Selenium: evidence grade D+, dosage & worth it

What it does

Selenium is an essential trace element built into about 25 human selenoproteins, including glutathione peroxidases and thioredoxin reductases (antioxidant defense) and iodothyronine deiodinases (which convert T4 to active T3). Adequacy matters for thyroid function, immunity, and reproduction. The supplement question is different from the nutrient question: most adults in North America already get enough from food, and above adequacy the dose-response turns from benefit to harm faster than for almost any other mineral we grade.

What the research says

Corrects true selenium deficiency and prevents deficiency disease
Evidence: strong. Deficiency is causally linked to Keshan disease (cardiomyopathy) and contributes to Kashin-Beck disease in low-selenium regions, and repletion reverses the deficiency state. This is a repletion claim, not a longevity claim: it applies where intake or status is genuinely low, which is uncommon in selenium-replete North American adults.
Lowers thyroid autoantibodies (TPOAb) in Hashimoto thyroiditis in the short term
Evidence: moderate. A systematic review and meta-analysis (Huwiler et al., Thyroid 2024) of 35 randomized studies found selenium supplementation significantly lowered TPO antibodies (standardized mean difference -0.96, with 2,358 participants in that analysis) and lowered TSH slightly only in patients not on thyroid hormone replacement. An earlier meta-analysis (Toulis et al., J Clin Endocrinol Metab 2010) found the same direction at 3 months, mostly at 200 mcg/day selenomethionine. No review establishes benefit for symptoms, thyroid function outcomes, or disease progression; the clinical significance of the antibody change remains unclear.
Reduces cancer risk or overall mortality in generally nourished adults
Evidence: weak. The claim fails its biggest tests. SELECT (Lippman et al., JAMA 2009) randomized 35,533 men to selenium 200 mcg/day, vitamin E, both, or placebo and found no reduction in prostate cancer, with follow-up stopped early; extended follow-up (Klein et al., JAMA 2011) confirmed no benefit. The updated Cochrane review of selenium for preventing cancer (83 studies, 27,232 randomized participants) found risk ratios at or near 1.0 for cancer incidence in low-bias trials (RR 1.01) and for cancer mortality (RR 1.02), concluding there is no convincing evidence selenium supplements prevent cancer. The older NPC trial's prostate signal was a secondary analysis concentrated in men with low baseline selenium and did not survive replication in SELECT.

Selenium the nutrient is essential; selenium the supplement, for a generally nourished adult, has one of the clearest negative records we grade. Harvard Health's supplement scorecard concludes plainly that selenium is not for you. SELECT, one of the largest supplement trials ever run (35,533 men, 200 mcg/day), found no prostate cancer prevention, and the updated Cochrane synthesis (83 studies, 27,232 randomized participants) found risk ratios at or near 1.0 for cancer incidence and mortality in low-bias trials, alongside more alopecia and dermatitis in selenium arms and a type 2 diabetes signal: significant in the NPC trial analysis (HR 1.55 at 200 mcg/day, strongest at high baseline selenium) and directional in SELECT (RR 1.07, not significant). The plus in D+ is for the two uses that survive: repleting genuine deficiency, and a real but clinically uncertain antibody effect in Hashimoto thyroiditis. For everyone else the expected value of adding selenium is zero to negative, and the gap between a typical 200 mcg pill and the 400 mcg upper limit is uncomfortably small.

Grades move when the evidence moves.

This grade for Selenium rests on the record above. When a new result changes that record, the grade changes with it, and the Thursday email is where the change lands first: the week's longevity and biotech news, in plain English, linked back to the reasoning on this page.

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Every grade follows a published rubric, and sponsors never change one. When the evidence moves a grade, Thursday is where you hear it first, before you renew a stack that stopped earning its cost.

What the experts say

Every position links to its source. We include skeptics alongside supporters.

Bryan Johnson
Source, 2026-09-10 Takes 28 mcg/day of selenium in his Blueprint Essential Capsules, in the morning with food, per the published Blueprint protocol (compiled in our expert-stacks.json). Note the dose: roughly half the RDA from the pill, a food-level top-up, not the 200 mcg pharmacologic dose SELECT tested and found wanting.

Is it worth it?

D

Derived under the October 2026 worth-it model: D+ evidence (1.3 points) at $5 to $10 a month scores 0.52, which bands to D. The low price does not rescue it: there is no demonstrated benefit for a selenium-replete adult, and the record carries diabetes and selenosis signals. Deficiency repletion is a medical use, not a value purchase. An earlier stored grade of D+ was an editorial override and has been corrected to the derivation.

Typical cost

$5-10/month

Common form, not a brand recommendation

Dosage contextestablished

For deficiency repletion, 55 to 100 mcg/day restores status; food-first is preferred where diet allows. Hashimoto antibody trials used 200 mcg/day selenomethionine, typically for 3 to 6 months, under clinician supervision. There is no evidence-based longevity dose: the 200 mcg/day dose was tested at scale in SELECT and failed. Forms (selenomethionine, selenium yeast, sodium selenite) differ in retention, but no form has outcome evidence the others lack. Do not exceed 400 mcg/day from all sources.

Worth-it leaderboard

Selenium sits outside the worth-it top 20.

Selenium sits outside the current top 20 on the gated leaderboard. The leaders below clear an evidence floor of B- before price counts at all, so cheap alone never earns a rank.

See the full worth-it leaderboard →

When the evidence moves a grade, this ranking moves with it. Thursday is where you hear it first.

Safety notes

RDA is 55 mcg/day for adults; the US tolerable upper intake level is 400 mcg/day (one source reviewed lists an EFSA upper limit of 255 mcg/day, so the US figure is not universal), and typical food intake in North America already runs about 100 to 130 mcg/day, so a standard 200 mcg pill plus diet plus a multivitamin can approach the ceiling. Chronic excess causes selenosis: garlic breath, metallic taste, hair loss, brittle or lost nails, skin rash, GI upset, fatigue, irritability, and at higher levels peripheral neuropathy. SELECT found significantly more alopecia and mild dermatitis in the selenium arm, and Cochrane notes increased high-grade prostate cancer risk in SELECT participants with the highest background selenium status. The diabetes signal (NPC significant at HR 1.55, SELECT directional) argues against supplementation in people with high baseline status or diabetes risk. Brazil nuts are a hidden dose: a single nut can carry 50 to 90 mcg, so daily Brazil-nut habits plus a pill stack up fast. Do not use selenium to prevent prostate cancer: that claim was tested in SELECT and failed.

Longevity Lab Report

Selenium news, graded when it happens.

The grade on this page stays free. The email carries the news on selenium: expert moves, dose changes, and what is working, in plain English every Thursday.

Frequently asked questions

Is Selenium safe?

RDA is 55 mcg/day for adults; the US tolerable upper intake level is 400 mcg/day (one source reviewed lists an EFSA upper limit of 255 mcg/day, so the US figure is not universal), and typical food intake in North America already runs about 100 to 130 mcg/day, so a standard 200 mcg pill plus diet plus a multivitamin can approach the ceiling. Chronic excess causes selenosis: garlic breath, metallic taste, hair loss, brittle or lost nails, skin rash, GI upset, fatigue, irritability, and at higher levels peripheral neuropathy. SELECT found significantly more alopecia and mild dermatitis in the selenium arm, and Cochrane notes increased high-grade prostate cancer risk in SELECT participants with the highest background selenium status. The diabetes signal (NPC significant at HR 1.55, SELECT directional) argues against supplementation in people with high baseline status or diabetes risk. Brazil nuts are a hidden dose: a single nut can carry 50 to 90 mcg, so daily Brazil-nut habits plus a pill stack up fast. Do not use selenium to prevent prostate cancer: that claim was tested in SELECT and failed.

What is the right dose of Selenium?

For deficiency repletion, 55 to 100 mcg/day restores status; food-first is preferred where diet allows. Hashimoto antibody trials used 200 mcg/day selenomethionine, typically for 3 to 6 months, under clinician supervision. There is no evidence-based longevity dose: the 200 mcg/day dose was tested at scale in SELECT and failed. Forms (selenomethionine, selenium yeast, sodium selenite) differ in retention, but no form has outcome evidence the others lack. Do not exceed 400 mcg/day from all sources.

Is Selenium worth taking?

We grade Selenium D for worth-it at a typical cost of $5-10 per month. Derived under the October 2026 worth-it model: D+ evidence (1.3 points) at $5 to $10 a month scores 0.52, which bands to D. The low price does not rescue it: there is no demonstrated benefit for a selenium-replete adult, and the record carries diabetes and selenosis signals. Deficiency repletion is a medical use, not a value purchase. An earlier stored grade of D+ was an editorial override and has been corrected to the derivation.

What do experts say about Selenium?

Bryan Johnson: Takes 28 mcg/day of selenium in his Blueprint Essential Capsules, in the morning with food, per the published Blueprint protocol (compiled in our expert-stacks.json). Note the dose: roughly half the RDA from the pill, a food-level top-up, not the 200 mcg pharmacologic dose SELECT tested and found wanting.