Protein is the main component of tissues and cells, and protein is also contained in body fluids. The nutritional function of protein lies in its various amino acids. There are more than 20 kinds of amino acids that make up food protein, some of which cannot be synthesized by humans and animals and must be obtained from food. These amino acids are called "essential amino acids", namely methionine, lysine, tryptophan, threonine, valine, phenylalanine, leucine and isoleucine. In addition, children need histidine for growth, and birds such as chickens need arginine and glycine. Amino acids other than these essential amino acids are called "nonessential amino acids" because they can be synthesized in vivo.

The types and contents of amino acids in protein are different. Some protein lacks certain essential amino acids, such as gelatin protein without tryptophan and corn gluten without lysine. Therefore, evaluating the nutritional value of a food protein mainly depends on whether the amount of various essential amino acids it contains can meet the needs of the body. When it is insufficient, the body cannot effectively synthesize protein, and other amino acids can only produce sugar (glycogen heterogenesis) through deamination metabolism and provide heat energy as fuel. Therefore, the amino acid pattern of food protein is the key to determine its quality. At present, the nutritional value of food in protein is evaluated based on the whole egg essential amino acid model, or the essential amino acid model in human milk, or the hypothetical model based on the amount of essential amino acids in human body. This is the so-called evaluation method of nutritional value in protein. In addition, there is a biological evaluation method, which evaluates the nutrition according to the utilization rate of food protein in the body. Commonly used are "protein physiological value" (percentage of nitrogen retained and absorbed in the body), "net protein utilization rate" (percentage of nitrogen retained and absorbed in the body), or "protein efficiency ratio" (weight gain per gram of protein).