Nutrition is required by a person throughout his/her life. It is an integrated process for the human body to get and make use of food to maintain a normal physiological, biochemical, immunologic functions, growth and development, metabolism and so on.

The nutritional substance in the food is called nutrient. There are six categories of nutrients needed by human body: protein, fat, carbohydrate (sugar), inorganic salt, vitamin and water. Among the six categories, only the first three categories of nutrients can produce heat, therefore, they are also called heat-production nutrients.

Every nutrient has a certain kind of physiological function, and sometimes, a certain nutrient can have several kinds of physiological functions. Physiological functions of these nutrients include body tissue structuring, heat energy supplying and physiological function adjusting.