(1) Structural material of animal body Various tissues and organs of animal body use protein as structural material. Protein is the substance with the highest content in the animal body besides water, and generally accounts for about 50% of the solid matter in the animal body. The protein content of some tissues and organs such as muscle, liver and spleen can be as high as 80% or more. The reason why various tissues have specific physiological functions is mainly due to the different types and forms of proteins in their organs. Physiological and biochemical processes such as pregnancy, growth, lactation, hair production and egg production of animals all use specific proteins as their material basis. Among the three major nutrients of protein, fat and carbohydrate, only protein contains nitrogen. It is the only source of nitrogen for the animal body and cannot be replaced by other nutrients.

(2) Maintain normal metabolism. Tissue cells are renewed through the continuous decomposition and synthesis of proteins, which is the most basic feature of life. Even adult animals need to continuously consume protein to supplement the protein synthesis needs of body tissues when their body protein is basically constant. This is because not all the amino acids generated by tissue protein decomposition during the renewal process can be used to resynthesize protein. A small part of the amino acids undergo a series of changes and are decomposed into urea, uric acid and other metabolites and are excreted from the body. According to measurements, about 0.25 to 0.30 of the total animal protein is updated every day. Based on this calculation, all body tissue proteins are renewed every 12 to 14 months.

(3) Functional substances in the animal body. There are many important functional substances in the animal body, including enzymes and hormones that catalyze and regulate metabolic processes, immune bodies that enhance defense functions and disease resistance, and oxygen-bearing Transport carriers, etc., are mainly composed of proteins. In addition, the maintenance of acid-base balance, normal distribution of water, transmission of genetic information, and transport of many important substances in animals are all related to proteins.

(4) As an energy source, although the main nutritional and physiological function of protein is not oxidation and energy supply, due to the continuous destruction and decomposition of old tissues in the metabolism process of the animal body, during this process, protein can be oxidized to produce part of the energy. . In addition, if the animal body eats too much protein or insufficient energy, the protein may be oxidized to release energy. These energies eventually become the supplementary source of energy required by the body. In addition to direct oxidation to provide energy, protein can also convert the nitrogen-free part into body fat through deamination, which can be used as an energy reserve for emergency needs.