Protein

Main article: Protein in nutrition
Proteins are the basis of many animal body structures (e.g. muscles, skin, and hair). Each molecule is composed of amino acids, sometimes many thousands, which are characterized by inclusion of nitrogen and sometimes sulphur.

The body requires amino acids to produce new proteins (protein retention) and to replace damaged proteins (maintenance). Excess amino acids are discarded, typically in the urine. For all animals, some amino acids are essential (an animal cannot produce them internally) and some are non-essential (the animal can produce them from other nitrogen-containing compounds).

About twenty amino acid are found in the human body, and about ten of these are essential, and therefore must be included in the diet. A diet that contains adequate amounts of amino acids (especially those that are essential) is particularly important when there is greater need: in early development and maturation, pregnancy, lactation, or injury.

A complete protein source contains all the essential amino acids; an incomplete protein source lacks one or more essential amino acid. It is possible to combine two incomplete protein sources (e.g. rice and beans) to make a complete protein source. Sources of dietary protein include meats, tofu and other soy-products, eggs, grains, legumes, and dairy products such as milk and cheese. A few amino acids from protein can be converted into glucose and used for fuel through a process called gluconeogenesis. The amino acids remaining after such conversion are discarded.