Protein is an essential nutrient that the body needs in order to grow, repair and form new tissue and cells, which includes muscles, blood, skin, hair, nails and internal organs. Protein also aids in wound healing (important after surgery), maintains lean body mass (which increases metabolism) and is necessary to prevent protein malnutrition. Protein is one of the three energy-producing nutrients, which provides 4 calories per gram of protein. High-protein foods slow the movement of food from the stomach to the intestine, which means you feel full longer. The body also uses more energy to digest protein than it does to digest carbohydrate or fat.