---

Background and Objective: Golestan province, located in north of Iran has been known as a high risk area for esophageal cancer. The relationship between esophageal cancer and Selenium (Se) has been assessed in previous studies. This study was conducted to assess the relationship between Soil selenium levels and development of esophageal cancer in Golestan province of Iran. Materials and Methods: In this ecologic study, Golestan province in northern Iran was divided into 135 blocks based on geographical altitude and longitude on the map. One Soil sample was collected from the center of each block. Selenium level in Soil samples was determined by flame atomic absorption spectrometry. Statistical analysis was performed by Pearson correlation and T-Student tests. Results: The mean±SD of Soil Selenium level in Golestan province was 3.7±1.61 mg/kg. There was a positive correlation between Soil level of Selenium and esophageal cancer rates in this area (P=0.03), (Pearson correlation coefficient=0.19). The Soil Selenium level for high risk and low risk area for esophageal cancer were (4.13 mg/kg) and (3.39 mg/kg) respectively (P=0.01). Conclusion: This study showed that there is high Soil Selenium level in Golestan province in north of Iran. Also it is found to be a significant positive relationship between Soil Selenium level and esophageal cancer rate in this area.

---

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.