Calories (energy) are contained in the protein, carbohydrates, and fats you consume every day. And it’s true that a calorie is not a calorie as far as how these macronutrients are metabolized in the body.
What that means is that when a certain amount of calories are consumed from fat, carbs or protein, each one induces a different thermic response.
Thermic means the number of calories required (burnt) to actually digest the nutrients.
For example, when 100 calories from carbohydrates are consumed the body burns 5 calories to digest these 100 calories.
The same amount of calories from fat induces a thermic response equivalent to 10 calories. Your body burns 10 calories digesting 100 calories of fat.
However, when you consume 100 calories from protein, this gives you a thermic response of 18 calories. It takes your body 18 calories to digest 100 grams of protein. That’s almost double the thermic response from fat and almost 4 times the calories or thermic response from carbohydrates.
This is why the amount of protein in your diet is so critical not only to building muscle, but also shedding body fat. It’s this difference in thermic response among the macronutrients that underlines the reason why all calorie sources are not the same.