Proteins After water, proteins are the most abundant substances in the body. –Used to repair bone, muscle, skin and blood cells, and are key elements of antibodies. –Proteins help transport iron, oxygen, and nutrients to all body cells. Structure and Sources of Proteins –Amino acids are the building blocks of proteins; 9 of the 20 are essential because they must be obtained from food; the other 11 can be produced by the body. –Dietary protein that supplies all 9 essential amino acids is known as complete (high-quality) protein. –Proteins from plant sources lack one or more amino acids and are known as incomplete proteins.