Selenium

Selenium is a trace mineral required in small amounts but plays an outsized role in maintaining cellular and hormonal balance. It is incorporated into selenoproteins, a special class of proteins that include selenium as the amino acid selenocysteine. These selenoproteins are crucial for several biological functions:

• Antioxidant defence – Selenium is a key component of glutathione peroxidase (GPx) and thioredoxin reductase (TrxR), enzymes that neutralise free radicals and protect cells from oxidative damage.
  

• Thyroid hormone metabolism – Selenium helps convert the inactive thyroid hormone thyroxine (T4) into its active form triiodothyronine (T3), directly influencing metabolism, energy levels, and mood.
  

• Immune system support – Selenium enhances the production and activity of immune cells, strengthening resistance to infections.
  

The body carefully regulates selenium through intestinal absorption and urinary excretion. However, since selenium content in foods depends on soil concentration, dietary intake can vary widely between regions. Good food sources include Brazil nuts, seafood, organ meats, poultry, eggs, and whole grains.

By supporting antioxidant enzymes, DNA repair, thyroid hormone activation, and immune cell signalling, selenium plays a foundational role in maintaining cellular vitality, metabolic health, and longevity.

Selenium’s role in human health extends beyond simple nutrition — it’s central to cellular defence, hormonal balance, and chronic disease prevention.

Antioxidant and anti-inflammatory protection:
Selenium-containing enzymes such as GPx and TrxR protect cells from oxidative stress caused by free radicals — unstable molecules generated through metabolism, stress, and environmental toxins. By reducing oxidative damage, selenium supports the prevention of cellular aging, cardiovascular disease, and neurodegenerative disorders such as Alzheimer’s and Parkinson’s disease.

Thyroid health:
The thyroid gland contains more selenium per gram than any other organ in the body. Selenium-dependent enzymes convert T4 into T3 — the hormone that regulates metabolism, temperature, and energy production. Deficiency can slow this conversion, leading to fatigue, weight gain, and hypothyroid symptoms even when iodine intake is adequate. Selenium also protects the thyroid from autoimmune damage in conditions like Hashimoto’s thyroiditis and Graves’ disease.

Immune function:
Adequate selenium supports the activation and proliferation of immune cells, helping the body fight off viral infections and modulate inflammation. Studies show low selenium status is linked to weakened immune response and higher susceptibility to infections, while optimal levels improve recovery and resilience.

Cognitive and cardiovascular health:
Selenium contributes to brain function and reduces oxidative damage in neurons. Chronic low selenium has been linked to cognitive decline, while excessive intake has been associated with type 2 diabetes and cardiometabolic risk.

Deficiency vs. toxicity:

• Deficiency can lead to fatigue, brain fog, weakened immunity, thyroid dysfunction, infertility, and oxidative stress–related damage.
  

• Excess intake (selenosis) may cause hair loss, brittle nails, nausea, fatigue, and in severe cases, neurological issues.
  

Maintaining balanced selenium levels is key — both too little and too much can disrupt vital physiological processes.

Dietary Factors:

• Rich sources: Brazil nuts (1–2 nuts can meet daily needs), seafood, meat, poultry, eggs, and whole grains.

• Deficiency: Occurs in regions with selenium-depleted soil, diets low in animal protein, or limited food diversity.

• Excess intake: Can occur from overconsumption of Brazil nuts, supplements, or selenium-fortified foods.
  

Lifestyle Factors:

• Smoking and alcohol use can reduce selenium absorption and increase oxidative stress, depleting stores.

• Certain medications (e.g., chemotherapy drugs, corticosteroids, statins) can affect selenium metabolism.
  

Related Biomarkers:

• Zinc and Vitamin E: Work synergistically with selenium in antioxidant defence. Low zinc or vitamin E may increase oxidative stress and diminish selenium’s protective effects.

• Iodine and Thyroid Hormones: Selenium and iodine are co-dependent for healthy thyroid function. Low selenium may worsen iodine deficiency symptoms.
  

Micronutrient Interactions:

• Vitamin A supports the expression of selenoproteins; deficiencies in either can impair immune and antioxidant function.

• Iron status may also influence selenium absorption and utilisation.
  

Environmental Influences:

• The selenium content of soil varies by geography. Regions with low soil selenium (such as parts of China, New Zealand, and Europe) produce foods lower in selenium.

• Occupational exposure to selenium compounds or supplements exceeding recommended doses can raise levels to toxic ranges.
  

If Your Results Are High

If your selenium levels are elevated, this may suggest over-supplementation or high dietary intake.

Diet

• Limit high-selenium foods such as Brazil nuts (to no more than 1–2 nuts per week), seafood, and organ meats until levels normalise.
  

Lifestyle

• Avoid using selenium supplements or multivitamins containing selenium. Ensure drinking water is not contaminated with selenium from environmental sources.
  

Supplements

• Discontinue selenium supplementation and review all multivitamins or trace mineral blends.

If Your Results Are Low

Low selenium may indicate poor dietary intake, regional soil deficiency, or malabsorption.

Diet

• Include selenium-rich foods daily — such as seafood (tuna, sardines), eggs, poultry, whole grains, and 1 Brazil nut per day.
  

Lifestyle

• Reduce alcohol and smoking, which can deplete selenium and impair absorption.
  

Supplements

• A low-dose selenium supplement (50–100 µg daily) may help restore optimal levels, under medical supervision.

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2. Bai S, Zhang M, Tang S, et al. Effects and Impact of Selenium on Human Health, a Review. Molecules (Basel, Switzerland). 2024;30(1):50. doi:10.3390/molecules30010050.
   
   

3. Shahidin, Wang Y, Wu Y, et al. Selenium and Selenoproteins: Mechanisms, Health Functions, and Emerging Applications. Molecules (Basel, Switzerland). 2025;30(3):437. doi:10.3390/molecules30030437.
   
   

4. Hu W, Zhao C, Hu H, Yin S. Food Sources of Selenium and Its Relationship With Chronic Diseases. Nutrients. 2021;13(5):1739. doi:10.3390/nu13051739.