What Is Selenium?
Selenium is an essential trace mineral that functions as a powerful antioxidant in the body. It's incorporated into proteins to make selenoproteins, which are important antioxidant enzymes that protect cells from damage. Selenium is found in soil, water, and certain foods, and is necessary for proper health and functioning of multiple body systems.
What It's Used For (Functions in the Body)
Selenium serves several critical functions:
Antioxidant protection - Component of glutathione peroxidase and other antioxidant enzymes that protect cells from oxidative damage
Thyroid hormone metabolism - Essential for converting T4 (inactive thyroid hormone) to T3 (active form)
DNA synthesis - Involved in creating and repairing genetic material
Immune function - Supports proper immune system response
Reproduction - Important for sperm motility and reproductive health
Selenoprotein production - Creates 25+ selenoproteins with various functions
Inflammation regulation - Helps control inflammatory responses
Cardiovascular health - Protects blood vessels from oxidative damage
Benefits of Adequate Intake
Proven benefits:
Thyroid function support: Essential for proper thyroid hormone production and metabolism
Immune system enhancement: Improves immune response and may reduce infection severity
Antioxidant protection: Reduces oxidative stress and cellular damage
Reproductive health: Supports fertility in both men and women; improves sperm quality
Cancer risk reduction: Some studies suggest adequate selenium may lower risk of certain cancers (prostate, lung, colorectal)
Heart health: May reduce cardiovascular disease risk
Cognitive function: May help preserve brain health and reduce cognitive decline
Anti-inflammatory effects: Reduces inflammatory markers
Potential benefits (more research needed):
May reduce asthma severity
Could improve mood disorders
Possible role in HIV progression
May help with autoimmune thyroid conditions (Hashimoto's, Graves')
Potential Negatives/Side Effects
At appropriate doses, selenium is safe. However:
Excess intake can cause:
Garlic breath odor (selenosis)
Metallic taste in mouth
Brittle nails or nail loss
Hair loss
Skin rashes and lesions
Digestive issues (nausea, diarrhea)
Fatigue and irritability
Nervous system problems
In severe cases: liver and kidney damage
Other considerations:
Narrow therapeutic window: The difference between beneficial and toxic doses is relatively small
May increase diabetes risk: Some studies suggest high-dose selenium supplementation might increase type 2 diabetes risk in those with adequate levels
Possible increased cholesterol: Very high doses may raise LDL cholesterol
Pro-oxidant effects: Excessive amounts can paradoxically increase oxidative stress
Recommended Intake/Serving Size
Recommended Dietary Allowances (RDA):
Adults (19+ years): 55 mcg/day
Pregnant women: 60 mcg/day
Breastfeeding women: 70 mcg/day
Teenagers (14-18): 55 mcg/day
Children (9-13): 40 mcg/day
Children (4-8): 30 mcg/day
Children (1-3): 20 mcg/day
Infants (7-12 months): 20 mcg/day
Infants (0-6 months): 15 mcg/day
Upper Tolerable Limit:
Adults and children over 14: 400 mcg/day
Children (9-13): 280 mcg/day
Children (4-8): 150 mcg/day
Children (1-3): 90 mcg/day
Therapeutic/Supplemental doses typically range from 50-200 mcg, with most supplements containing 100-200 mcg.
Important: Soil selenium content varies dramatically by region, affecting food content. Some areas have selenium-deficient soil (parts of China, Europe, New Zealand), while others have selenium-rich soil (parts of US Great Plains).
What to Take With It
Synergistic nutrients:
Vitamin E: Works synergistically with selenium for antioxidant protection; they enhance each other's effects
Vitamin C: Another antioxidant that works well with selenium
Zinc: Supports immune function alongside selenium
Iodine: Important for thyroid function (but see cautions below)
Coenzyme Q10: Works with selenium for cardiovascular and antioxidant benefits
Supportive nutrients:
B-vitamins (especially B6 and B12): Support overall metabolism with selenium
Vitamin A: Works with selenium in immune function
What NOT to Take With It (or Use Caution)
Problematic combinations:
High-dose vitamin C: Very high amounts (1,000+ mg) may reduce selenium absorption; space them apart by 2-4 hours if taking both in high doses
Antacids: May reduce selenium absorption; take 2 hours apart
Heavy metals: Mercury, lead, and cadmium can bind to selenium and reduce its effectiveness
High-dose iodine WITH selenium: In people with iodine deficiency, adding selenium before correcting iodine deficiency can worsen thyroid problems. Address iodine deficiency first, then add selenium, or add both together under medical supervision
Medication interactions:
Chemotherapy drugs: Selenium may interfere with some chemotherapy medications; consult oncologist
Anticoagulants (blood thinners): High doses may increase bleeding risk
Statins: Selenium may reduce effectiveness of some cholesterol medications
Niacin: Large doses of niacin may reduce selenium levels
Oral contraceptives: May reduce selenium levels
Corticosteroids: Long-term use may reduce selenium levels
Who Should Take It
Good candidates for supplementation:
People in selenium-deficient regions: Areas with low soil selenium (parts of Europe, China, New Zealand, certain US regions)
Vegetarians and vegans: Especially if not eating Brazil nuts or selenium-rich plant foods regularly
People with digestive disorders: Crohn's disease, celiac disease, ulcerative colitis (malabsorption issues)
Dialysis patients: Kidney disease patients on dialysis lose selenium
HIV-positive individuals: Often have lower selenium levels
Smokers: Increased oxidative stress increases selenium needs
Those with autoimmune thyroid disease: May benefit under medical supervision (Hashimoto's thyroiditis)
Older adults: May have lower intake and absorption
People trying to conceive: Particularly men with fertility issues
Those with increased oxidative stress: Athletes, people with chronic diseases
Who Should NOT Take It (or Use Caution)
Avoid or limit supplementation:
People with adequate selenium levels: No benefit to supplementing if levels are already sufficient
Those with history of non-melanoma skin cancer: Some research suggests high selenium may increase risk
People at risk for type 2 diabetes: High-dose supplementation may increase diabetes risk in susceptible individuals
Individuals with thyroid disorders: Should only supplement under medical supervision
Pregnant women: Should not exceed 60 mcg/day without medical advice
Those with kidney disease: May not excrete selenium properly (except dialysis patients who lose selenium)
People taking certain medications: Especially chemotherapy, blood thinners (consult doctor)
Those already consuming high-selenium foods regularly: Risk of excess intake
Deficiency Symptoms
Selenium deficiency is uncommon in developed countries but can occur. Symptoms include:
Mild to moderate deficiency:
Fatigue and weakness
Mental fog or confusion
Hair loss or thinning
Weakened immune system (frequent infections)
Muscle weakness or pain
Infertility (particularly in men)
Hypothyroidism symptoms
White spots on nails
Skin discoloration
Severe deficiency can cause specific diseases:
Keshan disease: A potentially fatal cardiomyopathy (heart muscle disease)
Occurs in selenium-deficient regions of China
Heart enlargement, irregular heartbeat, heart failure
Affects primarily children and women of childbearing age
Kashin-Beck disease: A type of osteoarthropathy (joint and bone disorder)
Joint deformities, stunted growth
Chronic joint pain and stiffness
Also occurs in selenium-deficient regions
Risk factors for deficiency:
Living in selenium-poor soil regions
Digestive disorders affecting absorption
Dialysis
HIV/AIDS
Prolonged parenteral (IV) nutrition without selenium
Severe gastrointestinal disorders
Toxicity Symptoms (Selenosis)
Selenium toxicity occurs when intake exceeds 400 mcg/day regularly. Symptoms include:
Early signs (acute toxicity):
Garlic breath odor (hallmark symptom)
Metallic taste in mouth
Nausea and vomiting
Diarrhea
Abdominal pain
Fatigue and irritability
Dizziness
Progressive symptoms (chronic toxicity):
Hair loss (often dramatic)
Brittle nails or nail loss
Skin rashes, lesions, or dermatitis
Tooth decay and discoloration
Neurological issues:
Tremors
Numbness and tingling
Paralysis (severe cases)
Mottled teeth
Nervous system abnormalities
Severe toxicity:
Respiratory distress
Heart attack
Kidney failure
Liver damage or failure
Death (in extreme cases)
Note: Toxicity typically occurs from:
Over-supplementation
Taking multiple supplements containing selenium
Consuming large amounts of Brazil nuts daily (2-3 nuts can provide 200+ mcg)
Manufacturing errors in supplements (rare but has occurred)
Living in high-selenium regions with selenium-rich water
Timing and Food Considerations
Best time to take:
Any time of day - selenium absorption is not significantly affected by timing
Consistency matters more than timing: Take at the same time daily for routine
Morning or with largest meal is convenient for most people
Some people prefer morning to avoid any potential digestive upset at night
With food or without:
Can be taken with or without food
Taking with food may reduce stomach upset (though uncommon with selenium)
Fat-containing meals may slightly enhance absorption of some selenium forms
If taking a multivitamin containing selenium, follow food instructions for optimal absorption of all nutrients
Form-specific considerations:
Selenomethionine: Well-absorbed with or without food
Sodium selenite/selenate: May be better absorbed with food
Selenium from yeast: Generally well-absorbed regardless of food
Splitting doses:
If taking higher therapeutic doses (200+ mcg), consider splitting into two doses
Not necessary for standard supplemental doses (50-200 mcg)
Food Sources
Most people can meet selenium needs through diet:
Excellent sources (very high):
Brazil nuts: 68-91 mcg per nut (1-2 nuts meets daily needs; don't overeat!)
Seafood: Yellowfin tuna (92 mcg per 3 oz), sardines, oysters, clams, shrimp, salmon, halibut
Organ meats: Kidneys, liver
Good sources (high):
Meat: Beef, turkey, chicken (20-30 mcg per 3 oz)
Eggs: 20 mcg per large egg
Cottage cheese: 20 mcg per cup
Brown rice: 19 mcg per cup cooked
Moderate sources:
Whole wheat bread: 13 mcg per slice
Oatmeal: 13 mcg per cup
Baked beans: 13 mcg per cup
Mushrooms (particularly cremini): 10-15 mcg per cup
Spinach: 11 mcg per cup cooked
Variable sources (depends on soil):
Grains and cereals (varies widely by growing region)
Vegetables grown in selenium-rich soil
Animal products (reflects selenium content of animal feed)
Low sources:
Most fruits
Most vegetables (unless grown in selenium-rich soil)
Processed foods
Forms of Selenium in Supplements
Organic forms (generally better absorbed):
Selenomethionine: Most common, well-absorbed (90%+), from yeast, stored in body proteins
Selenium-enriched yeast: Contains multiple selenium forms, mimics food
Selenocysteine: Highly bioavailable, less common in supplements
Inorganic forms (less preferred):
Sodium selenite: Moderately well absorbed (50-60%), less expensive
Sodium selenate: Similar to selenite, may have pro-oxidant effects at high doses
Best choice: Selenomethionine or selenium-enriched yeast for supplementation.
Additional Important Information
Selenium Status Testing:
Blood tests measure selenium levels
Normal range: 70-150 mcg/L (varies by lab)
Optimal range often considered: 120-150 mcg/L
Testing useful if deficiency or excess suspected
Factors Affecting Selenium Levels:
Increase selenium needs:
Smoking
Alcohol consumption
Intense physical activity
Chronic stress
Pregnancy and breastfeeding
Inflammatory conditions
Exposure to heavy metals
Decrease selenium levels:
Digestive disorders
Kidney dialysis
Certain medications (oral contraceptives, corticosteroids)
Low-protein diets
Advanced age
Selenium and Cancer:
Mixed research results
Some studies show protective effects against certain cancers (prostate, lung, colorectal)
Other studies show no benefit or potential harm
SELECT trial found no benefit for prostate cancer prevention; possible increased diabetes risk
Current consensus: Adequate intake from food is beneficial; high-dose supplementation not recommended for cancer prevention
Selenium and Thyroid Health:
Critical for thyroid hormone conversion (T4 to T3)
May reduce thyroid antibodies in Hashimoto's thyroiditis
Important for Graves' disease management
Should not supplement without also ensuring adequate iodine
Work with endocrinologist for thyroid-related supplementation
Selenium and COVID-19:
Some research suggests adequate selenium status associated with better outcomes
Not a treatment or prevention for COVID-19
Maintaining adequate levels supports overall immune function
Geographic Variations:
Selenium-poor regions: Parts of China (where Keshan disease occurs), Finland (before soil supplementation), New Zealand, Scotland, parts of Eastern Europe
Selenium-rich regions: Parts of Great Plains (US), Venezuela, parts of China
Fortification programs: Some countries (Finland) add selenium to fertilizers
Selenium and Fertility:
Important for male fertility: improves sperm motility and morphology
May reduce miscarriage risk in women with adequate levels
Protects reproductive organs from oxidative damage
Often included in fertility supplements
Interactions with Health Conditions:
May benefit:
Autoimmune thyroid disease
Asthma
Cardiovascular disease
Cognitive decline
HIV/AIDS
Male infertility
May worsen or complicate:
Type 2 diabetes (at high doses)
Non-melanoma skin cancer history
Certain thyroid conditions (without medical supervision)
Quality Considerations:
Choose reputable supplement brands
Look for third-party testing (USP, NSF, ConsumerLab)
Check for form of selenium (selenomethionine preferred)
Verify dosage matches label
Be aware of manufacturing errors (rare but serious)
Signs You May Need More Selenium:
Frequent infections
Fatigue despite adequate sleep
Thyroid issues (with normal TSH)
Poor wound healing
Muscle weakness
Hair thinning
White spots on fingernails
Living in selenium-poor region
Signs You May Be Getting Too Much:
Garlic breath (without eating garlic)
Metallic taste
Hair loss
Brittle nails
Skin rashes
Digestive issues
Fatigue and irritability
Bottom Line
Selenium is an essential mineral with powerful antioxidant and metabolic functions, particularly important for thyroid health, immune function, and reproductive health. Most people in North America get adequate amounts from food, but supplementation may benefit those with increased needs, malabsorption issues, or living in selenium-poor regions.
Key takeaways:
RDA is 55 mcg/day for adults; easily obtained from 1-2 Brazil nuts or seafood
Supports thyroid function, immune health, fertility, and antioxidant protection
Deficiency is rare but can cause serious health issues (Keshan disease, weakened immunity)
Toxicity can occur above 400 mcg/day with distinctive symptoms (garlic breath, hair loss)
Narrow margin between beneficial and harmful doses - don't over-supplement
Brazil nuts are extremely high in selenium - limit to 1-2 per day maximum
Works synergistically with vitamin E and other antioxidants
Best supplemental forms: selenomethionone or selenium-enriched yeast
Can be taken any time, with or without food
If supplementing, stick to 50-200 mcg/day unless under medical supervision
Consider testing selenium levels if you have thyroid issues, fertility concerns, or frequent infections
Most people should focus on getting selenium from food rather than supplements unless there's a specific medical need or risk factor for deficiency.