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Selenium is an essential mineral found naturally in soil, water, and some foods. Today, selenium, as an antioxidant, is one of the vital elements in the human body. Although the human body needs a small amount of selenium daily, recent studies have shown that the same small amount of selenium is essential for maintaining healthy body function. It is challenging to choose the right dose of selenium. According to several studies in this present review, a daily supplement of 31.5-200 micrograms seems beneficial since the amount of selenium in various diseases is less than the amount in healthy people. The articles published in PubMed, Web of Science, Google Scholar and SCOPUS databases were screened and collected based on the keywords including selenium, oxidant, antioxidant, diabetes, cancer, toxicity, heart disease and liver disease. These keywords were chosen because of the antioxidant role of selenium as well as the prominent role of oxidation in the progression of these diseases. 150 articles published over a period of 20 years from 2000 to 2020 were used in this study. Articles about nano-seleniums, non-dietary supplements, and diseases caused by the underlying disease of metabolic syndrome were excluded and finally 121 articles were included in the study. A limited number of studies show that patients with multiple disorders may need to take higher doses of selenium (82.4 to 200 micrograms) than healthy people. In this study, selenium's antioxidant role in chronic metabolic disorders, including hyperlipidemia and hyperglycemia, and some cancers, were investigated. Also, the clinical significance of selenium deficiency in metabolic diseases, as well as clinical and experimental observations on the effect of selenium dietary supplements in the treatment of chronic metabolic diseases such as diabetes and atherosclerosis, and cancers, are briefly mentioned. Besides, the recommended toxicity and dose of selenium have been discussed.