Protein is an important component of every cell in the body. Hair and nails are mostly made of protein. Your body uses protein to build and repair tissues. You also use protein to make enzymes, hormones, and other body chemicals. Protein is an important building block of bones, muscles, cartilage, skin, and blood.
Along with fat and carbohydrates, protein is a “macronutrient,” meaning that the body needs relatively large amounts of it. Vitamins and minerals, which are needed in only small quantities, are called “micronutrients.” But unlike fat and carbohydrates, the body does not store protein, and therefore has no reservoir to draw on when it needs a new supply.