Exogenous substances required by organisms to maintain normal life activities and ensure growth and reproduction are called nutritional elements, which are composed of six categories: water, minerals, carbohydrates (sugar), fats, proteins and vitamins. Among them, water and mineral salts are inorganic substances, while fats, proteins and vitamins are organic substances. In addition to the macroelements with a high content in mineral salts, some of them are very small but also participate in many life activities of the body, which are called trace elements. The above-mentioned nutritional elements enter the body through the act of eating. The normal eating behavior is controlled by many life activities at the behavioral level such as nerves and endocrines, among which the hypothalamus plays a very important role. But in fact, in human society, eating behavior is also restricted by culture, family, personal experience, personal economic conditions and market supply. The content of food is absorbed through the gastrointestinal tract. Most nutrients with larger molecules are first converted into soluble unit nutrients with smaller molecular weight in the digestive tract, and then absorbed by intestinal epithelial cells. Many nutritional elements need to bind to transport proteins when absorbed, and the activity of the latter is often affected by the concentration of the substance in the body and the related hormones involved in the metabolism of the substance. The absorbed nutrients can be sent to various organs and tissues for utilization. In addition, many nutrients can be stored in the body, especially energy substances. When the body needs them, these stored substances can be released into the circulation for utilization.