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"...the burn order for macronutrients is protein > glucose > fats."

I'm a little confused about this statement. Doesn't the burn order depend on the ratio of the nutrients in the your blood serum at a given time? I also thought it was glucose, protein, then fat. If no glucose, then your body starts to cannibalize it's muscles for protein, while converting fat also.

When you eat protein (and almost all Americans eat protein far in excess of amino acid needs for protein synthesis), the body uses what it needs for protein synthesis and metabolizes the rest for energy since there is no storage form for protein. The body is constantly recycling proteins to amino acids and synthesizing them back to proteins anyway. If your protein needs exceed the amount available in this recycled pool plus dietary protein, then your body will start using the protein in skeletal muscle to get what it needs. Some call skeletal muscles amino acid stores, but they're really not a storage form any more than a frame house is a lumber warehouse. This kind of wasting doesn't occur until you get into starvation territory (look at the Minnesota Starvation Experiment for some interesting reading). Protein metabolism involves both the glycolytic and lipolytic pathways since different amino acids can be oxidized in different ways. Because a typical American diet has excess protein, the portion in excess of amino acid needs is burned in the fuel cycle. Glucose can be stored via glycogen and doesn't necessarily have to be used immediately.

If you are consistently taking in more energy than you expend, the substrate usage shifts toward glucose use and away from fat use: you can store hundreds of pounds of fat; you can't store much glucose before starting to screw up your liver (think force-feeding of geese to make the special kind of liver used in pate).

If you entirely excluded glucose from your diet (or any other sugar that could be converted into glucose or an intermediate in the glycolytic pathway), you'd have to start using skeletal muscle to get the amino acids that can be burned in glycolytic pathway (so your brain can stay alive).

And where does the glycemic valuation of a given carb fit into your burn order - if at all? Weight lifters are told to eat more carbs then protein after workouts because of glycogen depletion and that certain carbs will restore the ATP quicker than protein. Of course, they need additional protein to repair the muscle fiber breakdown. Maybe I'm mixing up the terms or relying on old info.

The glycemic value of carbs is how quickly they are available as glucose. Saltine crackers have a really high glycemic index. Others, like grains of wheat, have lower glycemic index. This quickness of availability is in the gut during digestion. Starches are broken down into mono and disaccharides. Once monosaccharides are transported across the intestinal lumen, it's irrelevant where they originally came from. The only time it would make any real difference is if a diabetic is crashing from low blood sugar. You'd want to give something that required as few digestive enzyme steps as possible to get it into the bloodstream, so you wouldn't say, "Here's a baked potato." A glass of orange juice with some table sugar takes someone nearly passed out on the ground to up on his feet and bitching about how sweet it is in a matter of minutes because the fructose can be absorbed as soon as it reaches the brush border of the small intestine and sucrose (glucose + fructose) is almost as fast (I know this from experience with a friend who is a type 1 diabetic).

If someone is doing a workout to build muscles, he's using mostly aerobic, fat oxidation. The basic metabolism of muscles is fat oxidation. When you consider that long distance runners and cyclists expend far more energy over far longer periods of time than body builders without depleting their ATP (if they did, they'd die--think of why you die from cyanide), body builders are in no danger from this. And why would they think they have to quickly replenish ATP after a workout? Or else their muscles that they're trying to build will get catabolized to get the fuel to make the ATP? As long as they have any kind of nutrients in their last meal, it's going to be used to make ATP. This ATP synthesis is going on all the time. If you suddenly interrupt the process (think cyanide poisoning), you have enough ATP available for a few minutes of (unconscious) life. So their recently-exercised muscles aren't going to get eaten up to provide substrate for ATP synthesis. Of course, the necessary muscle tissue damage that ensues from the workout that triggers the synthesis of more muscle provides some amino acids that are used both for fuel and for amino acid synthesis.