The basic structural units, or building blocks, of protein. The body uses protein to build up and repair tissue. Protein is found in muscles, organs, bones, and skin and in many of the body’s hormones, or chemical messengers. There are about 20 amino acids in the human body, and they can combine in different ways to form a vast array of proteins with very different properties. For example, the insulin molecule is composed of a chain of 51 amino acids. Interestingly, human insulin varies from pig insulin by only a single amino acid along this chain.
Plants and bacteria can manufacture all the amino acids they need, but there are about nine that the human body needs but cannot make. These are called essential amino acids, because they must be obtained from food. Sources of dietary protein, including meat, poultry, fish, eggs, cheese, milk, grains, legumes, and nuts, can provide the essential amino acids.