Protein is a high-molecular compound， especially high in nitrogen ， existing in all the living creatures and consists of many amino acids. It is the foundation of all life and important element of forming， upgrading， and repairing organization and cellular， which affects the material metabolism and control and regulate physiological function. The element provides engergy for human body and is one of the three majjor nutrients for human body.

Human body is composed of 20 types of anino acids， 8 of which （9 for infants） are not produced by synthesis inside the body or its generating speed can not meet the need of organism， and must be replenished from food. Such amino acids are called essential amino-aicd.

Inorganic salt， also be called mineral substance， is a ingorganic nutrients mainly found in food. It is a key factor of organism such as bone， teeth and others， which plays an adjusting role in maintaining our body function.

In the opinion of chemists and nutritionsts， no matter how diversified the food， in essence， they are combinations of various nutrients， six major of which are protein， fat， carbohydrate， vitamin， ingrganic salt and water. Some believe that dietary fiber（DF） is the seventh element of major nutrients.

Carbohydrate（sugar）， with another name saccharides， is a organic compound with widespread existence in nature and the largest amount of all compounds. The element， mainly consisting of carbon， hydrogen and oxygen， is one of the three major nutrients and key factor of organization and cell.

Nutrient， also named nutritive substance， is a chemical substance serving as providing engergy and organic components to human body， as well as repairing organism and adjusting physiological treatment， and also a food composition that has to be intaken to maintain normal life activities.

Vitamin is a joint name of a series of organic compounds， and an essential trace element to maintain normal life activities， which plays an important part in the process of organisms metabolism， growth and development. Most vitamin can not be compounded inside human body， nor can be stored in large amount， so they must be provided by food.

According to the chemical structure and physiological function， as well as on the basis of solubility， vitamin falls into two main categories： lipid-soluble vitamin（ vitamin A， D， E， K） and water-soluble vitamin（ vitamin B complex and vitamin C）

Lipid is a compound which is soluble in organic solvent but insoluble in water， and can fall into fat and lipoid， also one of the three major nutrients for human body. It serves as helping the absorbing of lipid-soluble vitamin， protect and fix viscera， prevent the loss of heat， keeping normal temperature and others.

Water is the major element of body fluid and essential factor to sustain life， playing the role of adjusting temperature， transporting substances， promoting chemical reaction and lubricating.