Selenium
Selenium is built into about 25 selenoproteins as the amino acid selenocysteine, including the glutathione peroxidases, thioredoxin reductases, and the deiodinases that convert T4 to T3. Intake depends on the soil where food was grown, and the margin between enough and too much is narrower than for most nutrients.
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What It Does
Selenoprotein P carries selenium from the liver to other tissues and is the most useful marker of status. In UK adults with modest intakes, plasma selenoprotein P stopped rising once about 50 mcg a day was added to the diet (Hurst 2010), which suggests needs are met somewhere near 100 mcg of total intake. Most people in the US and Canada already eat that much. Parts of Europe, China, and New Zealand have low-selenium soil, and severe deficiency in China caused Keshan disease, a cardiomyopathy.
Cancer prevention was the great hope and the great disappointment. In the Nutritional Prevention of Cancer trial, 200 mcg a day of selenium yeast did not reduce skin cancer, its primary endpoint, though secondary analyses showed fewer total cancers (Clark 1996). SELECT then gave 200 mcg of selenomethionine to more than 35,000 men and found no reduction in prostate cancer (Lippman 2009). The NPC data also showed more type 2 diabetes in the selenium group, especially among people who started with high selenium levels (Stranges 2007).
Thyroid evidence is mixed. In mild Graves' orbitopathy, 200 mcg a day of sodium selenite for six months improved quality of life and slowed eye disease progression in a European population with marginal intake (Marcocci 2011). Trials in Hashimoto's thyroiditis lower antibody levels, but evidence that this changes thyroid function or symptoms is weak.
Toxicity is well characterized. A mislabeled liquid supplement containing about 200 times its stated selenium poisoned 201 people, causing diarrhea, fatigue, hair loss, joint pain, and nail changes (MacFarquhar 2010). Chronic intake above the 400 mcg upper limit produces the same picture more slowly, along with garlic breath.
Forms and Bioavailability
Absorption ratings are broad tiers. Head-to-head human trials rarely support a finer ranking among well-absorbed forms.
| Form | Elemental % | Absorption | Typical dose | Cost | Labels |
|---|---|---|---|---|---|
| SelenomethionineL-selenomethionine | Varies | HighAbsorbed like methionine, more than 90%. Nonspecifically incorporated into body proteins in place of methionine, which creates a storage pool and higher blood selenium. | 55-200 mcg daily | Low | 237 |
| Selenium Yeastselenized yeast, high-selenium yeast | Varies | HighYeast grown on selenium; selenium is present mostly as selenomethionine with smaller amounts of other compounds. | 55-200 mcg daily | Low | 120 |
| Sodium Seleniteselenite | 45.7% | ModerateInorganic form; about 50% absorbed. Not stored in proteins, but directly available for selenoprotein synthesis. | 55-200 mcg daily | Low | 43 |
| Sodium Selenateselenate | 41.8% | HighInorganic form; nearly completely absorbed, but a large share is excreted in urine rather than retained. | 55-200 mcg daily | Low | 13 |
Labels column: number of labels named as selenium products that list this form. A product can list more than one form, and some labels do not state a form at all.
Selenomethionine is absorbed at about 90% and is incorporated into body proteins in place of methionine, which builds a reserve. That helps repletion and raises the cost of chronic overuse. Selenium yeast is mostly selenomethionine, with batch-to-batch variation. Sodium selenite is less well absorbed but goes straight to selenoprotein synthesis without building a reserve. Selenate is well absorbed and largely excreted in urine.
Brazil nuts are the most concentrated food source. Two nuts a day, about 53 mcg, raised plasma selenium as well as 100 mcg of selenomethionine (Thomson 2008), but content varies severalfold with the soil they grew in, and eating them by the handful daily can exceed the upper limit.
Typical Dosing
| Group | RDA |
|---|---|
| Adults 19+ | 55 mcg |
| Pregnancy | 60 mcg |
| Lactation | 70 mcg |
| Upper limit (UL) | 400 mcg per day from all sources |
Supplements commonly provide 55-200 mcg. A single Brazil nut can contain more than the RDA, and content varies widely with soil.
Food sources: Brazil nuts, tuna, halibut, sardines, ham, shrimp, beef, eggs.
What to Look For
Find selenium in mcg on every product you take; multivitamins commonly carry 55-200 mcg. Most people eating a mixed North American diet do not need more. A supplement makes sense for people with documented low status, those living in low-selenium regions, and specific prescribed uses.
If you supplement, 50-100 mcg a day is enough to saturate selenoprotein P for nearly anyone. Avoid long-term doses of 200 mcg or more, which was the dose in the trials with no cancer benefit and a diabetes signal.
Red flags: liquid selenium products, "thyroid support" blends with unstated amounts, and stacking a selenium supplement on a multivitamin and daily Brazil nuts.
Selenomethionine
Well absorbed and the form used in SELECT. It builds up in body protein, which helps repletion and makes chronic overuse more consequential.
What to look for: 50-100 mcg per serving, counting any multivitamin, and only when diet or region suggests low status.
Find on iHerb Affiliate linkSodium Selenite
Less absorbed, but it feeds selenoprotein synthesis directly without building a reserve. The form used in the Graves' orbitopathy trial.
What to look for: Usually a prescribed, time-limited course at a defined dose.
Find on iHerb Affiliate linkProducts on Titrate
16,483 of the 214,745 labels in the NIH Dietary Supplement Label Database list selenium as an active ingredient (7,594 on market). 554 are sold as selenium products by name.
Top on-market labels, ranked by label completeness and simplicity:
- Selenium 200 mcg
- Selenium 200 mcg
- Selenium 200 mcg
- Albion Selenium 200 mcg
- Albion Selenium Complex 200 mcg
Selenium does not have its own search category yet. Each product above links to its full label in the NIH database.
Related Tools
References
NIH Office of Dietary Supplements. Selenium: Fact Sheet for Health Professionals.
Clark LC, et al. Effects of selenium supplementation for cancer prevention in patients with carcinoma of the skin. A randomized controlled trial. JAMA. 1996;276(24):1957-1963.
Stranges S, et al. Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. Ann Intern Med. 2007;147(4):217-223.
Thomson CD, et al. Brazil nuts: an effective way to improve selenium status. Am J Clin Nutr. 2008;87(2):379-384.
Lippman SM, et al. Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. 2009;301(1):39-51.
Hurst R, et al. Establishing optimal selenium status: results of a randomized, double-blind, placebo-controlled trial. Am J Clin Nutr. 2010;91(4):923-931.
MacFarquhar JK, et al. Acute selenium toxicity associated with a dietary supplement. Arch Intern Med. 2010;170(3):256-261.
Marcocci C, et al. Selenium and the course of mild Graves' orbitopathy. N Engl J Med. 2011;364(20):1920-1931.
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