Selenium | Supplementopedia

Overview

Selenium is an essential trace mineral that functions primarily as a co‑factor for a family of selenoproteins, many of which are antioxidant enzymes and thyroid‑hormone regulators. In humans, it is required in minute amounts but is critical for maintaining cellular redox balance, immune competence, and proper thyroid function.

Benefits

Antioxidant protection: Selenium‑dependent glutathione peroxidases (GPx) and thioredoxin reductases neutralise hydrogen peroxide and lipid peroxides, reducing oxidative stress (Hoffmann et al., 2022).
Thyroid health: Deiodinases (DIO1, DIO2) convert T₄ to active T₃, supporting metabolism and neurodevelopment (Kharazmi et al., 2021).
Immune support: Selenium enhances T‑cell proliferation and NK‑cell activity, lowering infection risk in low‑status individuals (Miller et al., 2020).
Reproductive function: Selenomethionine improves sperm motility and reduces DNA fragmentation in men (Safarinejad et al., 2023).
Cognitive and mood health: Adequate status correlates with better memory scores and lower incidence of mood disorders (Gao et al., 2022).
Cardiovascular protection: Observational data link higher selenium status to lower coronary artery calcification and reduced inflammatory markers (Rutschmann et al., 2021).

How It Works

Selenoprotein incorporation: Selenium is incorporated as selenocysteine—the 21st amino acid—into selenoproteins via a unique UGA codon recoding mechanism.
Redox activity: In its active form, the selenol (–SeH) of selenocysteine exhibits lower pKa and higher nucleophilicity than cysteine, allowing rapid reduction of peroxides (GPx) and regeneration of thioredoxin.
Thyroid hormone conversion: Selenium also participates in the iodothyronine deiodinase cascade, enabling the conversion of T₄ → T₃ through redox‑dependent cleavage of iodine.
Inflammation modulation: By maintaining intracellular redox potential, selenium modulates NF‑κB signaling, attenuating pro-inflammatory cytokine production.
DNA repair and mitochondrial function: Selenium influences DNA repair through selenoprotein H and regulates mitochondrial function via the selenoprotein P (SeP) transport system, ensuring tissue‑specific selenium delivery.

Dosage

Recommended Dietary Allowance (RDA): 55 µg/day for adults.
Upper Limit (UL): 400 µg/day.
Common supplemental doses: Range from 50–200 µg/day, taken with a meal to improve absorption (selenium is lipophilic in organic forms).
Thyroid support: 100–200 µg/day is often used.
Antioxidant or immune enhancement: 100 µg/day is common.
Therapeutic protocols: Short‑term therapeutic protocols for thyroid auto‑immunity may use 200 µg/day for 3–6 months under clinician supervision.
Pregnancy/Lactation: Should not exceed 200 µg/day to avoid fetal toxicity.
Split dosing: Split dosing (e.g., 50 µg twice daily) can reduce gastrointestinal irritation, though single‑dose regimens are equally effective when total daily intake stays below the UL.

Safety & Side Effects

Tolerance: Selenium is well‑tolerated at ≤200 µg/day.
Selenosis: Excess intake (>400 µg/day) can cause selenosis: garlic‑odor breath, hair/ nail brittleness, skin lesions, gastrointestinal upset, and, rarely, neurotoxicity.
Chronic high doses: May impair glucose metabolism and increase insulin resistance.
Contraindications:
- Pregnancy (high doses linked to fetal loss).
- Thyroid disorders requiring careful monitoring of hormone levels.
Drug interactions:
- High‑dose selenium may antagonise anticoagulants (e.g., warfarin).
- May reduce efficacy of certain chemotherapeutics (e.g., cisplatin).
Special populations:
- Individuals with autoimmune thyroid disease should adjust dosing under medical guidance.
- Renal‑impaired patients may accumulate selenium, necessitating lower doses (≤50 µg/day).

Chemistry

Elemental selenium: Atomic number 34, symbol Se.
Molecular formula: For elemental selenium is Se₈ (a polymeric ring).
Common supplement forms:
- Sodium selenite (Na₂SeO₃) and sodium selenate (Na₂SeO₄) for inorganic.
- Selenomethionine (C₅H₁₁NO₂Se) for organic.
IUPAC name for selenomethionine: (2S)-2-amino-4-(selenyl)butanoic acid.
Valence electron configuration: Selenium possesses a 4p⁴ valence electron configuration, enabling multiple oxidation states (+4, +6) that underpin its redox activity.
Atomic mass: 78.96 g mol⁻¹.
Melting point: 217 °C.
Solubility: Low solubility in water (inorganic salts) dictates formulation choices.
Selenocysteine pKa: Selenocysteine’s selenol group has a pKa ≈5.2, making it a potent nucleophile in enzyme active sites.

Sources & Quality

Natural dietary sources:
- Brazil nuts (up to 800 µg per nut).
- Seafood (e.g., tuna, 60–120 µg/100 g).
- Organ meats (liver, 30–50 µg/100 g).
- Cereals (1–5 µg/100 g).
Commercial supplements:
- Extraction: Extraction of selenium from sulfide ores (e.g., copper‑sulfide) followed by chemical conversion to sodium selenite/selenate.
- Biotechnological synthesis: Biotechnological synthesis of selenomethionine via fermentation of Saccharomyces cerevisiae grown in selenium‑enriched media, yielding a highly bio-available organic form.
Quality control:
- GMP manufacturing.
- Verification of selenium species (organic vs inorganic) via ICP‑MS.
- Absence of heavy-metal contaminants.
- Third‑party certifications (USP, NSF) assure potency, purity, and consistent bioavailability across batches.