Selenium Vitamin Essential For Human Health

Selenium is an essential trace mineral - often referred to as a “vitamin” in popular contexts, though technically a mineral - that plays a vital role in human health. Although required in very small amounts (typically micrograms per day), selenium is incorporated into selenoproteins such as glutathione peroxidases, thioredoxin reductases, and iodothyronine deiodinases - critical enzymes for antioxidant defense, thyroid hormone metabolism, and cellular redox balance.

Because selenium must be encoded into proteins via the amino acid selenocysteine, it occupies a unique position in biology. Its physiological effects range across the endocrine system, cardiovascular health, immunity, reproduction, and neurodevelopment.

What does selenium do in the body?

Biochemical roles and selenoproteins

Selenium is being integrated into over 25 known human selenoproteins. They include:

• Glutathione peroxidases (GPXs) – detoxify hydrogen peroxide and lipid peroxides, rescuing the cells from oxidative injury.

• Thioredoxin reductases (TrxRs) – reduce antioxidant disulfide systems and are involved in DNA synthesis and repair.

• Iodothyronine deiodinases (DIOs) – they transform the thyroid hormone T4 into active T3 and thus regulate metabolic rate and thermogenesis.

Collectively, these enzyme systems have roles in antioxidant protection, modulation of immune function, and hormone regulation. Selenium status also influences epigenetic processes, such as DNA methylation and one-carbon metabolism.

Immune and metabolic support

Optimal selenium status optimizes natural and acquired immunity with enhanced T-cell growth and natural killer (NK) cell function. It can decrease susceptibility to life-threatening infections as well as modulate inflammatory processes.

Observational studies have suggested that moderate selenium intake is related to lower cardiovascular mortality, possibly as a result of influence on endothelial function and lipid metabolism, but causality is less clear.

In addition, there are a few hints of protective effects against chronic metabolic disorders like type 2 diabetes and fatty liver disease but with inconsistent and dose-dependent evidence.

Selenium Benefits

Thyroid metabolism and hormone function

Selenium plays an essential role in the functioning of deiodinase enzymes, which convert inactive T4 hormone to active T3. Selenium is also responsible for the detoxification of hydrogen peroxide produced during thyroid hormone synthesis, thereby preserving thyroid tissue and reducing antibody-mediated autoimmune responses.

Antioxidant and anti-inflammatory activities

By assisting GPX and TrxR systems, selenium lowers oxidative stress in cells. The action is behind less inflammation, preserved cell integrity, and potentially delayed aging processes.

Cardiovascular protection

Population evidence shows that mild selenium intake (e.g., ~135 µg/day) is linked with reduced stroke, heart failure, and coronary event risks—approximately 16% risk reductions reported in a large NHANES-based review. However, benefits plateau or become adverse for excessive intake.

Immune support

Aside from antioxidant defense, selenium facilitates effective immune response and can potentially reduce severity of certain infections. Some local studies correlated higher soil/biomarker selenium concentrations with reduced mortality from COVID-19, although the evidence remains preliminary.

Reproductive health and fertility

Selenium status influences reproductive organs and gamete quality in both sexes. Adequate selenium has been associated with increased sperm motility and ovarian follicular integrity; deficiency is associated with implantation failure and miscarriage risk.

Cognitive and neurological effects

Some evidence associates selenium deficiency with decreased cognitive function and mental disturbances, including depression, anxiety, and predisposition to neurodegenerative disease. Evidence is mostly observational.

What are the symptoms of selenium deficiency?

Selenium deficiency is uncommon in well-nourished populations but can occur in areas with low soil selenium or in individuals with malabsorption, pregnancy, HIV, or on long-term dialysis.

Clinical manifestations include:

• Cardiomyopathy, notably Keshan disease: congestive heart failure, arrhythmias, especially in selenium-deficient regions, classically northern China.

• Osteoarthropathy—Kashin-Beck disease—with cartilage degeneration and joint deformities.

• Hypothyroid symptoms: fatigue, goiter, recurrent miscarriage, cognitive slowing, driven by impaired T4→T3 conversion.

• Immune dysfunction: higher susceptibility to infections; slower recovery.

• Mental health and cognitive impairments, as well as reproductive disorders such as infertility and miscarriage risk increases.

Is it safe to take selenium every day? Daily Requirements by Age and Sex

Recommended intakes (RDA, EAR, UL)

According to the U.S. Institute of Medicine:

• EAR (Average Requirement): 45 µg/day for those aged ≥14

• RDA (Recommended Dietary Allowance): 55 µg/day for adults

• Pregnancy RDA: 60 µg/day

• Lactation RDA: 70 µg/day

• Tolerable Upper Intake Level (UL): 400 µg/day for adults

For children 1–13 years the RDA increases with age, ranging from 20 to 40 µg/day.

Daily safety and selenosis risk

Daily selenium intake at or below the RDA is considered safe. In clinical studies, supplementation up to 200 µg/day (e.g., as selenomethionine) was well tolerated even in pregnancy, provided total intake remained well below the 400 µg UL.

Chronic overconsumption above 400 µg/day may cause selenosis, typified by hair and nail brittleness, gastrointestinal upset, fatigue, irritability, neurological symptoms, and a characteristic garlic odour on the breath.

Consistency of daily use

Routine intake of selenium through diet or supplements at recommended levels is generally safe and beneficial. However, long-term high-dose supplementation beyond UL increases risk. It’s advisable to monitor total selenium intake from all sources.

Is selenium good for pregnancy?

Importance during gestation

Pregnancy increases oxidative stress and demands on thyroid and immune systems. Plasma selenium levels commonly decline during pregnancy, partly due to increased red cell mass and fetal demand.

Low maternal selenium status has been correlated with adverse outcomes: miscarriage, preeclampsia, preterm birth, gestational diabetes, low birth weight, and delayed neurodevelopment.

Supplementation evidence

Multiple randomized controlled trials have examined selenium supplementation in pregnant women. Outcomes include reduced postpartum thyroid dysfunction and improvements in thyroid-related antibody titres—but evidence remains limited and of variable quality.

Systematic reviews conclude that while selenium supplementation may be promising in high-risk or deficient populations, current evidence is insufficient for broad recommendation. Well-designed clinical trials are still needed.

Safety considerations

Supplementation up to 200 µg/day during pregnancy appears safe when total intake remains under the 400 µg UL. Side effects are rare at these doses; however, routine supplementation is only recommended when deficiency is documented or risk is high.

What food is highest in selenium?

Dietary selenium content depends heavily on the selenium concentration of the soil where plants are grown or animals graze. Foods highest in selenium include:

• Brazil nuts – extremely rich and often cited as the richest natural source (soil-dependent).

• Seafood – fish, shellfish, and seafood tend to contain meaningful selenium levels.

• Meat and poultry – organ meats and muscle meats.

• Eggs and dairy – moderate but consistent supplies.

• Cereals and grains, mushrooms, and legumes – varying levels depending on soil selenium.

Just a few Brazil nuts per day (e.g., 2–4 nuts) may provide an adult’s entire RDA, but intake should be moderated to avoid exceeding the UL.

Selenium Supplementation

Supplements are available in several forms:

• Selenomethionine (organic form)—widely used in supplements and better retained in body tissues.

• Sodium selenite or selenate (inorganic forms)—also effective but may differ in bioavailability and toxicity profiles.

Typical supplement doses range from 50 µg up to 200 µg per dose. Multivitamins often carry ~55 µg, while standalone supplements may offer 100–200 µg.

Use should be tailored based on individual dietary intake, clinical lab tests of plasma/whole-blood selenium, and consultation with a healthcare provider.