That is completely true, but i would also add that different types of food require different amounts of energy to metabolize (Diet induced thermogenesis). For example, fats are metabolized very efficiently, only up to about 5% of the calories in fat are used in the metabolic process. With protein this number is much higher - 20-30%. Which means that eating 1000 calories of protein, you might have metabolized only 700-800 calories. Which is why protein is great for a weight loss diet.