Why can't I eat 1200 calories of junk food??

You would lose a lot initially. But in the long term, the body would react to the nutrient defiencies to preserve itself from starvation.

Sugar is pure carbs. Carbs are the only thing that you don't *need* to eat, you body will produce glucose for itself out of protein and to a lesser extent fat. So if you are eating pure sugar, your body will have to cannibalize itself to get it's protein requirements. When you eat sugar (carbs) your body produces lots of insulin, whose job it is to get the sugar into the blood so it can be used for energy. One of insulins other jobs is to STORE excess energy as fat. So eating 1200 calories of pure sugar a day will spike your insulin and cause what can't be used immediately to be stored as fat, limiting weight loss.

I don't believe that you wouldn't lose weight, though. It all boils down to calories in vs. calories out, even if all calories are not created equal. Even if it were just muscle loss, you would still notice weight loss on the scale.

I don't want to do the Kreb's Cycle lesson again!!!! CARBS ARE NOT OPTIONAL! EAT THEM!

What you just said is the equivalent of

Sugar is pure carbs. Carbs are the only thing that you don't *need* to eat, you body will produce glucose for itself out of protein and to a lesser extent fat. So if you are eating pure sugar, your body will have to cannibalize itself to get it's protein requirements. When you eat sugar (carbs) your body produces lots of insulin, whose job it is to get the sugar into the blood so it can be used for energy. One of insulins other jobs is to STORE excess energy as fat. So eating 1200 calories of pure sugar a day will spike your insulin and cause what can't be used immediately to be stored as fat, limiting weight loss.

I don't believe that you wouldn't lose weight, though. It all boils down to calories in vs. calories out, even if all calories are not created equal. Even if it were just muscle loss, you would still notice weight loss on the scale.

I don't want to do the Kreb's Cycle lesson again!!!! CARBS ARE NOT OPTIONAL! EAT THEM!

Of course carbs are optional. Do I eat them? Yes. Can you live without them? Yes. Have you heard of glucogenisis? It is a metabolic pathway where (very, very basically) your body turns excess amounts of protein, and to a lesser extent fat, into glycogen and glucose in the blood. Glucogenesis is a less effective form of making glycogen and glucose, which raises the amount of calories burned to produce the blood glucose the brain needs. Now if we are talking about how to eat the least amount of calories and not lose weight, I might say eat carbs. But since this is a weightloss forum, I am going out on a limb to say that we are looking for the opposite effect. You would still have to eat protein and fat, as the body can not produce these macros itself (at least not protein and only incompletely fat).

Fat can only be stored in the presence of insulin, as insulin is the primary hormonal regulator of adipose tissue and fat metabolism. When insulin levels are raised (excess insulin, for example Type II diabetes) fat is stored in adipose tissue. When insulin levels decrease, fat is realeased into the blood as energy. Now obviously, starches and sugars (carbs) provoke high insulin secretion. In the absence of carbs insulin is still released, but in much smaller quantities to account for the much smaller sugar load. Now, excess glucose will result in fat storage, decreasing energy available for burning and resulting in hunger. Eating fat and protein results in a much smaller insulin release, which results in much less energy storage in adipose tissue (near nil), increasing available energy in the blood. This is the reason why you feel hungry an hour after a 500 calorie donut and not hungry an hour after a 350 calorie steak.

Regarding the Krebs Cycle. Instead of sitting here trying to put it all into my own words, I will quote an easily understandable passage from wisegeek.com:

"Within the Krebs cycle, energy in the form of ATP is usually derived from the breakdown of glucose, although fats and proteins can also be utilized as energy sources. Since glucose can pass through cell membranes, it transports energy from one part of the body to another. The Krebs cycle affects all types of life and is, as such, the metabolic pathway within the cells. This pathway chemically converts carbohydrates, fats, and proteins into carbon dioxide, and converts water into serviceable energy."
http://www.wisegeek.com/what-is-the-krebs-cycle.htm

So basically, I am not sure what you are getting at insinuating that I don't know what I'm talking about because I didn't mention the Krebs Cycle, which explains how ALL MACROS are metabolised. It has nothing to do with carbs per se and does not support your theory of "CARBS ARE NOT OPTIONAL!"