This article is about protein as a nutrient. For protein protein per oz of beef a class of molecule, see Protein.

For other uses, see Bodybuilding supplement. Amino acids are the building blocks of protein. Present in every cell, they are also precursors to nucleic acids, co-enzymes, hormones, immune response, repair and other molecules essential for life. Proteins are essential nutrients for the human body. They are one of the building blocks of body tissue and can also serve as a fuel source. Proteins are polymer chains made of amino acids linked together by peptide bonds. Protein is a nutrient needed by the human body for growth and maintenance.

Aside from water, proteins are the most abundant kind of molecules in the body. Protein can be found in all cells of the body and is the major structural component of all cells in the body, especially muscle. This also includes body organs, hair and skin. Proteins are also used in membranes, such as glycoproteins. Protein occurs in a wide range of food. Meat, dairy, eggs, soy, fish, whole grains, and cereals are sources of protein. Vegan sources of proteins include legumes, nuts, seeds and fruits.