Selenium: Benefits, Forms, Dosing, and Safety

Selenium is a trace mineral essential for the biosynthesis of 25 distinct selenoproteins in the human body [1][2]. These selenoproteins incorporate selenium in the form of selenocysteine — the 21st amino acid in the genetic code — and play critical roles in antioxidant defense, thyroid hormone metabolism, DNA synthesis, reproduction, and protection from oxidative damage and infection [1][3][4]. The selenium concentration in the thyroid gland is higher than in any other organ in the body, reflecting its importance in thyroid function [5].

Selenium deficiency is uncommon in the U.S. but more prevalent in regions with selenium-depleted soil, including Northern and Central Europe (England, Wales, Scotland, Ireland, Germany, Poland) and certain areas of China [10][11]. Importantly, while deficiency is associated with health risks, supplementation in people who already have adequate selenium levels may increase the risk of certain conditions including type 2 diabetes and prostate cancer.

Table of Contents

• Overview
• Key Selenoprotein Families
• Forms and Bioavailability
• Evidence for Health Benefits
• Recommended Dosing
• Safety and Side Effects
• Drug Interactions
• Dietary Sources
• References

Overview

Selenium works with vitamin E to protect cell membranes from free radical damage [1]. Deficiency is uncommon in North America, where the average daily selenium intake from foods and beverages is approximately 108 mcg — well above the 55 mcg Recommended Dietary Allowance (RDA) [9]. However, deficiency is more prevalent in regions with selenium-depleted soils, including Northern and Central Europe and certain areas of China [10][11]. Globally, an estimated 0.5 to 1 billion people are at risk of selenium deficiency [12]. Vegetarians and vegans are at particular risk in low-selenium regions — research has noted that "the most pronounced difference in nutritional status between British and American vegetarians is selenium status" [13].

Selenium deficiency alone rarely causes overt illness but produces biochemical changes that predispose individuals to additional health problems [1][14]. Severe deficiency is associated with two endemic diseases:

• Keshan disease: An endemic cardiomyopathy first identified in 1935 in selenium-poor areas of China, mainly affecting women of childbearing age and preschool children. A 2018 systematic review and meta-analysis of 41 studies found that selenium supplementation reduces the risk of Keshan disease by 86% [15].
• Kashin-Beck disease: An osteoarthritis-like condition affecting cartilage and bone, commonly presenting in childhood and puberty in low-selenium regions of China, Tibet, Siberia, and North Korea [1][16].

Subclinical selenium deficiency may contribute to weakened immune function (reduced T-cell proliferation and antibody production), hypothyroidism (particularly when compounded by iodine deficiency), and impaired thyroid hormone metabolism [1][5][17]. Plasma and serum selenium concentrations are the most commonly used measures of selenium status. Concentrations of 8 mcg/dL or higher are generally considered sufficient for selenoprotein synthesis [1][8]. The average serum selenium concentration in the US population is approximately 12.7 mcg/dL [18].

Key Selenoprotein Families

The body uses selenium to synthesize 25 human selenoproteins. Key families include:

• Glutathione peroxidases (GPx): A family of antioxidant enzymes that reduce hydroperoxides and lipid hydroperoxides, protecting cells from oxidative damage. GPx4 is particularly important in spermatogenesis, protecting developing sperm and contributing to the structural integrity of the sperm midpiece [6][7].
• Thioredoxin reductases (TrxR): Maintain the redox state of thioredoxin and other proteins, supporting DNA synthesis, apoptosis regulation, and antioxidant defenses [6].
• Iodothyronine deiodinases (DIO1, DIO2, DIO3): Essential for thyroid hormone metabolism — specifically the conversion of the prohormone thyroxine (T4) to the biologically active triiodothyronine (T3), or its inactivation [5][7].
• Selenoprotein P: The primary selenium transport protein in plasma, used as a functional biomarker of selenium status alongside glutathione peroxidase 3 [1][8].

Forms and Bioavailability

Selenium compounds are generally very efficiently absorbed, with the body absorbing up to 90% from most supplement forms [3][21]:

Form	Absorption	Key Characteristics
Selenomethionine	>90%	Organic form; the predominant form in plant foods. Non-specifically incorporated into body proteins (methionine pool). Most studied supplemental form. Used in the SELECT and PREADVISE trials [1][22].
Selenium-enriched yeast	>90%	Brewer's yeast grown in selenium-rich broth; contains predominantly selenomethionine. Used in the Nutritional Prevention of Cancer Trial. Believed slightly better absorbed than inorganic selenite [10][23].
Sodium selenite	>80%	Inorganic form. Well absorbed but metabolized differently — rapidly reduced to selenide rather than being incorporated into the methionine pool. Used in some thyroid trials [1][21].
Sodium selenate	>90%	Inorganic form. High absorption, then reduced to selenite in vivo. Used in fertilizer fortification programs (e.g., Finland) [1][21][24].
Selenium glycinate	~80%	Selenium complexed with the nonessential amino acid glycine. Promoted as better absorbed, but this has not been demonstrated in human studies. Research in broiler chickens showed results similar to selenite in raising muscle selenium levels [10].
Se-methyl-L-selenocysteine	High (animal data)	Naturally occurring organic form representing ~80% of total selenium in cruciferous vegetables. Readily absorbed based on animal studies; limited human supplement data — EFSA advises caution [10][25].

Key Differences Between Organic and Inorganic Forms

Organic selenium (selenomethionine, selenium yeast) is incorporated non-specifically into body proteins wherever methionine is used, creating a selenium "reserve" in tissues. This means organic selenium has a longer biological half-life and can be released when needed for selenoprotein synthesis [1][22]. Inorganic selenium (selenite, selenate) is rapidly metabolized to a common intermediate (selenide) that is used directly for selenoprotein synthesis. It does not accumulate in the methionine pool and has a shorter half-life [1][22].

A randomized clinical trial comparing selenomethionine with selenium yeast (both at 200 mcg/day) in healthy men found that both forms increased plasma selenium, but selenium yeast produced more favorable changes in oxidative stress biomarkers [22].

Practical Form Selection

For general supplementation, selenomethionine and selenium-enriched yeast have the most clinical evidence. Selenite is commonly used in multivitamins and has adequate absorption. Selenium glycinate is a reasonable option, though its claimed absorption advantages over other forms lack human evidence [10]. Factors such as digestive conditions (Crohn's disease, short-bowel syndrome, ulcerative colitis) and acid-reducing medications may impair selenium absorption of some forms [10].

Evidence for Health Benefits

Cancer

The relationship between selenium and cancer has evolved significantly over three decades of research.

Early evidence suggesting benefit: The observation that cancer incidence is much higher in parts of China with selenium-depleted soil gave rise to the hypothesis that selenium deficiency promotes cancer [10][11]. The landmark Nutritional Prevention of Cancer Trial (NPC Trial) randomized 1,312 men and women with histories of nonmelanoma skin cancer to 200 mcg/day selenium as selenium yeast or placebo for a mean of 4.5 years. While selenium did not prevent recurrence of skin cancer, a secondary analysis found that men who took selenium had a 49% lower risk of prostate cancer over ~7.5 years of follow-up. The effect was strongest in men with the lowest baseline selenium concentrations (below 10.6 mcg/dL), with no benefit in men whose plasma selenium exceeded 12.3 mcg/dL (Clark, JAMA, 1996; Reid, Cancer Epidemiol Biomarkers Prev, 2002) [26][27]. The NPC Trial also reported a 58% reduction in colorectal cancer incidence among participants with low baseline selenium levels [29].

A study of African-American women found that those with the highest supplemental selenium intake (≥20 mcg/day) had approximately 30% lower risk of ovarian cancer compared to non-supplementers, after adjusting for age, contraceptive use, and family history. This association did not exist for selenium from foods nor for other antioxidants (Terry, J Nutr, 2017) [28].

Large trials showing no benefit: The Selenium and Vitamin E Cancer Prevention Trial (SELECT) — the largest trial of selenium for cancer prevention — randomized 35,533 men aged 50+ to 200 mcg/day selenium (as selenomethionine), 400 IU/day vitamin E, both, or placebo. The trial was discontinued after 5.5 years when analyses showed no association between selenium supplementation and prostate cancer risk [30]. An additional 1.5 years of follow-up confirmed the null finding [31]. Crucially, a case-cohort analysis found that selenium supplementation actually increased the risk of high-grade prostate cancer by 91% among men with already-high selenium levels (≥60th percentile), while having no effect in men with low baseline selenium [32].

Evidence of potential harm with high-dose supplementation: A study following men with non-metastatic prostate cancer found that those supplementing with 140 mcg or more of selenium per day were 160% more likely to die from prostate cancer compared to non-users (Kenfield, J Natl Cancer Inst, 2015) [33].

Systematic review: A 2018 Cochrane Review of 83 studies on selenium and cancer prevention found mixed results [34]. Among 70 observational cohort studies, participants with the highest baseline selenium status had 28% lower overall cancer risk, 24% lower cancer mortality, 33% lower bladder cancer risk, 18% lower lung cancer risk, 18% lower colorectal cancer risk, and 16% lower prostate cancer risk. However, among 10 randomized, placebo-controlled trials (27,232 participants), supplementation with 200–500 mcg/day selenium had little to no effect on cancer incidence or mortality (high certainty evidence for prostate, colorectal, lung, and bladder cancers) [34]. A separate Cochrane Review concluded that selenium supplements do not decrease the risk of skin or colorectal cancer regardless of baseline selenium status (Filippini, Cochrane Database Syst Rev, 2018) [35].

The US FDA has allowed qualified health claims that selenium "may reduce the risk of certain forms of cancer" but specifies that this evidence is "limited and not conclusive." In 2009, FDA determined it is "highly unlikely" that selenium supplements reduce prostate cancer risk [36].

Summary: The evidence suggests that correcting genuine selenium deficiency may reduce cancer risk, but supplementing beyond adequate levels offers no cancer prevention benefit and may increase risk of certain cancers — particularly high-grade prostate cancer in men with already-adequate selenium.

Thyroid Disease

The thyroid gland contains more selenium per gram of tissue than any other organ, reflecting selenium's central role in thyroid hormone synthesis and metabolism [5]. Selenoproteins — specifically the deiodinases (DIO1, DIO2, DIO3) — catalyze the conversion of T4 to the active T3. Additionally, glutathione peroxidase and thioredoxin reductase protect the thyroid from hydrogen peroxide generated during thyroid hormone synthesis [5][7].

Epidemiological associations: Epidemiological studies have found inverse associations between selenium status and thyroid disease risk, with this effect observed only in women [37][38]. A study of 1,900 middle-aged adults found that higher serum selenium was protective against goiter in women but not men [39]. A cross-sectional study in Denmark (n=805) found the same sex-specific pattern — lower selenium was associated with larger thyroid volume only in women [38].

Hashimoto's thyroiditis: A systematic review and meta-analysis of 16 trials (n=1,494, mainly women) evaluated selenium supplementation (80–200 mcg/day as selenomethionine or sodium selenite for 3–12 months) for Hashimoto's thyroiditis [41]. Key findings:

• In patients already on levothyroxine, selenium supplementation lowered thyroid peroxidase antibody (TPOAb) levels.
• Selenomethionine was more effective than sodium selenite at lowering TPOAb, possibly due to higher absorption.
• In newly diagnosed patients not on levothyroxine, selenium lowered both TPOAb and thyroglobulin antibody (TgAb) levels at 3 months, but not at 6 or 12 months.
• Baseline selenium status was deficient to marginal (3.7–8.5 mcg/dL) in the five trials that reported it.

However, a systematic review of 9 controlled trials (n=679) found that selenium supplementation (80–200 mcg/day for 3–12 months) did not improve TSH levels, thyroid echogenicity, or health-related quality of life [40]. The American Thyroid Association issued a weak recommendation against selenium supplementation for TPOAb-positive pregnant women, based on moderate quality evidence (2017) [46].

Selenium-iodine interaction: Selenium deficiency can exacerbate iodine deficiency, increasing the risk of congenital hypothyroidism by impairing deiodinase enzymes needed for thyroid hormone activation [1][48].

Summary: Selenium is clearly important for normal thyroid function, and correcting deficiency in selenium-poor populations may protect against thyroid disease. For Hashimoto's thyroiditis, selenium supplementation can lower antibody markers (particularly TPOAb), but this does not consistently translate to improved thyroid function or quality of life.

Cardiovascular Disease

Selenoproteins help reduce inflammation and prevent lipid oxidation and platelet aggregation — mechanisms that theoretically could reduce cardiovascular risk [49][50].

A 2006 meta-analysis of 25 observational studies (primarily men over 40) found an inverse association between selenium concentrations and coronary heart disease risk: men with the highest selenium had 15% lower risk (Flores-Mateo, Am J Clin Nutr, 2006) [51]. A 2025 meta-analysis of cohort studies reported that selenium intake above 55 mcg/day correlated with a 20–30% lower cardiovascular disease risk, with glutathione peroxidase activity identified as a key mediator of endothelial protection [55]. However, a Cochrane Review of SELECT and 11 smaller trials (total n=19,715) found that selenium supplementation did not reduce fatal or nonfatal cardiovascular events [50]. A trial of 474 healthy adults found that selenium supplementation at 100 and 200 mcg/day lowered total cholesterol by 8.5–9.7 mg/dL, though the clinical significance was noted as unclear [56].

One notable finding: a meta-analysis of 43 trials found that antioxidant formulas containing selenium reduced cardiovascular mortality by 23% compared to antioxidant formulas without selenium, though selenium alone had no significant effect [57]. This suggests potential synergistic benefits when selenium is combined with other antioxidants.

Selenium does not appear to be helpful for preventing heart disease when used alone. One fairly large double-blind study also failed to find that selenium supplementation improved general sense of well-being (Rayman, Biol Psychiatry, 2006) [58].

Cognitive Function

Selenoproteins have antioxidant and anti-inflammatory activities relevant to brain health, and serum selenium concentrations decline with age [59][60]. Chronic selenium deficiency has been correlated with cognitive decline [61][62].

An analysis of NHANES data showed that higher whole blood selenium and higher selenium intakes were associated with higher cognitive scores in older adults [18][63]. A study of 1,012 men and women in Italy (mean age 75 years) found that those in the lowest quartile of plasma selenium (<6.67 mcg/dL) had worse neuro-motor performance than those in the highest quartile (>8.23 mcg/dL) [60]. A 2024 meta-analysis of trace elements in Alzheimer's disease patients revealed significantly lower serum selenium levels (by 15–25 mcg/L) compared to controls [64].

However, the PREADVISE trial — the first large-scale primary prevention trial — randomized 7,540 cognitively healthy men aged 60+ to 200 mcg/day selenomethionine, 400 IU/day vitamin E, both, or placebo for 5.4 years. No difference in the incidence of Alzheimer's disease or dementia was found among any of the groups (Kryscio, JAMA, 2017) [67]. A 2022 systematic review and meta-analysis of 11 clinical trials evaluating selenium supplements for 12–24 weeks in adults aged 69–89 with mild cognitive impairment or Alzheimer's disease found no improvement in cognitive test scores [69].

Summary: The mechanistic rationale for selenium in brain health is strong, and observational studies consistently show associations between low selenium and cognitive decline. However, clinical trial evidence does not support selenium supplementation for preventing or treating cognitive decline in adults with adequate selenium levels.

Immune Function and Infectious Disease

Selenium deficiency may contribute to a weakened immune system through impaired T-cell proliferation and antibody production [1][17]. A small study in Germany found that selenium deficiency was common among people diagnosed with COVID-19, with approximately 39–43% having levels below the 2.5th percentile. Selenium deficiency was more common in individuals who died from COVID-19 compared to survivors (64–70% vs 32–39%, respectively). However, it is not known whether selenium deficiency is a risk factor for COVID-19 or a consequence of the illness (Moghaddam, Nutrients, 2020) [70]. Recent research on selenium nanoparticles has demonstrated strong antiviral activity against SARS-CoV-2 in vitro, with up to 90% inhibition of viral replication [71].

People living with HIV often have low selenium concentrations, possibly due to malabsorption or inadequate intake [72]. Two Cochrane Reviews concluded there is insufficient evidence that selenium supplementation benefits people with HIV [77][78].

Summary: Adequate selenium status supports immune function, and deficiency clearly impairs immunity. However, selenium supplementation above adequate levels does not appear to provide meaningful immune benefits in well-nourished populations.

Male Fertility

The selenoprotein GPx4 is a major structural component of mature sperm, and both low and high semen selenium concentrations have been associated with male infertility [80][81].

A 3-month trial in Scotland in 69 men with reduced sperm motility and low selenium plasma concentrations found that 100 mcg/day selenomethionine improved plasma selenium, sperm motility, and the odds of conception versus placebo [82]. Another 3-month trial of 200 mcg/day selenium yeast in 70 men with idiopathic infertility reported improved total and progressive sperm motility [83]. However, a trial in 54 healthy men given 300 mcg/day selenium yeast for 48 weeks found no differences in sperm concentration or motility versus placebo [84].

Summary: Selenium supplementation may improve sperm motility in selenium-deficient men with reduced fertility, but does not appear to benefit men with adequate selenium status.

Bone Health

Greater bone mineral density has been associated with higher selenium levels in the blood of postmenopausal women (Hoeg, J Clin Endocrinol Metab, 2012) [85]. This is consistent with selenium's role in reducing inflammation and oxidative stress that contribute to bone loss.

Rheumatoid Arthritis

Low selenium levels have been associated with the development of rheumatoid arthritis, although selenium supplements do not appear to help once the disease has developed (Tarp, Analyst, 1995) [86].

Diabetic Neuropathy

There is weak preliminary evidence that selenium might be helpful for diabetic neuropathy (Kahler, Z Gesamte Inn Med, 1993) [87].

Nutrient Interactions

Selenium and iodine: Selenium deficiency can exacerbate iodine deficiency, increasing the risk of congenital hypothyroidism by impairing the deiodinase enzymes required for thyroid hormone activation [1][48].

Selenium and vitamin E: These nutrients work synergistically in antioxidant protection. Selenium (via glutathione peroxidase) helps regenerate vitamin E from its oxidized form, amplifying defense against lipid peroxidation [88]. The SELECT trial revealed a complex interaction: vitamin E increased prostate cancer risk by 46–111% in men low in selenium, while having no such effect in men with adequate selenium — suggesting that selenium status modulates the risk profile of vitamin E supplementation [32].

Selenium and coenzyme Q10 (CoQ10): Selenium facilitates CoQ10 recycling and supports the formation of antioxidant enzymes. A randomized trial found that combined supplementation with selenium and CoQ10 improved cardiovascular health outcomes, with more pronounced effects in selenium-deficient populations [89][90].

Recommended Dosing

The RDA for selenium is 55 mcg/day for adults 14+ (60 mcg during pregnancy, 70 mcg while nursing) [3]. Most U.S. adults already consume adequate selenium from diet (average intake ~108 mcg/day from food alone). Supplementation primarily benefits those with documented low levels.

Recommended Dietary Allowances (RDAs)

Age Group	Male	Female	Pregnancy	Lactation
0–6 months	15 mcg*	15 mcg*	—	—
7–12 months	20 mcg*	20 mcg*	—	—
1–3 years	20 mcg	20 mcg	—	—
4–8 years	30 mcg	30 mcg	—	—
9–13 years	40 mcg	40 mcg	—	—
14–18 years	55 mcg	55 mcg	60 mcg	70 mcg
19–50 years	55 mcg	55 mcg	60 mcg	70 mcg
51+ years	55 mcg	55 mcg	—	—

*Adequate Intake (AI)

Tolerable Upper Intake Levels (ULs)

The UL for selenium is 400 mcg/day for adults and adolescents 14+ (combined dietary and supplemental intake) [1][91]. In 2023, the European Food Safety Authority (EFSA) set a more conservative UL of 255 mcg/day for all adults, including pregnant and lactating women, based on systematic reviews linking excess selenium to alopecia [92]. Children's ULs range from 70 to 230 mcg/day depending on age.

Practical Dosing Considerations

Most adults in the US and Canada do not need selenium supplements. The average US adult already consumes approximately 108 mcg/day from food alone — nearly double the RDA [9]. Adding a selenium supplement on top of adequate dietary intake could push total intake toward or above the UL.

For general supplementation (to fill potential gaps): A modest dose of 25–55 mcg from a multivitamin is reasonable and unlikely to cause harm. Since it is selenium deficiency that is associated with disease risk, meeting the RDA should be sufficient to capture any benefit [10].

For Hashimoto's thyroiditis: Studies have used 80–200 mcg/day (as selenomethionine or sodium selenite), though benefits have been most evident at 200 mcg/day in populations with low baseline selenium [41][42].

For male fertility (in deficient men): 100–200 mcg/day for at least 3 months [82][83].

Important safety note: When considering selenium supplementation, it is critical to account for dietary intake. A study in Denmark found that adults who took 300 mcg/day selenium for 5 years had an 11% greater risk of all-cause mortality compared to placebo. The researchers concluded that even 200 mcg supplemental selenium could result in toxicity in populations where selenium levels are already high, such as in the US (average blood selenium 137 mcg/L) (Rayman, Free Radic Biol Med, 2018) [93]. Total selenium intake (diet + supplements) should remain well below the Tolerable Upper Intake Level of 400 mcg/day.

Safety and Side Effects

The overall evidence points to a U-shaped relationship between selenium status and health outcomes: both too little and too much selenium are harmful. The therapeutic window is narrow [101].

Selenosis (Chronic Toxicity)

Chronically high selenium intake leads to selenosis. The earliest clinical signs are:

• Hair and nail brittleness and loss — the primary indicator used to establish the UL of 400 mcg/day [1][91]. Can occur above 400 mcg/day total intake [20].
• Garlic odor on the breath and metallic taste in the mouth — early warning signs [1][94].
• Skin rash, nausea, diarrhea, fatigue, irritability.
• Nervous system abnormalities (depression, nervousness, emotional instability) — typically beginning at approximately 900 mcg/day [10].

A case report from Turkey described a 48-year-old woman consuming 150 mcg/day from a supplement plus an estimated 650–850 mcg/day from nuts, resulting in blood selenium of 1,300 mcg/L (normal range: 23–190 mcg/L). Within 6 months of discontinuing selenium and nut consumption, hair loss stopped, hair density increased, and nail abnormalities improved dramatically (Civas, J Cosmet Dermatol, 2023) [95].

Diabetes Risk

• The SELECT trial found that among people with higher baseline selenium, those given 200 mcg/day were 2.7 times more likely to develop type 2 diabetes compared to placebo. No increase was seen in those with lower baseline selenium (Stranges, Ann Intern Med, 2007) [13][99].
• A study of 21,334 men and women in Italy found that those with the highest dietary selenium intakes had a 64% greater risk of being hospitalized for type 2 diabetes over 8 years, independent of BMI (Vinceti, Nutr Metab Cardiovasc Dis, 2021) [14][100].

Prostate Cancer Risk

• 200 mcg/day increased high-grade prostate cancer by 91% in men with already-high selenium levels [10][32].
• In men with non-metastatic prostate cancer, 140+ mcg/day was associated with 160% greater prostate cancer mortality [11][33].

All-Cause Mortality at High Doses

• 300 mcg/day for 5 years was associated with 11% greater all-cause mortality compared to placebo (Rayman, Free Radic Biol Med, 2018) [21][93].

People with adequate-to-high selenium (serum ≥122 mcg/L) should generally avoid selenium supplements [23][101].

Acute Toxicity

Acute selenium toxicity can result from consuming misformulated supplements. In 2008, 201 people experienced severe adverse reactions from a liquid dietary supplement containing 200 times the labeled amount of selenium [96]. Acute toxicity can cause severe gastrointestinal and neurological symptoms, respiratory distress syndrome, myocardial infarction, kidney failure, cardiac failure, and, in rare cases, death [1][96].

Special Populations

• Brazil nut caution: Brazil nuts contain 68–91 mcg per nut (average 544 mcg per ounce of 6–8 nuts). Regular consumption can easily exceed the UL [1][96].
• Kidney dialysis patients: Selenium concentrations are often significantly lower in patients undergoing hemodialysis, partly because the procedure removes selenium from blood [102][103].
• Maximum safe doses for individuals with severe liver or kidney disease have not been established [10].

Drug Interactions

• Acid-reducing medications: PPIs, H2 receptor antagonists, and antacids may potentially reduce absorption of some selenium forms [10].
• Cisplatin and other chemotherapy: Cisplatin, used for ovarian, bladder, lung, and other cancers, can reduce selenium levels in hair and serum. Some small studies suggest selenium supplementation may reduce cisplatin's toxicity, but a Cochrane Review concluded the evidence is insufficient [104][105][106]. Consult oncologist.
• Fluoroquinolone antibiotics: Prophylactic selenium with vitamin E may reduce Achilles tendinopathy severity (but not incidence) caused by fluoroquinolones such as ciprofloxacin and levofloxacin [10].

Individuals taking medications on a regular basis should discuss their selenium status with their healthcare provider, particularly those on chemotherapy regimens or long-term acid-suppression therapy [1][106].

Dietary Sources

Most individuals in the US and Canada consume adequate selenium from their diets. Foods high in protein tend to be the best sources. The selenium content of plant-based foods varies by soil selenium content in the growing region [1][9].

Food	Serving	Selenium (mcg)	% DV (55 mcg)
Brazil nuts	1 oz (6–8 nuts)	544	989%
Yellowfin tuna, cooked	3 oz	92	167%
Rockfish, cooked	1 fillet	113.5	206%
Halibut, cooked	½ fillet	88.1	160%
Sardines, canned	3 oz	45	82%
Shrimp, cooked	3 oz	42	76%
Salmon, sockeye	1 fillet	82	149%
Chicken liver	3 oz	75	136%
Whole wheat flour	1 cup	74.2	135%
Pork chop, broiled	3 oz	37	67%
Beef steak, roasted	3 oz	37	67%
Turkey, roasted	3 oz	36	65%
Shiitake mushrooms	1 cup	36	65%
Eggs, hardboiled	1 cup	41.9	76%
Beef liver, pan fried	3 oz	28	51%
Spaghetti, cooked	1 cup	33	60%
Oatmeal, cooked	1 cup	13	24%
Mushrooms, portabella	1/2 cup	13	24%
Brown rice, cooked	1 cup	12	22%
Whole-wheat bread	1 slice	8	15%
Egg, hard-boiled (single)	1 large	15	27%
Cottage cheese, 1%	1 cup	20	36%
Yogurt, plain, low fat	1 cup	8	15%
Lentils, boiled	1 cup	6	11%
Spinach, frozen, boiled	1/2 cup	5	9%

Source: USDA FoodData Central [107].

Practical Notes

• Brazil nuts are uniquely rich in selenium but highly variable — a single nut provides roughly 68–91 mcg. Just 1–2 Brazil nuts daily can meet or exceed the RDA without supplementation, but regular consumption of a full ounce can exceed the UL [1][96].
• Animal sources are more consistent than plant sources because animals maintain predictable tissue selenium concentrations through homeostatic mechanisms, and formulated livestock feeds contain consistent selenium levels [1][108].
• Selenium in plant foods varies widely by soil selenium content. Wheat grown in selenium-rich Midwestern US soils contains substantially more selenium than wheat from selenium-poor European soils [1][9].
• Selenium in plant-based foods is more bioavailable than that in animal products, though total amounts from plants are less predictable due to soil variability [10].
• In selenium-deficient regions such as Finland, national programs have fortified fertilizers with sodium selenate since 1984, effectively raising population selenium intake without exceeding safe limits [24].

Groups at Higher Risk of Inadequacy

• People in selenium-deficient regions (parts of Europe, China, sub-Saharan Africa)
• Vegetarians and vegans (especially in low-selenium regions like the UK)
• People undergoing kidney dialysis (hemodialysis removes selenium from blood)
• People living with HIV (possibly due to malabsorption or inadequate intake)
• Smokers (lower selenium status, possibly due to increased oxidative stress)

References

1. NIH Office of Dietary Supplements. Selenium Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/
2. Gladyshev VN, et al. Selenoprotein Gene Nomenclature. J Biol Chem. 2016;291:24036-40. https://pubmed.ncbi.nlm.nih.gov/27758862/
3. Sunde RA. Selenium. In: Ross AC, et al., eds. Modern Nutrition in Health and Disease. 11th ed. 2014.
4. Lei XG, Rayman M, Sunde RA. Selenium. In: Tucker KL, et al., eds. Modern Nutrition in Health and Disease. 12th ed. 2024.
5. Winther KH, Rayman MP, Bonnema SJ, Hegedus L. Selenium in thyroid disorders. Nat Rev Endocrinol. 2020;16:165-76. https://pubmed.ncbi.nlm.nih.gov/32001830/
6. Grokipedia. Selenium — Essential functions and enzymes. https://grokipedia.com/page/Selenium
7. Hong LK, Diamond AM. Selenium. In: Marriott BP, et al., eds. Present Knowledge in Nutrition. 11th ed. 2020.
8. Burk RF, et al. Effects of chemical form of selenium on plasma biomarkers. Cancer Epidemiol Biomarkers Prev. 2006;15:804-10. https://pubmed.ncbi.nlm.nih.gov/16614127/
9. U.S. Department of Agriculture. What We Eat in America by Gender and Age, 2017-March 2020. 2022.
10. ConsumerLab. Selenium Supplements Review. https://www.consumerlab.com/reviews/selenium-supplements-review-ratings/selenium/
11. Levander OA. A global view of human selenium nutrition. Annu Rev Nutr. 1987;7:227-50.
12. Grokipedia. Selenium — Deficiency symptoms.
13. Gibbs J. Selenium status in British vegetarians. Dietetics. 2024.
14. Chen J. An original discovery: selenium deficiency and Keshan disease. Asia Pac J Clin Nutr. 2012;21:320-6. https://pubmed.ncbi.nlm.nih.gov/22705420/
15. Zhou H, et al. Prevention of Keshan Disease by Selenium Supplementation. Biol Trace Elem Res. 2018;186:98-105. https://pubmed.ncbi.nlm.nih.gov/29524054/
16. Jirong Y, et al. Sodium selenite for treatment of Kashin-Beck disease in children. Osteoarthritis Cartilage. 2012;20:605-13. https://pubmed.ncbi.nlm.nih.gov/22405744/
17. Grokipedia. Selenium — Deficiency symptoms.
18. Bastola MM, et al. Selenium, copper, zinc and hypertension: NHANES analysis. BMC Cardiovasc Disord. 2020;20:45. https://pubmed.ncbi.nlm.nih.gov/32013860/
19. Kafai MR, Ganji V. Factors influencing serum selenium concentrations in the USA. J Trace Elem Med Biol. 2003;17:13-8. https://pubmed.ncbi.nlm.nih.gov/12755896/
20. Terry EN, Diamond AM. Selenium. In: Erdman JW, et al., eds. Present Knowledge in Nutrition. 10th ed. 2012.
21. FAO/WHO. Vitamin and Mineral Requirements in Human Nutrition. 2004.
22. Richie JP Jr, et al. Comparative effects of two different forms of selenium on oxidative stress biomarkers. Cancer Prev Res (Phila). 2014;7:796-804. https://pubmed.ncbi.nlm.nih.gov/24938534/
23. Ferrari L, et al. Advances in selenium supplementation. Anim Nutr. 2023;14:193-203. https://pubmed.ncbi.nlm.nih.gov/37635935/
24. Grokipedia. Selenium — Dietary sources and intake. Finland fertilizer fortification program.
25. European Food Safety Authority. Se-methyl-L-selenocysteine — Scientific opinion. 2009.
26. Clark LC, et al. Effects of selenium supplementation for cancer prevention. JAMA. 1996;276:1957-63. https://pubmed.ncbi.nlm.nih.gov/8971064/
27. Reid ME, et al. Selenium supplementation and lung, colon, and prostate cancer: further analysis of the NPC trial. Cancer Epidemiol Biomarkers Prev. 2002.
28. Terry P, et al. Selenium intake and ovarian cancer risk in African-American women. J Nutr. 2017.
29. Grokipedia. Selenium — Health effects. NPC trial colorectal cancer findings.
30. Lippman SM, et al. Effect of selenium and vitamin E on risk of prostate cancer (SELECT). JAMA. 2009;301:39-51. https://pubmed.ncbi.nlm.nih.gov/19066370/
31. Klein EA, et al. Vitamin E and the risk of prostate cancer (SELECT follow-up). JAMA. 2011;306:1549-56. https://pubmed.ncbi.nlm.nih.gov/21990298/
32. Kristal AR, et al. Baseline selenium status and effects of selenium and vitamin E supplementation on prostate cancer risk. J Natl Cancer Inst. 2014;106:djt456. https://pubmed.ncbi.nlm.nih.gov/24563519/
33. Kenfield SA, et al. Selenium supplementation and prostate cancer mortality. J Natl Cancer Inst. 2015.
34. Vinceti M, et al. Selenium for preventing cancer. Cochrane Database Syst Rev. 2018;1:CD005195. https://pubmed.ncbi.nlm.nih.gov/29376219/
35. Filippini T, et al. Selenium for preventing skin and colorectal cancer. Cochrane Database Syst Rev. 2018.
36. U.S. Food and Drug Administration. Selenium and a Reduced Risk of Site-specific Cancers. 2009.
37. Di Dato C, et al. Profiling of selenium absorption in healthy subjects after L-selenomethionine supplementation. J Endocrinol Invest. 2017;40:1183-90. https://pubmed.ncbi.nlm.nih.gov/28612306/
38. Cross-sectional study in Denmark (n=805) on selenium and thyroid volume. Cited in NIH ODS.
39. Study of 1,900 middle-aged adults on serum selenium and thyroid volume (sex-specific). Cited in NIH ODS.
40. Systematic review of 9 controlled trials (n=679) on selenium and autoimmune thyroiditis. Cited in NIH ODS.
41. Systematic review and meta-analysis of 16 trials (n=1,494) on selenium and thyroid antibodies. Cited in NIH ODS.
42. Grokipedia. Selenium — Health effects. Hashimoto's thyroiditis evidence.
43. Trial of 200 mcg/day selenomethionine in 151 TPOAb-positive pregnant women. Cited in NIH ODS.
44. Trial of 83 mcg/day selenomethionine in TPOAb/TgAb-positive pregnant women. Cited in NIH ODS.
45. Trial of 60 mcg/day selenium yeast in TPOAb-positive pregnant women with iodine deficiency. Cited in NIH ODS.
46. American Thyroid Association. 2017 guidelines on selenium supplementation in pregnancy.
47. Randomized trial of 100/200/300 mcg/day selenium yeast in 368 healthy adults. Cited in NIH ODS.
48. Grokipedia. Selenium — Nutrient interactions. Selenium-iodine interaction.
49. Selenoproteins reduce inflammation and prevent lipid oxidation. Cited in NIH ODS.
50. Rees K, et al. Selenium supplementation for the primary prevention of cardiovascular disease. Cochrane Database Syst Rev. 2013;2013:CD009671. https://pubmed.ncbi.nlm.nih.gov/23440843/
51. Flores-Mateo G, et al. Selenium and coronary heart disease: a meta-analysis. Am J Clin Nutr. 2006;84:762-73. https://pubmed.ncbi.nlm.nih.gov/17023702/
52. Stranges S, et al. Higher selenium status associated with adverse blood lipid profile. J Nutr. 2010;140:81-7. https://pubmed.ncbi.nlm.nih.gov/19906807/
53. Bleys J, et al. Serum selenium levels and all-cause, cancer, and cardiovascular mortality. Arch Intern Med. 2008;168:404-10. https://pubmed.ncbi.nlm.nih.gov/18299496/
54. Xun P, et al. Longitudinal association between toenail selenium levels and subclinical atherosclerosis (CARDIA). Atherosclerosis. 2010;210:662-7. https://pubmed.ncbi.nlm.nih.gov/20138620/
55. Grokipedia. Selenium — Health effects. 2025 meta-analysis on selenium intake and CVD risk.
56. Cold F, et al. Randomised controlled trial of selenium supplementation on plasma cholesterol. Br J Nutr. 2015;114:1807-18. https://pubmed.ncbi.nlm.nih.gov/26420473/
57. Jenkins DJA, et al. Selenium, antioxidants, cardiovascular disease, and all-cause mortality. Am J Clin Nutr. 2020;112:1642-52. https://pubmed.ncbi.nlm.nih.gov/32780093/
58. Rayman M, et al. Selenium and well-being. Biol Psychiatry. 2006.
59. Savarino L, et al. Serum concentrations of zinc and selenium in elderly people. Exp Gerontol. 2001;36:327-39. https://pubmed.ncbi.nlm.nih.gov/11226746/
60. Shahar A, et al. Plasma selenium is positively related to neurological task performance. Mov Disord. 2010;25:1909-15. https://pubmed.ncbi.nlm.nih.gov/20687207/
61. Cardoso BR, et al. Selenium, selenoproteins and neurodegenerative diseases. Metallomics. 2015;7:1213-28. https://pubmed.ncbi.nlm.nih.gov/26072072/
62. Loef M, et al. Selenium and Alzheimer's disease: a systematic review. J Alzheimers Dis. 2011;26:81-104. https://pubmed.ncbi.nlm.nih.gov/21709373/
63. Ferdous KA, et al. Association between selenium intake and cognitive function among older adults. J Nutr Sci. 2023;12:e57. https://pubmed.ncbi.nlm.nih.gov/37251752/
64. 2024 meta-analysis of trace elements in Alzheimer's disease. Cited in Grokipedia.
65. 2024 study linking higher selenium status to better cognitive function. Cited in Grokipedia.
66. Cardoso BR, et al. Selenium Status Is Not Associated with Cognitive Performance in 154 Older Australian Adults. Nutrients. 2018;10:1847. https://pubmed.ncbi.nlm.nih.gov/30501088/
67. Kryscio RJ, et al. Selenium and vitamin E for prevention of dementia (PREADVISE). JAMA. 2017.
68. SU.VI.MAX post hoc analysis of antioxidant supplementation and cognitive function. Cited in NIH ODS.
69. 2022 systematic review and meta-analysis of 11 clinical trials of selenium for cognitive decline. Cited in NIH ODS.
70. Moghaddam A, et al. Selenium deficiency and COVID-19. Nutrients. 2020.
71. Grokipedia. Selenium — Health effects. 2024–2025 selenium nanoparticle research for SARS-CoV-2.
72. Stone CA, et al. Role of selenium in HIV infection. Nutr Rev. 2010;68:671-81. https://pubmed.ncbi.nlm.nih.gov/20961298/
73. Campa A, et al. Mortality risk in selenium-deficient HIV-positive children. J Acquir Immune Defic Syndr Hum Retrovirol. 1999;20:508-13. https://pubmed.ncbi.nlm.nih.gov/10225235/
74. Kupka R, et al. Selenium levels in children born to HIV-infected mothers. Eur J Clin Nutr. 2005;59:1250-8. https://pubmed.ncbi.nlm.nih.gov/16015248/
75. Trial of 200 mcg/day selenium in 300 HIV-positive adults in Rwanda. Cited in NIH ODS.
76. Trial of 200 mcg/day selenium in 146 HIV-positive adults in Iran. Cited in NIH ODS.
77. Cochrane Review of micronutrient supplementation in children with HIV. Cited in NIH ODS.
78. Cochrane Review of selenium supplementation in adults with HIV. Cited in NIH ODS.
79. Trial of 200 mcg/day selenium in 90 pregnant women with HIV in Nigeria. Cited in NIH ODS.
80. GPx4 as a structural component of mature sperm. Cited in NIH ODS.
81. 5-year prospective study of semen selenium and male infertility. Cited in NIH ODS.
82. 3-month trial of 100 mcg/day selenomethionine in 69 men with reduced sperm motility (Scotland). Cited in NIH ODS.
83. 3-month trial of 200 mcg/day selenium yeast in 70 men with idiopathic infertility. Cited in NIH ODS.
84. Trial of 300 mcg/day selenium yeast in 54 healthy men for 48 weeks. Cited in NIH ODS.
85. Hoeg A, et al. Bone mineral density and selenium in postmenopausal women. J Clin Endocrinol Metab. 2012.
86. Tarp U. Selenium and rheumatoid arthritis. Analyst. 1995.
87. Kahler W, et al. Selenium and diabetic neuropathy. Z Gesamte Inn Med. 1993.
88. Grokipedia. Selenium — Nutrient interactions. Selenium-vitamin E synergy.
89. Grokipedia. Selenium — Nutrient interactions. Selenium-CoQ10 synergy.
90. Improved cardiovascular health by supplementation with selenium and coenzyme Q10. Cited in Grokipedia.
91. Institute of Medicine. Dietary Reference Intakes: Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington, DC: National Academy Press; 2000.
92. European Food Safety Authority. Scientific opinion on tolerable upper intake levels for selenium. 2023.
93. Rayman MP, et al. Long-term selenium supplementation and all-cause mortality. Free Radic Biol Med. 2018.
94. Grokipedia. Selenium — Toxicity mechanisms.
95. Civas E, et al. Selenium toxicity and hair loss. J Cosmet Dermatol. 2023.
96. Acute selenium toxicity from misformulated supplement (2008 outbreak, 201 cases). Cited in NIH ODS.
97. Grokipedia. Selenium — Toxicity mechanisms. Thiol group binding.
98. Grokipedia. Selenium — Toxicity mechanisms. Oxidative stress from excess selenide.
99. Stranges S, et al. Selenium supplementation and type 2 diabetes. Ann Intern Med. 2007.
100. Vinceti M, et al. Dietary selenium intakes and type 2 diabetes hospitalization risk. Nutr Metab Cardiovasc Dis. 2021.
101. Rayman MP. Selenium and human health. Lancet. 2012;379:1256-68. https://pubmed.ncbi.nlm.nih.gov/22381456/
102. Tonelli M, et al. Trace elements in hemodialysis patients. BMC Med. 2009;7:25. https://pubmed.ncbi.nlm.nih.gov/19460136/
103. Tonelli M, et al. Concentrations of Trace Elements in Hemodialysis Patients. Am J Kidney Dis. 2017;70:696-704. https://pubmed.ncbi.nlm.nih.gov/28754457/
104. Cisplatin and selenium level reduction. Cited in NIH ODS.
105. Cisplatin and selenium level reduction in hair and serum. Cited in NIH ODS.
106. Cochrane Review of selenium supplementation to alleviate chemotherapy side effects. Cited in NIH ODS.
107. U.S. Department of Agriculture. FoodData Central. 2019.
108. Animal tissue selenium homeostasis. Cited in NIH ODS.
109. Regional variation in US selenium status. Cited in NIH ODS.