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29: Selenium (Chapter 25b)

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    Abstract

    In only a few decades, the trace element selenium (Se) was added to the Nutrition agenda as an essential nutrient with potential to reduce cancer risk. Found in the 1950's to prevent vitamin E deficiency disorders in animals, initial investigations in animal nutrition spawned studies of metabolic mechanisms starting in the 1960's. It is now known that Se functions as an essential constituent of some 25 proteins that contain a previously unrecognized Se-containing amino acid, selenocysteine. These selenoproteins are thought to discharge the nutritional functions of Se. The relationship of one, glutathione peroxidase, to the level of Se intake was used in 2000 to establish an RDA for Se. Se is typically obtained from protein-rich foods, particularly those produced on Se-rich soils. Low Se status occurs in areas of low soil Se, and among individuals consuming low-protein diets. Many consequences of Se deprivation are sub-clinical in nature, requiring other precipitating factors (vitamin E deficiency, low iodine intake, viruses, carcinogens) to reveal the effects of sub-optimal expression of selenoenzymes and/or insufficient amounts of active Se-metabolites. Studies that started in the 1960's showed that Se can prevent carcinogenesis in animal and cell models, and that anti-tumorigenic activities required Se doses greater than those required to support selenoenzyme expression. Only a few clinical intervention trials have been conducted to test the hypothesis that Se supplementation can reduce cancer risk in humans; most, but not all have shown positive results. Hence, whether Se may be a risk modifier for cancer remains unclear.

    • 29.1: Overview (25b.1)
      This page discusses Selenium (Se), an essential nutrient with historical significance dating back to the 1930s. Its health benefits were noted in the 1950s, finding a role in preventing diseases related to vitamin E deficiency. Research in the 1980s connected low Se levels to heart disease, underscoring its importance for human health.
    • 29.2: Selenium Chemistry (25b.2)
      This page discusses selenium's chemical similarities with sulfur, its metabolic relevance through compounds like seleno­cysteine, and outlines its primary anionic forms: selenide, selenite, and selenate. It highlights the differing metabolic processes, with selenite being easily reduced and selenate requiring activation. In cells, seleno­cysteine and seleno­methionine are the main forms, with seleno­methionine used by plants, microorganisms, and incorporated into animal proteins.
    • 29.3: Health Effects (25b.3)
      This page highlights the importance of selenium (Se) as an essential nutrient for health, preventing diseases in both animals and humans while noting the risks of deficiency and excess. It discusses the serious health issues caused by chronic selenosis in Enshi County, China, and mentions the correlation between high plasma selenium levels and increased type 2 diabetes risk, alongside inconsistent study findings. Animal studies suggest high selenium exposure can lead to insulin resistance.
    • 29.4: Selenium Requirements (25b.4)
      This page discusses the Recommended Dietary Allowance (RDA) for selenium, set at 55µg/day for adults based on studies of Glutathione Peroxidase 3 activity. It includes recommendations for infants and pregnant or lactating women. The World Health Organization proposed lower intake levels, while the FDA established a no observed adverse effect level at 1000ng/mL (853µg/day). The Institute of Medicine also set a tolerable upper intake level of 400µg/day for adults.
    • 29.5: Food Sources (25b.5)
      This page discusses the variability of selenium (Se) content in plant and animal foods, influenced by soil Se levels and absorption capabilities of plants. Cruciferous vegetables and animal products reflect local soil conditions, with livestock often receiving Se supplements. Primary dietary sources include cereals, meats, and fish, while some countries experience low Se intake, posing health risks.
    • 29.6: Enteric Absorption (25b.6)
      This page discusses the efficient absorption of selenium (Se) from food, noting that Se-amino acids need protein digestion and active transport, while inorganic forms are absorbed via diffusion. It highlights research findings indicating high bioavailability, with absorption rates of 98% for selenomethionine-Se and 84% for selenite-Se in humans.
    • 29.7: Metabolism (25b.7)
      This page explains selenium metabolism, highlighting dietary sources like selenomethionine and selenocysteine. It details how selenocysteine is converted to selenide for incorporation into selenoproteins, while selenomethionine can replace methionine in proteins. The main excretion route is through urinary metabolites, influenced by selenium intake, with limited enterohepatic circulation resulting in fecal excretion mainly of unabsorbed selenium.
    • 29.8: Biochemical Functions (25b.8)
      This page discusses the importance of selenoproteins, which contain selenocysteine and are vital for nutrition and regulation in organisms, particularly eukaryotes. Humans recognize 25 selenoproteins, such as Glutathione Peroxidases and Thioredoxin Reductases, that are crucial for redox signaling and metabolism. Genomic studies also reveal several other selenoproteins with potential yet unidentified roles in cells, underscoring their biological significance.
    • 29.9: Assessing Selenium Status (25b.9)
      This page discusses selenium (Se) status, emphasizing the importance of accurate food analysis for intake assessment, tissue retention, and specific seleno­proteins for fulfilling Se requirements. Plasma is a key tissue for evaluating Se levels, influenced by factors including gender, age, and diet. Genetic variability and obesity can also affect Se biomarkers.
    • 29.10: Questions Remain (25b.10)
      This page discusses ongoing research on selenium (Se), highlighting its essential role in preventing deficiencies and its debated potential in cancer prevention, with mixed trial results. There are concerns regarding safe dosage and the effect of high levels of Se on diabetes risk. Supplementation is noted to benefit adults with low baseline Se levels (< 50µg/d), while emerging evidence suggests those with plasma levels between 80-120ng/mL may also see reduced cancer risk.

    This page titled 29: Selenium (Chapter 25b) is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Rosalind S. Gibson via source content that was edited to the style and standards of the LibreTexts platform.