selenium compounds
Also known as: Se, selenomethionine, selenate, selenite, selenoproteins, selenized yeast, Selenium
Overview
Selenium is an essential trace element (atomic number 34) naturally present in soil, water, and various foods such as Brazil nuts, seafood, and meats. It is crucial for human health as it is incorporated into selenoproteins, which play vital roles in antioxidant defense, thyroid hormone metabolism, and immune system function. Common forms used in supplements include organic selenomethionine (SeMet) and inorganic selenite (SeO3^2−) and selenate (SeO4^2−), with selenomethionine generally exhibiting higher bioavailability. Selenium supplements are primarily used to prevent and correct selenium deficiency, and are also investigated for their potential in reducing cancer risk, supporting cardiovascular health, and regulating mood. While extensive research, including numerous randomized controlled trials and meta-analyses, has been conducted on selenium, the evidence quality varies, and some findings regarding its benefits and risks remain conflicting.
Benefits
Selenium offers several evidence-based health benefits. A 2022 systematic review and meta-analysis indicated that selenium supplementation significantly reduced depressive symptoms, suggesting a beneficial impact on mood regulation. For cancer prevention, the Nutritional Prevention of Cancer (NPC) trial showed that 200 μg/day of selenium (as selenized yeast) reduced the risk of prostate, lung, and colorectal cancers. However, the larger SELECT trial did not confirm prostate cancer prevention benefits, indicating mixed evidence. Some research suggests selenium may reduce cardiovascular disease risk and mortality, likely due to its antioxidant properties. Selenium supplementation has been confirmed effective in treating metaphyseal lesions in children with Kashin-Beck disease, an osteoarthropathy linked to selenium deficiency. While evidence is mixed, some studies suggest selenium may improve insulin sensitivity in type 2 diabetes, though high doses might increase diabetes risk. Other potential benefits, with lower to moderate evidence quality, include reduced all-cause mortality, improved sperm quality, and benefits in autoimmune thyroid disease and polycystic ovary syndrome.
How it works
Selenium exerts its biological effects primarily by being incorporated into selenoproteins, which are enzymes critical for various physiological processes. Key selenoproteins include glutathione peroxidases and thioredoxin reductases, which function as powerful antioxidants, protecting cells from oxidative damage caused by free radicals. Selenium also plays a role in modulating immune responses and is essential for thyroid hormone metabolism through its involvement in iodothyronine deiodinases. Furthermore, selenium influences neurotransmitter pathways, which may contribute to its observed effects on mood regulation. The bioavailability of selenium depends on its chemical form; organic selenium, such as selenomethionine, is absorbed efficiently via amino acid transport pathways, while inorganic forms are absorbed through different mechanisms.
Side effects
Selenium is generally considered safe at recommended doses, but toxicity, known as selenosis, can occur at high intakes, typically above 400 μg/day. Common side effects, though rare (occurring in less than 5% of users), may include mild gastrointestinal discomfort. Uncommon effects (1-5% frequency) can include hair and nail brittleness, and mild neurological symptoms. Rare but severe side effects (<1% frequency) associated with selenosis include a characteristic garlic breath odor, dermatitis, fatigue, irritability, and peripheral neuropathy. Long-term high-dose selenium supplementation has been linked to an increased risk of developing type 2 diabetes and non-melanoma skin cancer. Caution is advised when combining selenium supplements with anticoagulants due to potential interactions, and with certain chemotherapy agents. Selenium supplementation is generally not recommended for individuals who are not deficient or once a deficiency has been corrected, as excess intake can be harmful.
Dosage
The recommended daily allowance (RDA) for selenium, which is the minimum effective dose for preventing deficiency, is approximately 55 μg/day for adults. For supplementation aimed at achieving therapeutic benefits, optimal doses observed in clinical trials typically range from 100 to 200 μg/day. The upper tolerable intake level (UL) for selenium, representing the maximum safe dose that is unlikely to cause adverse effects in most individuals, is 400 μg/day. Selenium supplements are generally taken orally, once daily. Organic forms, particularly selenomethionine, are often preferred due to their higher bioavailability and better absorption. Absorption of selenium can be enhanced by adequate protein intake and may be antagonized by heavy metals and certain medications.
FAQs
Is selenium supplementation necessary for everyone?
No, selenium supplementation is primarily beneficial for individuals with a confirmed deficiency. For those with adequate dietary intake, excess supplementation can be harmful and is generally not recommended.
How quickly do benefits appear?
The time frame for observing benefits can vary. Effects on mood or antioxidant status may become noticeable within weeks to a few months of consistent supplementation, depending on the individual's baseline status and the specific health concern.
Can selenium prevent cancer?
Evidence regarding selenium's role in cancer prevention is mixed. Some studies suggest a benefit, particularly in individuals with low baseline selenium status, but large trials have yielded inconsistent results. The effectiveness may depend on the specific cancer type and the form of selenium used.
Is selenium safe during pregnancy?
Selenium is generally considered safe during pregnancy when taken within recommended dietary allowances. However, pregnant individuals should consult their healthcare provider before starting any supplement to ensure appropriate dosing and monitoring.
Research Sources
- https://www.nature.com/articles/s41598-022-05078-1 – This systematic review and meta-analysis by Sajjadi et al. (2022) investigated the effect of selenium supplementation on depressive symptoms. The study found that selenium supplementation significantly reduced depressive symptoms across various populations, indicating a beneficial role in mood regulation. Despite high heterogeneity across studies, the pooled effect sizes consistently supported a positive impact.
- https://pmc.ncbi.nlm.nih.gov/articles/PMC10534049/ – Wang et al. (2023) conducted an umbrella review of 76 meta-analyses to assess the associations between selenium intake and various health outcomes. The review linked selenium intake to reduced risks of digestive cancers, overall mortality, and depression. However, it also highlighted a potential increased risk of diabetes, emphasizing the need for caution and further research, especially given the prevalence of observational data.
- https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2021.685317/full – This review by Ferreira et al. (2021) synthesized findings from various studies, including large randomized controlled trials like the NPC and SELECT trials. It noted that the NPC trial showed a reduction in cancer risk with selenium supplementation, while the SELECT trial did not confirm prostate cancer prevention benefits. The review also discussed mixed data regarding selenium's impact on diabetes risk, highlighting the complexity and variability of findings across different populations and study designs.
- https://pubmed.ncbi.nlm.nih.gov/29511006/ – Xie et al. (2018) performed a systematic review and network meta-analysis focusing on the efficacy of selenium supplementation in children with Kashin-Beck disease. The study concluded that all forms of selenium supplementation were superior to placebo in promoting the repair of metaphyseal lesions in affected children. This research provides strong evidence for selenium's therapeutic role in this specific deficiency-related osteoarthropathy.
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