Selenium (Selenomethionine)
1. Introduction – What is the product, chemical/botanical basics, mechanism of action
Selenium is an essential trace mineral naturally present in many foods and available as a dietary supplement [1]. It is critical for human health, primarily through its incorporation into a group of proteins known as selenoproteins.
The form of selenium most commonly found in supplements and in plant-based foods is selenomethionine (SeMet), an organic compound where selenium replaces sulfur in the amino acid methionine [1]. Other forms include selenocysteine (SeCys), which is considered the 21st amino acid and is the biologically active form at the core of selenoproteins [2].
Mechanism of Action
Selenium’s biological activity is mediated by the approximately 25 known selenoproteins in humans [1]. These proteins play critical roles in various physiological processes:
- Antioxidant Defense: Selenoproteins like glutathione peroxidases (GPx) and thioredoxin reductases (TrxR) are powerful antioxidants that protect cells from oxidative damage caused by free radicals [3].
- Thyroid Hormone Metabolism: Selenoproteins, particularly iodothyronine deiodinases, are essential for the synthesis, activation, and metabolism of thyroid hormones [4].
- Immune Function: Selenoproteins are involved in regulating immune responses and protecting against infection [1].
- DNA Synthesis and Reproduction: Selenium is necessary for proper DNA synthesis and fertility [1].
2. Chemical Composition/Key Bioactive Roles
The primary bioactive compounds of selenium are the selenoproteins, which contain the amino acid selenocysteine. Selenomethionine is the storage form that the body converts into selenocysteine for incorporation into these proteins [2].
| Key Bioactive Selenoproteins | Primary Role |
|---|---|
| Glutathione Peroxidases (GPx) | Catalyze the reduction of hydrogen peroxide and organic hydroperoxides, protecting cells from oxidative damage. |
| Thioredoxin Reductases (TrxR) | Regulate cellular redox balance and control the activity of various transcription factors and enzymes. |
| Iodothyronine Deiodinases (DIO) | Regulate the activation and deactivation of thyroid hormones (T4 to T3 conversion). |
| Selenoprotein P (SEPP1) | Major selenium transport protein in the plasma and an antioxidant. |
3. Health Benefits – Detailed health benefits with scientific evidence
Thyroid Disease
Selenium plays a vital role in thyroid function and hormone metabolism [4]. Low selenium status has been associated with an increased risk of thyroid disease in women [1]. Clinical trials have shown that selenium supplementation can lower the levels of certain thyroid antibodies in patients with chronic autoimmune thyroiditis (Hashimoto’s disease), though results on overall thyroid function are mixed [1].
Cancer Prevention
Epidemiological studies have suggested that higher selenium status may be associated with a lower risk of certain cancers [1]. However, large, randomized controlled trials, such as the Selenium and Vitamin E Cancer Prevention Trial (SELECT), found that selenium supplementation (200 mcg/day, primarily as selenomethionine) did not reduce the risk of prostate cancer or other forms of cancer in a generally well-nourished population [5]. The evidence remains inconclusive, and more research is needed to define the relationship between selenium supplementation and cancer risk [1].
Cardiovascular Disease (CVD)
Observational studies linking low selenium status to an increased risk of CVD have yielded conflicting results [1]. Clinical trial evidence does not currently support the use of selenium supplementation alone to reduce CVD risk, especially in individuals with sufficient dietary selenium intake. However, taking selenium as part of a multi-antioxidant formula has been suggested to potentially reduce the risk of cardiovascular mortality [1].
Immune Function and HIV
Selenium deficiency can increase the risk of morbidity and mortality in people with HIV [1]. While selenium supplementation may provide limited benefits, two Cochrane Reviews concluded that there was insufficient evidence to determine if supplementation is broadly beneficial for people with HIV. Some evidence suggests it may reduce the risk of preterm birth in pregnant women with HIV [1].
4. Dosage and Usage
Recommended Dietary Allowances (RDAs)
The Food and Nutrition Board at the National Academies of Sciences, Engineering, and Medicine has established the following RDAs for selenium [1]:
| Age Group | RDA (mcg/day) |
|---|---|
| Adults (19+ years) | 55 mcg |
| Pregnant Women | 60 mcg |
| Lactating Women | 70 mcg |
Supplementation Dosages
Selenium supplements commonly contain 50 to 400 mcg of selenium, with selenomethionine being one of the most common forms [1]. The body absorbs up to about 90% of selenium from selenomethionine [1]. In research trials, dosages of 200 mcg/day have been frequently used, often in the form of selenomethionine [5].
Food Sources
Selenium is found in protein-rich foods. The selenium content in plant-based foods varies significantly based on the soil’s selenium content.
| Food Source | Selenium Content (mcg per serving) |
|---|---|
| Brazil nuts (1 ounce, 6–8 nuts) | ~544 mcg (Highly variable) [1] |
| Yellowfin tuna (3 ounces, cooked) | 92 mcg [1] |
| Sardines (3 ounces, canned) | 45 mcg [1] |
| Pork chop (3 ounces, broiled) | 37 mcg [1] |
5. Safety and Precautions
Safety and Tolerable Upper Intake Level (UL)
The Tolerable Upper Intake Level (UL) for selenium in adults is 400 mcg (0.4 mg) per day [1]. This is the maximum daily intake unlikely to cause adverse health effects.
Side Effects (Selenosis)
Chronically high intakes of selenium can lead to a toxic condition called selenosis [1]. Symptoms include:
- Hair loss and nail brittleness or loss (most common signs)
- Garlic odor in the breath
- Metallic taste in the mouth
- Skin rash
- Nausea, diarrhea, fatigue, and irritability
- Nervous system abnormalities
Extremely high doses can be fatal [6].
Contraindications and Drug Interactions
- Contraindications: Individuals with a known hypersensitivity to selenium or those with a history of selenosis should avoid supplementation. People who live in regions with high selenium soil content should be cautious with supplementation [1].
- Drug Interactions: Selenium supplements may interact with certain medications. For example, the chemotherapy drug cisplatin can affect selenium levels in the body [1]. It is advisable to consult a healthcare provider before taking selenium with any prescription medication.
6. References
[1] National Institutes of Health, Office of Dietary Supplements. Selenium – Health Professional Fact Sheet. https://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/[2] Sunde, R. A. (2012). Selenium. In Modern Nutrition in Health and Disease (11th ed., pp. 225-237). Lippincott Williams & Wilkins.[3] Lei, X. G., Rayman, M., & Sunde, R. A. (2024). Selenium. In Modern Nutrition in Health and Disease (12th ed.). Jones & Bartlett Learning.[4] Winther, K. H., Rayman, M. P., Bonnema, S. J., & Hegedüs, L. (2020). Selenium in thyroid disorders – essential knowledge for clinicians. Nature Reviews Endocrinology, 16(3), 165–176. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7018870/[5] Lippman, S. M., Klein, E. A., Goodman, P. J., Lucia, M. S., Thompson, I. M., Jr., et al. (2009). Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA, 301(1), 39–51. https://jamanetwork.com/journals/jama/fullarticle/183244[6] WebMD. Selenium – Uses, Side Effects, and More. https://www.webmd.com/vitamins/ai/ingredientmono-1003/seleniumCategory: Mineral
