Wow, I don't think I've ever seen so many different theories about how your body works in one thread here at SP before, lol.
Basically, what happens to the protein you eat depends on what else you eat, and how much of it--not on some numerical limit on how much protein your body can handle.
Technically speaking, your body doesn't "burn" protein, glucose, OR fat as energy. The energy your cells actually use comes from breaking the chemical bonds of a molecule called ATP, which happens during something known as the "Krebs cycle" that takes place at the cellular level, in the mitochondria of your cells. Your body can use all 3 dietary macronutrients, fat, protein, and carbohyrdates, to generate ATP molecules, so it really doesn't make much practical sense to think in terms of whether your body turns protein into glucose, or into fat, etc.
In terms of weight loss, what matters is whether you're eating more or less than enough to satisy your body's energy requirements, as measured in "calories." When you eat more than your body can use either to generate ATP when your cells need immediate energy, or to restock the energy reserves stored as glycogen in your muscle cells and liver, then this excess energy will be stored as body fat. It will get taken out of storage when there is not enough "stuff" to make ATP out of floating around in your blood stream following the process of digesting your last meal.
As always, this stuff isn't really as simple as just doing the math of calories in versus calories out. Your body is very efficient, and does prefer to get what it needs from the nutrients best suited to provide it. It likes, for example, to "spare" dietary protein from being used to generate ATP for immediate energy needs, and use the amino acids in it instead for repairing and replacing various body tissues and other such purposes. It can do this if you're eating enough carbs and fat to meet your immediate energy needs. But if you eat a lot of protein, and not enough carbs and fat, it will use that protein to make ATP. If you don't eat enough period, your body will actually break down muscle and organ tissue so it can turn the protein in them into ATP for your immediate energy needs. That's why people lose muscle as well as fat when they are dieting. Likewise, your body has no way to store dietary protein as protein for future use (other than a very small pool of amino acids it keeps on hand), so the protein you eat either gets used right away, or it gets broken down and reassembled as fat. This is one of the reasons exercise is so important--it forces your body to keep repairing your muscles because you need them, instead of just letting them get broken down and turned into ATP.
Here's a relatively non-technical explanation of how this all works: recipes.howstuffworks.com/food2.htm
Hope this helps.
Edited by: SP_COACH_DEAN at: 1/31/2009 (23:13)