There have been studies that show increased protien intake increased thermogenosis temporarily. After 6-8 weeks of high protien intake, the body adapts, and protien no longer yields this effect in requiring more energy expenditure to break down and use than other micronutrients per calorie. If we are talking about calories consumed verses calories actually absorbed and able to utilize by the body, fat calories are best. If you ate a meal that had 500 calories for example that was entirely carbs, other than fiber, you will absorb almost, if not all 500 calories. Protien will be a little lower, and you will not be able to absorb anywhere near all 500 of the fat calories. The calorie content of food is based on potential heat value. The body is not a blast furnace, and does not absorb, breakdown or utilize 100% of the calories present, and it varies from food type to food type. Thus the expression "a calorie is a calorie" is not exactally true when dealing with something as complex as a living organism. Want the greatest metabolic advantage for every calorie you eat? Do the Adkins diet, but just because you have a better metabolic advantage, does not mean you will achieve the best fat loss possible. Your tyroid will downregulate and your glycogen stores will never be full.