Abstract: Selenium is an essential trace element in both animals and humans, playing a crucial role in boosting immunity, promoting growth, and supporting reproduction. However, both excessive and insufficient intake of selenium can pose serious risks to health. Maintaining an appropriate level of selenium is vital for ensuring the well-being of living organisms. This article explores the impact of selenium deficiency and excess on human and animal health, examining how these conditions lead to specific diseases and symptoms. It also delves into the relationship between nutrition and immune function, shedding light on the mechanisms behind nutrient metabolism disorders. By enriching our understanding of animal nutrition and immunity, we aim to establish new guidelines for the responsible use of trace elements and prevent nutritional metabolic diseases. Keywords: trace element selenium health Selenium (Se) is a critical micronutrient for both animals and humans. It supports numerous physiological processes, including immunity, growth, and reproduction. Adding selenium to diets can enhance these processes, yet both overconsumption and underconsumption can be harmful. Proper selenium intake is essential for maintaining health in both humans and animals. 1 Selenium Deficiency and Health Since the discovery of selenium's benefits in the mid-20th century, researchers have learned much about its biological roles. In 1973, the World Health Organization recognized selenium as an essential trace element necessary for life processes. Adequate selenium levels are crucial for normal bodily functions, while deficiencies can lead to various health issues. 1.1 Selenium Deficiency and Animal Health Livestock and poultry deficient in selenium may experience several health problems. These include: (1) White muscle disease: This condition primarily affects lambs, yaks, and piglets. Characterized by degeneration and necrosis of skeletal and cardiac muscles, it is named for the pale appearance of the affected tissues. Animals display skeletal muscle and myocardial degeneration, with a light-colored hue resembling cooked meat, interspersed with yellow-white, gray-yellow, or gray-white streaks. The liver becomes swollen, hard, and brittle, with a coarse surface and a nut-like texture upon sectioning. (2) Horse endemic proteinuria: A selenium-deficiency disease in horses, this condition presents with locomotion difficulties, symmetrical muscle swelling, heart weakness, and brownish sputum containing myoglobin. Pathological changes involve degeneration, hemorrhage, necrosis, and edema of parenchymal organs like the liver and kidneys, along with myocardial and skeletal muscle degeneration. (3) Camel "wobbly" disease: This disorder causes movement problems in camels. Clinical signs include stiffness in the hindquarters, difficulty walking with forelimbs stuck, dragging of hind legs, and swaying. The camel’s gait becomes uncoordinated during turns or sharp movements, increasing the risk of falls and fractures. Descending slopes is particularly challenging, and the head and neck often tilt to one side. Some muscles may tremble, and the hind legs may stand in an "eight" formation. (4) Yak "big belly" disease: Common in Xinjiang brown calves introduced to Qinghai, this condition is marked by diarrhea, heart failure, ascites, and pneumonia. The abdomen swells, giving the disease its name. Similar to yak white muscle disease, it features mild dyskinesia but severe heart failure, often accompanied by complications such as concurrent or secondary pneumonia and pleurisy. (5) Subcutaneous edema in chicks: This condition is a localized selenium deficiency causing subcutaneous swelling in chicks. The head, neck, chest, abdomen, wings, and thighs exhibit cerebral edema, appearing yellow or light blue due to denatured hemoglobin. Puncture reveals fluid discharge, and late-stage symptoms include diarrhea, anemia, lethargy, and eventual death. Embryos may also experience subcutaneous edema, reducing hatching rates. This condition is also called chick edema. Beyond these major diseases, selenium deficiency can lead to diarrhea in young animals, mastitis in cows, infertility, miscarriages, colibacillosis, porcine mulberry heart, fertility decline, embryo degradation, reduced immune function, decreased survival rates of young animals, and slower growth in livestock and poultry. 1.2 Selenium Deficiency and Human Health When humans lack selenium, they may develop Kashin-Beck disease and Keshan disease, which are among the most prominent manifestations of selenium deficiency. In 1972, Yang Xingyu confirmed that selenium deficiency is the primary cause of these diseases. Urine selenium concentrations were significantly lower in affected individuals compared to healthy populations. Residents in affected areas had average serum selenium levels below 11 μg/L, whereas normal areas had levels around 60-105 μg/L. Keshan disease predominantly affects women of childbearing age and preschool children, presenting as congestive heart failure and occasional diffuse cardiac thrombosis, sometimes resulting in sudden death or paralysis. Grain and soil selenium levels in affected areas were notably lower than in unaffected regions, with daily selenium intake as low as 10 μg/d. Additionally, low selenium nutrition increases the incidence and mortality of coronary heart disease. When serum selenium levels fall below 45 μg/L, the risk of coronary heart disease doubles or triples. Selenium deficiency also harms nerve cells and thyroid function and correlates with cancer incidence. High colorectal cancer rates in Jiashan, Zhejiang, are linked to low soil selenium levels (10 μg/kg) and low selenium content in rice (17-37 μg/kg). Research shows that cancers of the mouth, throat, nasopharynx, breast, colon, stomach, liver, lungs, and esophagus accompany declining blood selenium levels. Selenium levels significantly influence the expression of oncogenes and tumor suppressor genes. In cretinism studies, patients exhibited significantly lower serum selenium levels than healthy individuals. Long-term consumption of selenium-deficient foods can impair the immune system, digestive function, neurological health, vision, kidney function, reproductive health, and sexual function, leading to over 20 diseases including malnutrition, cataracts, mastitis, hepatitis, AIDS, infertility, hypertension, and hemolytic anemia. 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