calories, calorie sources and weight management

1. calories in >calories out= weight gain
2. calories in < calories out = weight loss
3. calories in=calories out = weight stability

But aren't calories from different macronutrient sources processed differently by the body?
They are processed differently. Glucose is used as energy, fructose is turned into fat. Excess protein is flushed out in the urine. Since you NEED glucose, if you avoid carbohydrates, your body will turn the protein from its own tissue into glucose. This would result in death IF you didn't eat more protein to replenish your tissues. 

However, as far as weight loss/gain/stability is concerned, the basic rule of calories in/calories out still holds.

if all your calories came from fructose and got converted into fat, if they were still less calories than you needed that day, your body would then burn the the fat that the fructose had been converted into, plus some of the extra fat you already had.)

If all your calories came from protein, your body would disassemble its tissues to turn protein into glucose and would then reassemble some tissue from the new protein you ate.

If your calories all came from fat, your body would use fat stores for energy, and would also (after using up stored glucose) disassemble its tissues to turn protein into glucose.

So the basic rule of 
1. calories in >calories out= weight gain
2. calories in < calories out = weight loss
3. calories in=calories out = weight stability

holds true