The protein of the element, its role in food nutrition is obvious, but it can not be used directly in the human body, but by becoming the amino acid small molecule to be used. That is, in the human gastrointestinal tract is not directly absorbed by the body, but in the gastrointestinal tract through a variety of digestive enzymes, the polymer protein decomposition into a low molecular polypeptide or amino acids, in the small intestine is absorbed, along the liver portal vein into the liver. Some amino acids are decomposed or synthesized in the liver, while the other part of the amino acid continues to distribute with blood to various tissues and organs, allowing it to be selected to synthesize a variety of specific tissue proteins. Under normal conditions, amino acids in the blood are almost equal to their output speed, so the amino acid content in normal blood is fairly constant. In the case of amino nitrogen, the content of each hundred ml plasma is 4~6 mg, the content of each hundred ml blood corpuscles is 6.5~9.6 mg. After a meal of protein, a large number of amino acids were absorbed, the blood amino acid level temporarily elevated, after 6-7 hours, the content returned to normal. It is indicated that amino acid metabolism is in homeostasis, and the blood amino acid is the balance junction, and the liver is an important regulator of blood amino acid. As a result, food proteins are digested into amino acids and then absorbed by the body, which uses these amino acids to synthesize its own proteins. The human need for protein is actually the need for amino acids.