Selenium

What it is

Selenium is a trace mineral. The body builds it into about 25 selenoproteins, including glutathione peroxidases and thioredoxin reductases.

What the research says

Selenium is needed for thyroid hormone metabolism, DNA synthesis and protection from oxidative damage. The large trials do not support taking it to prevent cancer: SELECT, in 35,533 men, found no effect on prostate cancer, and a Cochrane review concluded the cancer-prevention hypothesis is not supported. The earlier Nutritional Prevention of Cancer trial missed its skin-cancer target but reported fewer total cancers in secondary analyses, which SELECT did not confirm. Selenium supplementation was linked to a higher risk of type 2 diabetes in the NPC trial (a nominal signal in SELECT). One trial found benefit in mild Graves' eye disease. The margin to toxicity is narrow: a mislabelled 2008 liquid supplement poisoned 201 people.

Evidence grade: Limited evidence.

What goes wrong

In a secondary analysis of the Nutritional Prevention of Cancer trial, selenium 200 ug/day was associated with more new cases of type 2 diabetes than placebo. (Source 1)

• Randomized trial, Moderate certainty.
• Size: 1,202 people without diabetes at baseline.
• Who: Dermatology patients in low-selenium areas of the eastern US.
• How long: Average 7.7 years.
• Result: HR 1.55 (95% CI 1.03-2.33); 12.6 vs 8.4 cases per 1000 person-years.
• Funding: Not stated in abstract.

Selenium supplementation does not seem to prevent type 2 diabetes, and it may increase risk for the disease.

In 2008, a liquid supplement containing 200 times its labelled selenium caused an outbreak of selenium poisoning, with hair loss, nail damage and fatigue, some lasting 90 days or more. (Source 2)

• Case series, High certainty.
• Size: 201 cases in 10 states.
• Who: US consumers of one liquid supplement.
• How long: Follow-up to 90 days.
• Result: Median dose 41,749 ug/day; diarrhoea 78%, fatigue 75%, hair loss 72%.
• Funding: Public health investigation.

The source of the outbreak was identified as a liquid dietary supplement that contained 200 times the labeled concentration of selenium.

NIH ODS (2025) states that chronically high selenium intake (selenosis) most commonly shows as hair loss and brittle or lost nails. (Source 3)

• Official position, Certainty not rated.
• Size: Not applicable.
• Who: General population.
• How long: Chronic.
• Result: Not applicable.
• Funding: US government.

The most common clinical signs of chronically high selenium intakes, or selenosis, are hair loss and nail brittleness or loss.

What the evidence supports

In people with mild Graves' orbitopathy, selenium 100 ug twice daily for 6 months improved quality of life and eye involvement and slowed progression compared with placebo. (Source 4)

• Randomized trial, Moderate certainty.
• Size: 159 patients.
• Who: Adults with mild Graves' orbitopathy in Europe.
• How long: 6 months treatment, 12 months follow-up.
• Result: Quality of life P<0.001; eye involvement P=0.01; progression P=0.01.
• Funding: Independent (University of Pisa, Italian Ministry)

At the 6-month evaluation, treatment with selenium, but not with pentoxifylline, was associated with an improved quality of life (P<0.001) and less eye involvement (P=0.01)

What the evidence does not support

In SELECT, selenium 200 ug/day did not prevent prostate cancer in relatively healthy men. (Source 5)

• Randomized trial, High certainty.
• Size: 35,533 men.
• Who: Men aged 50+ (African American) or 55+ in US, Canada and Puerto Rico.
• How long: Median 5.46 years.
• Result: Prostate cancer HR 1.04 (99% CI 0.87-1.24) for selenium vs placebo.
• Funding: Not stated in abstract (NCI-sponsored per public record; not verified here)

Selenium or vitamin E, alone or in combination at the doses and formulations used, did not prevent prostate cancer in this population of relatively healthy men.

A Cochrane review concluded that the hypothesis that more selenium lowers cancer risk is not supported. (Source 6)

• Systematic review, High certainty.
• Size: Randomised trials and observational studies (numbers not captured from page read)
• Who: Adults in cancer-prevention trials and cohorts.
• How long: Varied.
• Result: No reduction in overall or site-specific cancer in randomised trials (as summarised)
• Funding: Cochrane.

the hypothesis that increasing selenium intake may reduce cancer risk is not supported by epidemiological evidence

Where the evidence is mixed

SELECT recorded a statistically nonsignificant increase in type 2 diabetes in the selenium-alone group. (Source 5)

• Randomized trial, Moderate certainty.
• Size: 35,533 men.
• Who: Relatively healthy men.
• How long: Median 5.46 years.
• Result: Type 2 diabetes RR 1.07 (99% CI 0.94-1.22), P = .16.
• Funding: Not stated in abstract.

type 2 diabetes mellitus in the selenium group (relative risk, 1.07; 99% CI, 0.94-1.22; P = .16)

The Nutritional Prevention of Cancer trial found no effect on its primary end points, skin cancers, but reported fewer total cancers and cancer deaths as secondary end points, which the authors said needed confirmation. (Source 7)

• Randomized trial, Low certainty.
• Size: 1,312 patients.
• Who: People with prior basal or squamous cell skin cancer, eastern US.
• How long: Mean 4.5 years treatment, 6.4 years follow-up.
• Result: Basal cell RR 1.10 (0.95-1.28); total cancer mortality RR 0.50 (0.31-0.80); total cancer incidence RR 0.63 (0.47-0.85)
• Funding: Not stated in abstract.

selenium treatment did not significantly affect the incidence of basal cell or squamous cell skin cancer

Where the research disagrees

Whether selenium supplements reduce cancer

• Clark et al., NPC trial 1996, RCT, 1,312 patients, secondary end points: "results from secondary end-point analyses support the hypothesis that supplemental selenium may reduce the incidence of, and mortality from, carcinomas of several sites" (Source 7)
• SELECT investigators, 2009, RCT, 35,533 men, primary end point: "Selenium or vitamin E, alone or in combination at the doses and formulations used, did not prevent prostate cancer in this population of relatively healthy men." (Source 5)

How much

• Reference intake: NIH ODS (fact sheet updated 2025, from the Food and Nutrition Board) lists an RDA of 55 ug/day for adults aged 19 and over. (Source 3)
• Upper limit: NIH ODS (2025) lists a Tolerable Upper Intake Level of 400 ug/day for adults aged 19 and over. (Source 3)
• Studied: SELECT: 200 ug/day selenium as L-selenomethionine. (Source 5)
• Studied: Nutritional Prevention of Cancer trial: 200 ug/day selenium. (Source 7)
• Studied: Marcocci 2011: selenium 100 ug twice daily for 6 months. (Source 4)

A common belief, and what the research shows

The belief: Selenium is an antioxidant that prevents cancer.

What the research shows: The NPC trial's hopeful secondary result was followed by SELECT, which found selenium "did not prevent prostate cancer", and a Cochrane review concluded "the hypothesis that increasing selenium intake may reduce cancer risk is not supported by epidemiological evidence".

Questions and answers

What is it?

Selenium is a trace mineral that the body builds into around 25 proteins called selenoproteins. (Source 3)

What does it do in the body?

Selenoproteins are involved in thyroid hormone metabolism, making DNA, reproduction and protecting cells from oxidative damage. (Source 3)

Is it good or bad for you?

Essential in small amounts, harmful in large ones. Supplements did not prevent cancer in large trials and were linked to more type 2 diabetes in one trial; a mislabelled product caused mass poisoning. One trial found benefit in mild Graves' eye disease. (Source 1)

How do you get more of it?

Food is the usual source, especially Brazil nuts, seafood, meat, poultry and organ meats. Trials gave 200 ug/day as selenomethionine (SELECT) or 100 ug twice daily (Graves' trial). (Source 3)

If it is harmful, what reduces it?

The sources we read describe no specific treatment for excess selenium beyond stopping the source. In the 2008 outbreak, some symptoms were still present 90 days or more later. (Source 2)

Why might someone be low in it or missing it?

People on long-term kidney dialysis (hemodialysis) often have lower selenium than healthy people, partly because dialysis removes some selenium from the blood. NIH ODS also lists people living with HIV and people in selenium-deficient regions among the groups at risk of low intake. (Source 3)

Which whole foods contain it or feed it?

Brazil nuts, seafood, meat, poultry and organ meats are the richest sources; plant content depends on the soil. (Source 3)

What happens if you do not have it?

Severe deficiency is linked to Keshan disease, a heart muscle disease first described in low-selenium areas of China. (Source 3)

How can you test for it?

Plasma or serum selenium is the usual test; NIH ODS says a level of 8 ug/dL or higher in healthy people is considered sufficient for making selenoproteins. The Cochrane review warned that exposure indicators can misclassify people. (Source 3)

References

1. Annals of Internal Medicine. Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. 2007. PMID 17620655, DOI 10.7326/0003-4819-147-4-200708210-00175. Read the source
2. Archives of Internal Medicine. Acute selenium toxicity associated with a dietary supplement. 2010. PMID 20142570, DOI 10.1001/archinternmed.2009.495. Read the source
3. NIH Office of Dietary Supplements. Selenium: Fact Sheet for Health Professionals (updated September 4, 2025). 2025. Read the source
4. New England Journal of Medicine. Selenium and the course of mild Graves' orbitopathy. 2011. PMID 21591944, DOI 10.1056/nejmoa1012985. Read the source
5. JAMA (record in Duke Scholars). Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). 2009. PMID 19066370, DOI 10.1001/jama.2008.864. Read the source
6. Cochrane. Selenium for preventing cancer (Cochrane review, plain language summary page). 2018. PMID 29376219, DOI 10.1002/14651858.CD005195.pub4. Read the source
7. JAMA (record in Augusta University Research Profiles). Effects of selenium supplementation for cancer prevention in patients with carcinoma of the skin: A randomized controlled trial. 1996. DOI 10.1001/jama.276.24.1957. Read the source