Zinc is found in all tissues, organs, body fluids and secretions of the human body. 95% of the total zinc is found in cells and 60% to 80% of the total zinc is found in the cytoplasm. Zinc content in tissues is highest in the prostate, followed by the liver, bones and muscles. Zinc in bones is regulated by parathyroid hormone and vitamin D, which can supply the body with what it needs for growth when exogenous zinc is insufficient. Zinc plays an important role in growth, intellectual development, immune function, substance metabolism and reproductive function. Physiological role of zinc 1, component of enzymes or enzyme activator There are about 200 kinds of zinc-containing enzymes in the body, among which the main zinc-containing enzymes are superoxide dismutase, malate dehydrogenase, alkaline phosphatase, lactate dehydrogenase, etc. These enzymes play an important role in participating in tissue whistling, energy metabolism and antioxidant process. Zinc is an essential trace element for maintaining the activities of RNA polymutase, DNA polymutase and reverse transcriptase. 2.Promote growth and tissue regeneration Zinc is involved in protein synthesis, cell growth, division and differentiation processes. Zinc deficiency can cause RNA, DNA and protein synthesis disorders, cell division is reduced, resulting in growth arrest. Zinc is involved in the metabolism of luteinizing hormone, follicle stimulating hormone, gonadotropin and other related endocrine hormones, and has an important regulatory role in fetal growth and development, sex organ and sexual function development. 3.Maintain cell membrane structure Zinc can act with various groups and receptors on the cell membrane to enhance membrane stability and resistance to oxygen free radicals. Zinc deficiency can cause oxidative damage to the membrane, structural deformation, and functional changes in the membrane carriers and transport proteins. The effects of zinc on membrane function are also manifested in the effects on barrier function, transport function and receptor binding. 4.Promote the immune function of the body Zinc can promote lymphocyte mitosis and increase the number and vitality of T cells. The regulatory effect of zinc on the immune function of the body is mainly manifested in the control of peripheral blood mononuclear cells synthesis and secretion of a variety of immunomodulatory factors; zinc deficiency can cause thymus atrophy, thymic hormone reduction, impaired T-cell function and cell-mediated immune function changes. Zinc deficiency may affect taste and appetite, and may even lead to heterophagia. In addition, zinc has protective effects on skin and vision, and zinc deficiency may cause skin roughness and epithelial keratinization. Zinc absorption and metabolism Zinc is mainly absorbed in the duodenum and jejunum, but also in the ileum. The absorption rate is about 30%. The zinc absorbed from the intestine is first concentrated in the liver and then distributed to other tissues. Zinc in plasma is mainly bound to albumin, transferrin, alpha-2 macroglobulin and immunoglobulin G. It enters the portal circulation with the blood and is distributed to all organs and tissues, and is easily absorbed by the tissues after forming a complex with albumin. The absorption of zinc by the body is related to the concentration of zinc in the intestinal lumen, and the absorption rate increases in the absence of zinc in the body. The high concentration of zinc in the body induces an increase in the synthesis of hepatic metallothionein, which is bound and stored in the intestinal mucosa cells, and then released into the intestinal lumen when the zinc level decreases, thus regulating the balance of zinc in the body. After metabolism, zinc is mainly excreted from the intestine and to a lesser extent from the urine, and to a lesser extent from sweat and hair. Factors affecting zinc absorption: high protein, vitamin D3 and glucose can promote zinc absorption; dietary fiber and phytic acid can reduce zinc absorption; copper, calcium and ferrous ions can inhibit zinc absorption; zinc bioavailability is higher in animal foods. 2. Zinc deficiency and overdose Zinc deficiency The main factors causing zinc deficiency are: ① unbalanced dietary intake, low intake of animal food, partial eating habits, etc.; ② increased special physiological needs, such as the increased zinc needs of pregnant women, lactating mothers and infants, which can easily cause insufficient zinc intake; ③ diarrhea, acute infection, kidney disease, diabetes, trauma and certain diuretic drugs increase zinc excretion. Zinc deficiency can affect cellular nucleic acid protein synthesis, taste bud cell renewal, mucosal hyperplasia, keratinization insufficiency, and reduction of phosphatase in saliva, which can lead to symptoms such as loss of appetite, heterophagia, and growth arrest, and long-term zinc deficiency in children can lead to dwarfism. Long-term zinc deficiency in adults can lead to symptoms such as hypogonadism, reduced sperm count, fetal malformations, rough skin, and reduced immunity. Zinc overdose Blindly over-supplementing zinc or consuming food and beverages contaminated with zinc from galvanized containers and packaging materials can cause zinc overdose or zinc poisoning. Excess zinc can interfere with the absorption and utilization of copper, iron and other trace elements, affect neutrophil and macrophage activity, inhibit cell killing ability and impair immune function. Zinc toxicity can occur in adults with intake of >2g zinc, causing clinical symptoms such as acute abdominal pain, diarrhea, nausea and vomiting. Nutritional evaluation of zinc Currently, the nutritional evaluation of zinc is mainly determined by biochemical and functional indicators combined with dietary status surveys. The common clinical symptoms of zinc deficiency in humans are slow growth, poor healing of skin wounds, taste disorders, gastrointestinal disorders and reduced immune function. 1. Biochemical indicators Serum (plasma) zinc concentration is not considered as a good indicator to evaluate zinc nutritional status because it is more stable and does not change with zinc intake. Therefore, the detection of serum zinc, hair zinc and salivary zinc indicators are only used as a reference for the evaluation of zinc nutritional status. 2.Functional indicators The functional effect is evaluated by the changes of enzyme activity, taste and dark adaptation ability, such as plasma alkaline phosphatase is the most commonly used indicator to evaluate the zinc nutritional status. 3.Dietary survey Through scientific and reasonable dietary nutritional status survey, understanding dietary habits and food zinc intake can help the evaluation of zinc nutritional status, but it should be considered that food zinc is greatly influenced by the environment of growth, and the application of food composition table should be cautious. 4.Reference intake of zinc and food sources The maximum tolerable dose (UL) of zinc for adult males is 45 mg/day and 37 mg/day for females in the dietary reference intake of zinc recommended by the Chinese Nutrition Society. Zinc is available from a wide range of sources, including shellfish (e.g. oysters, oyster meat, razor clams, scallops), red meat and its offal. In addition, eggs, beans, cereal germ, oats and peanuts are also rich in zinc, while vegetables and fruits are lower in zinc. Foods with high zinc content, see Table 1.