), but it's not freeing it up. So, there is an "emergency dump", so to speak: Pyruvate + NADH + H+ <-> Lactate + NAD
And that's where lactic acid comes from. Now, the lactic acid reaction is completely reversible. So if the body's using up all the sugar with little oxygen (anaerobic), then it'll accumulate Lactate.
Anyway, the other way to obtain energy, fat, depends completely on the presence of oxygen. Why? Well, it creates reduced coenzymes too, but fat is broken down into Acetyl-Coenzyme A, always! And the way to use that is to burn it with oxygen (well, or create ketone bodies, but that's only mostly for transport, it'll need oxygen regardless). Because Pyruvate Dehydrogenase is irreversible there is no way to create sugar from Acetyl-Coenzyme A.
The point of all this? Think about what happens when you have depleted your supply of sugar.
Aerobic: You've burned it all into CO2 (together with some fat). But the body wants to keep some sugar around at all times, so it'll either take it from food, or it will try to regenerate it. However, there's no way to get sugar from fat (ironic, since fat from sugar is so easy), so it'll use whatever it can to regenerate that sugar (gluconeogenesis). And what can it use? Aminoacids. Several aminoacids can be converted into something in the first part of glucolysis that can be reversed and made into glucose.
If you go doing aerobics for a long time, way past your sugar reserves, then sure you'll burn fat. But your body will try to regenerate its standard level of sugar to have enough in blood, so it'll eat away at whatever aminoacids are available.
Anaerobic: You have a lot of lactic acid kicking about. So as soon as oxygen starts getting into the system your body will turn that lactic acid around and into glucose FRST, then it'll go after the aminoacids if it still needs more sugar.
The good news, no matter what way you went, gluconeogenesis will use the energy from fats to happen. It's a matter of what substrate it'll use to create those sugars.
Now, how does that concern you? How much aerobic you can do without eating at the muscle will depend on several things I cannot really predict, your overall level of fitness (how efficient your body is at moving while using as little energy as possible), how the regulation between the fat burning and sugar burning pathways in your metabolism is balanced, even if you've taken food and are digesting and absorbing sugar and free aminoacids while doing your cardio, etc etc. I'm a molecular biologist, so I'm not so good at the "big picture" of the overall body balance (there's hormonal and overall homeostasis processes involved, I can tell you a lot about what events are triggered by the different hormones in individual cells, but that's not what you'd likely be interested in!).
But bottom line: There is an unavoidable reason why Aerobic will eventually eat into the muscle, however how much of it you can do and reap benefits from will depend on quite a few factors, so it may work for you while not work so good for others.
Also note: Heart muscle is a bit on the special side, because it consumes fat with high preference over stored sugar. Probably evolved that way because of its origin (came from a specialization of vascular smooth muscle) and because it's always guaranteed to have the best supply of oxygen of the body. A strong heart beating away hard will use fat nicely.
The reason why steady cardio = muscle eating is the fact that the enzyme Pyruvate Dehydrogenase catalyzes an irreversible step in glucolysis. Let me explain.
When burning sugar, the chain of reactions can be divided into two parts: One that happens regardless of whether there's oxygen or not, and a second part where the destination depends on the presence of oxygen.
1. Glucose (6 cabon atoms, 6C) <-> <-> <-> Pyruvate (2 x 3C). This chain of reactions is completely reversible, going left to right creates energy (ATP) and some reduced coenzymes (NADH). If you go the other way, you need to use energy and oxydize the coenzymes.
Now, NADH is limited, since it's a coenzyme. That means it's supposed to be used as intermediary of the reacions but not be altered itself. But that's okay, in the presence of oxygen, NADH + H+ + O2 -> NAD + H2O + Energy! That energy can be harnessed and create more usable ATP. Good stuff all around.
2a. Next step. IF THERE IS OXYGEN: Pyruvate (3C) -> Acetyl-Coenzyme A (2C). Acetyl-Coenzyme A has one destination, be turned into CO2 and lots of energy, with the use of Oxygen.
2b. If there isn't oxygen, the body has a problem. That problem is that it's using up all the NAD coenzyme to create NADH (see 1.), but it's not freeing it up. So, there is an "emergency dump", so to speak: Pyruvate + NADH + H+ <-> Lactate + NAD
And that's where lactic acid comes from. Now, the lactic acid reaction is completely reversible. So if the body's using up all the sugar with little oxygen (anaerobic), then it'll accumulate Lactate.
Anyway, the other way to obtain energy, fat, depends completely on the presence of oxygen. Why? Well, it creates reduced coenzymes too, but fat is broken down into Acetyl-Coenzyme A, always! And the way to use that is to burn it with oxygen (well, or create ketone bodies, but that's only mostly for transport, it'll need oxygen regardless). Because Pyruvate Dehydrogenase is irreversible there is no way to create sugar from Acetyl-Coenzyme A.
The point of all this? Think about what happens when you have depleted your supply of sugar.
Aerobic: You've burned it all into CO2 (together with some fat). But the body wants to keep some sugar around at all times, so it'll either take it from food, or it will try to regenerate it. However, there's no way to get sugar from fat (ironic, since fat from sugar is so easy), so it'll use whatever it can to regenerate that sugar (gluconeogenesis). And what can it use? Aminoacids. Several aminoacids can be converted into something in the first part of glucolysis that can be reversed and made into glucose.
If you go doing aerobics for a long time, way past your sugar reserves, then sure you'll burn fat. But your body will try to regenerate its standard level of sugar to have enough in blood, so it'll eat away at whatever aminoacids are available.
Anaerobic: You have a lot of lactic acid kicking about. So as soon as oxygen starts getting into the system your body will turn that lactic acid around and into glucose FRST, then it'll go after the aminoacids if it still needs more sugar.
The good news, no matter what way you went, gluconeogenesis will use the energy from fats to happen. It's a matter of what substrate it'll use to create those sugars.
Now, how does that concern you? How much aerobic you can do without eating at the muscle will depend on several things I cannot really predict, your overall level of fitness (how efficient your body is at moving while using as little energy as possible), how the regulation between the fat burning and sugar burning pathways in your metabolism is balanced, even if you've taken food and are digesting and absorbing sugar and free aminoacids while doing your cardio, etc etc. I'm a molecular biologist, so I'm not so good at the "big picture" of the overall body balance (there's hormonal and overall homeostasis processes involved, I can tell you a lot about what events are triggered by the different hormones in individual cells, but that's not what you'd likely be interested in!).
But bottom line: There is an unavoidable reason why Aerobic will eventually eat into the muscle, however how much of it you can do and reap benefits from will depend on quite a few factors, so it may work for you while not work so good for others.
Also note: Heart muscle is a bit on the special side, because it consumes fat with high preference over stored sugar. Probably evolved that way because of its origin (came from a specialization of vascular smooth muscle) and because it's always guaranteed to have the best supply of oxygen of the body. A strong heart beating away hard will use fat nicely.
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