How many calories are in a gram of protein?

There are 4 Calories in a gram of protein, 4 calories in a gram or carbohydrates and 9 calories in a gram of fat. Protein is the last source of energy the body uses (otherwise you would be losing muscle). Fat is obviously the hardest to burn because it is only used once the carbs are used up, and fat contains more than 2x the calories per gram.