Selenium (Selenomethionine)

Mechanism of Action

Selenium is incorporated as selenocysteine (the 21st amino acid) into 25 selenoproteins. Key enzymes: glutathione peroxidases (GPx1-4, reduce hydrogen peroxide and lipid hydroperoxides), thioredoxin reductases (TrxR1-3, maintain cellular redox balance), iodothyronine deiodinases (DIO1-3, convert T4 to T3 and regulate thyroid hormone metabolism), selenoprotein P (selenium transport and brain antioxidant), and selenoprotein W (muscle antioxidant). Selenium is essential for thyroid function, immune defense, reproductive health, and cancer chemoprevention.

Dosing Protocol

Notes

Selenium has a NARROW therapeutic window — the UL is 400 mcg/day and toxicity (selenosis) can occur at only 2-3x the therapeutic dose. Baseline selenium status matters enormously: the SELECT trial showed prostate cancer risk INCREASE in men who were already selenium-sufficient. Test plasma selenium or selenoprotein P before supplementing. Optimal range: 100-150 ng/mL plasma selenium. The selenium-iodine synergy for thyroid health is critical — never supplement iodine without adequate selenium (Hashimoto's risk). Brazil nut content varies 10-fold by region — they are unreliable as a sole source.

Stacking

• Vitamin E (synergistic antioxidant)
• Iodine (for thyroid support)
• NAC
• Zinc

Interactions

• Cisplatin/chemotherapy [MEDIUM] — Selenium may reduce oxidative chemotherapy toxicity (protective but theoretical efficacy concern)
• Statins [LOW] — Selenium + statins: conflicting data on combined effect on selenoprotein expression
• Niacin [LOW] — High-dose selenium + niacin may reduce HDL-raising effect of niacin

Contraindications

• Selenium excess (>400 mcg/day — narrow therapeutic window)
• Non-melanoma skin cancer history (SELECT trial concern)

Side Effects

• Garlic breath/body odor (selenosis)
• GI upset
• Hair loss and nail brittleness (toxicity)
• Fatigue
• Peripheral neuropathy (chronic excess)
• Potential type 2 diabetes risk at high doses (SELECT trial observation)

Key Papers

• 10.1002/14651858.CD005195.pub4
• 10.1016/S0140-6736(12)60061-8
• 10.3390/nu11092068

Source Quality

Selenomethionine (organic, from Se-enriched yeast) is preferred for general supplementation — it is incorporated into body proteins as a selenium reserve. Sodium selenite (inorganic) is used for acute glutathione peroxidase upregulation. Se-methylselenocysteine has specific anti-cancer evidence. Brands: Thorne Selenomethionine, Pure Encapsulations, Life Extension. Brazil nuts (1-2/day) provide ~100-200 mcg but content varies enormously by origin.

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