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Selenium

Selenium was the great cancer-prevention hope of the 1990s and large randomised trials comprehensively killed it. In selenium-replete populations, supplementing achieves nothing useful and raises type 2 diabetes risk; the only defensible use is correcting genuine deficiency.

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What the science says

  • Does not prevent cancer Limited evidence

    A Cochrane review of 83 studies, including 10 RCTs with 27,232 participants, found no reduction in overall cancer incidence (RR 1.01, 95% CI 0.93-1.10, high-certainty evidence) or cancer mortality (RR 1.02), and no benefit for prostate (RR 1.01), colorectal (RR 0.99), lung (RR 1.16) or bladder cancer (RR 1.07). Observational data suggesting benefit are confounded; the trials settle it, and low-risk-of-bias RCTs even suggested increased melanoma risk.

  • Increases type 2 diabetes risk in already-replete people Moderate evidence

    In the Nutritional Prevention of Cancer trial, 200 mcg/day of selenised yeast over a mean 7.7 years increased type 2 diabetes incidence (HR 1.55, 95% CI 1.03-2.33), with the effect concentrated in participants in the highest tertile of baseline plasma selenium. The Cochrane pooled RCT data reach the same conclusion — supplementation slightly raises diabetes risk.

  • Lowers thyroid antibodies in Hashimoto's without proven clinical benefit Limited evidence

    Trials of 200 mcg/day selenium in autoimmune thyroiditis consistently reduce thyroid peroxidase antibody titres, but they are small, short and heterogeneous. None has shown that patients end up on less levothyroxine, feel better, or progress more slowly to overt hypothyroidism. Antibody titre is a surrogate, not an outcome, and treating it is not the same as treating the disease.

Dosage & safety

Studied doseRDA is 55 mcg/day. Most Western diets already supply 80-150 mcg/day; two Brazil nuts alone can exceed 200 mcg. Trials used 200 mcg/day as selenised yeast, selenomethionine or sodium selenite. The tolerable upper limit is 400 mcg/day from all sources — an unusually narrow margin above the RDA, and a strong argument against casual supplementation on top of an adequate diet.
SafetySelenium has one of the narrowest therapeutic windows of any nutrient. Chronic intake above roughly 400 mcg/day causes selenosis: garlic-like breath odour, brittle hair and nail loss or brittleness, skin rash, nausea, irritability and peripheral neuropathy. Brazil nut selenium content varies enormously with soil, so a daily handful can push intake into the toxic range unpredictably. SELECT reported increased alopecia and dermatitis at just 200 mcg/day, and in men with high baseline selenium, more high-grade prostate cancer. Avoid in anyone with adequate dietary intake — the diabetes signal is real and replicated. True deficiency (Keshan disease, Kashin-Beck disease) occurs only in specific low-selenium soil regions, severe malabsorption, and long-term parenteral nutrition.

How to take it

TimingTiming does not matter.
With food?With food.
Worth knowingCount your Brazil nuts before you count supplements — two can exceed the RDA several times over, and soil variation makes the amount per nut unpredictable.

Interactions

With other supplements

  • high-dose vitamin C

    Ascorbic acid reduces absorption of selenium given as sodium selenite (organic forms such as selenomethionine are less affected). separate by a few hours, or choose a selenomethionine product

Well-established interactions only — this is not a complete list. Always tell your doctor and pharmacist what you take.

Frequently asked questions

Does selenium prevent cancer?

No — the evidence is limited and negative. A Cochrane review of 10 randomised trials with 27,232 participants found selenium did not reduce cancer incidence or cancer death, and some low-bias trials even suggested more melanoma.

Can selenium raise diabetes risk?

Yes, with moderate evidence, in people who already get enough selenium. In one trial, 200 mcg/day of selenium for a mean 7.7 years raised type 2 diabetes incidence, and pooled trial data agree.

Does selenium work for Hashimoto's thyroiditis?

The evidence is limited: trials of 200 mcg/day of selenium lower thyroid antibody levels, but none has shown patients feel better, need less levothyroxine or progress more slowly.

How many Brazil nuts give you enough selenium?

Two Brazil nuts alone can exceed 200 mcg of selenium, well above the 55 mcg/day RDA, and content varies unpredictably with soil. The selenium upper limit is 400 mcg/day, so count your nuts before adding a supplement.

Scientific references

Where to buy

Pure Encapsulations, Selenium (Selenomethionine), 60 Capsules (200 mg per Capsule)

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