Selenium
Contributes to the normal function of the immune system and to the protection of cells from oxidative stress.
Also known as Selenium Methionine, Selenomethionine, Sodium Selenite
Common doses
55-200 mcg
Best timing
Morning
Food
Take with food
Interactions
2 known
Selenium is an essential trace mineral that functions primarily through selenoproteins, including glutathione peroxidases and thioredoxin reductases. These enzymes are critical for antioxidant defence, thyroid hormone metabolism, and immune function.
Selenium intake varies dramatically by geography because it depends on soil selenium content. People in selenium-rich regions (parts of the US, Canada) may get adequate intake from diet alone, while those in selenium-poor regions (parts of Europe, China, New Zealand) are more likely to benefit from supplementation.
The relationship between selenium and cancer is complex. The SELECT trial found no cancer prevention benefit and suggested a possible increase in prostate cancer risk for men with already-adequate selenium levels. This underscores the importance of not supplementing unnecessarily.
Key benefits
Thyroid function
The thyroid contains more selenium per gram than any other organ. Selenium is required for the enzymes that convert T4 to the active T3 hormone.
Antioxidant defence
Selenium is a cofactor for glutathione peroxidases, which protect cells from oxidative damage. It works synergistically with Vitamin E.
Immune function
Selenium contributes to the normal function of the immune system. Deficiency impairs both innate and adaptive immune responses.
Reproductive health
Selenium is important for male fertility. It is incorporated into the structural protein of sperm mitochondria.
Available forms
Selenomethionine
Organic form. Well-absorbed and stored in body proteins. The preferred form for general supplementation.
Sodium Selenite
Inorganic form. Lower bioavailability. Can be reduced by Vitamin C to poorly absorbed elemental selenium.
Selenium Yeast
Grown on selenium-enriched yeast. Contains mostly selenomethionine. Used in the NPC and SELECT trials.
Food sources
- Brazil nuts (1-2 nuts provide daily requirement)
- Yellowfin tuna
- Sardines
- Chicken and turkey
- Eggs
- Cottage cheese
Signs of deficiency
- Fatigue
- Weak immune function
- Thyroid dysfunction
- Hair loss
- Brain fog
- Muscle weakness
Upper intake limit
400 mcg/day (NIH). Selenium toxicity (selenosis) causes hair loss, nail brittleness, nausea, and nerve damage. Brazil nuts are so selenium-rich that 4-5 per day can approach the upper limit.
Research summary
Strong evidence for thyroid function support. Good evidence for antioxidant defence and immune function. The cancer prevention story is mixed: early trials (NPC) showed benefit, but the larger SELECT trial did not. Current advice: supplement only if dietary intake is likely inadequate. Brazil nuts are the simplest dietary solution.
Known interactions (2)
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1 flagZinc and Iron compete for the same absorption pathway. Take at least 2 hours apart.
Supplements
This information is for educational purposes only. It is not medical advice and does not diagnose, treat, cure, or prevent any disease. Always consult a qualified healthcare professional.