Atomic Elements, Trace Essential Elements, Human

Selenium (Se)

23
May

Selenium (Se) is a naturally occurring chemical element essential for human health in trace amounts, but toxic in excess. Unlike harmful metals like lead or mercury, selenium is a vital nutrient with critical roles in antioxidant defense and thyroid function. This article provides a clear, engaging, and scientifically accurate overview of selenium, empowering you to understand its properties, benefits, risks, and practical steps to maintain a healthy balance.

What Is Selenium?

Basic Chemical and Physical Properties

  • Atomic Number: 34
  • Symbol: Se
  • Appearance: Selenium exists as a gray, crystalline solid or a red amorphous form, with a metallic sheen in some forms.
  • Classification: A non-metal in the periodic table, often found in sulfide ores alongside sulfur and metals like copper.
  • Properties: Selenium is a semiconductor, used in electronics, glass, and solar cells. Its biological role in selenoproteins makes it essential, but its narrow therapeutic range requires careful management.

Selenium’s dual nature as a nutrient and potential toxin necessitates balanced intake for optimal health.

Where Is Selenium Found?

Natural Occurrence and Common Sources

  • Environmental Presence: Selenium is present in the Earth’s crust, primarily in sulfide ores and soils, and is released through weathering, volcanic activity, and human activities like mining and coal burning.
  • Exposure Routes:
    • Dietary Sources:
      • Brazil nuts, seafood (e.g., tuna, shrimp, sardines), and organ meats (e.g., liver, kidney).
      • Whole grains (e.g., wheat, oats), eggs, and dairy products from animals fed selenium-rich feed.
      • Vegetables (e.g., mushrooms, broccoli) grown in selenium-rich soils.
      • Drinking water, though levels vary by region.
    • Environmental Sources:
      • Soil and dust in selenium-rich areas (e.g., parts of the U.S. Great Plains, China).
      • Air pollution from coal combustion or industrial emissions.
    • Other Sources:
      • Selenium supplements (e.g., selenomethionine, sodium selenite) or multivitamins.
      • Occupational exposure in industries like electronics, glass manufacturing, or mining.
      • Consumer products like anti-dandruff shampoos (e.g., selenium sulfide) or fertilizers.

Most dietary selenium comes from animal products and grains, with supplements and environmental exposure contributing in specific cases.

Selenium in the Human Body

Biological Importance and Functions

  • Essential Role: Selenium is a key component of selenoproteins, including glutathione peroxidase (antioxidant defense) and iodothyronine deiodinases (thyroid hormone activation).
  • Metabolism: Selenium is absorbed in the small intestine (50–90% bioavailability), distributed to tissues like the liver, kidneys, and muscles, and excreted primarily via urine. It has a half-life of weeks to months in the body.
  • Functions: Supports antioxidant protection, thyroid function, immune response, and reproductive health.

How It Affects the Body

  • Selenium protects cells from oxidative damage, supports hormone metabolism, and enhances immune function.
  • Excess selenium can cause oxidative stress or disrupt normal protein function, leading to toxicity.

Health Impacts of Selenium Exposure

Health Benefits and Deficiency Symptoms

  • Health Benefits:
    • Enhances antioxidant defenses, reducing oxidative stress and inflammation.
    • Supports thyroid health by aiding hormone production and metabolism.
    • Boosts immune function, potentially reducing infection risk.
    • May lower the risk of certain cancers (e.g., prostate) and heart disease, though evidence is mixed.
  • Deficiency Symptoms:
    • Selenium deficiency is rare in well-nourished populations but common in areas with low soil selenium (e.g., parts of China).
    • Symptoms include:
      • Keshan disease (cardiomyopathy, heart failure).
      • Kashin-Beck disease (joint and bone disorders).
      • Weakened immune system or thyroid dysfunction (e.g., hypothyroidism).
      • Fatigue, infertility, or muscle weakness.

Health Risks and Toxicity

Selenium is safe in recommended amounts, but excessive intake is toxic. The International Agency for Research on Cancer (IARC) does not classify selenium as a carcinogen, but high doses may pose risks.

  • Selenosis:
    • Chronic excess causes symptoms like hair loss, brittle nails, and neurological issues (e.g., tremors, irritability).
  • Gastrointestinal Effects:
    • High doses may lead to nausea, diarrhea, or abdominal pain.
  • Other Effects:
    • Excess selenium may increase diabetes risk or impair liver function.
    • Rare cases of skin rashes or garlic-like breath odor.

Symptoms of Acute Exposure

  • High-level exposure (e.g., accidental overdose of supplements) may cause:
    • Nausea, vomiting, or abdominal pain.
    • Headache, dizziness, or irritability.
    • Garlic-like breath or metallic taste.

Symptoms of Chronic Exposure

  • Low-level, long-term excess may result in:
    • Hair loss or nail deformities.
    • Neurological symptoms like numbness or mood changes.
    • Fatigue or muscle pain.

Selenium in Supplements or Nutrition

Forms in Supplements

  • Selenium is available in supplements as selenomethionine, sodium selenite, or sodium selenate, often included in multivitamins or antioxidant formulas.
  • Fortified foods (e.g., cereals) may provide additional selenium.
  • Choose supplements from reputable brands with third-party testing to ensure accurate dosing and purity.

Recommended Intake and Exposure Limits

Recommended Intake

  • The Recommended Dietary Allowance (RDA) for selenium (U.S. National Institutes of Health):
    • Adults: 55 μg/day.
    • Pregnancy: 60 μg/day.
    • Lactation: 70 μg/day.
    • Children: 15–40 μg/day, depending on age.
  • Tolerable Upper Intake Levels (UL):
    • Adults: 400 μg/day.
    • Children: 45–300 μg/day, depending on age.
  • The World Health Organization (WHO) suggests a safe dietary range of 26–34 μg/day for adults, with adjustments for higher needs.

Typical Exposure

  • Dietary intake typically ranges from 50–150 μg/day in selenium-adequate regions, meeting needs for most people.
  • Higher intake occurs in areas with selenium-rich soils or with supplement use.

Minimizing Exposure

  • Dietary Choices:
    • Consume a balanced diet with selenium-rich foods like seafood, eggs, and grains to meet needs without excess.
    • Limit Brazil nut consumption (one nut can contain 50–90 μg of selenium) to avoid toxicity.
    • Diversify diet to avoid over-reliance on foods from selenium-rich regions.
  • Home Safety:
    • Test drinking water for selenium in areas with high natural levels or industrial contamination.
    • Store selenium supplements safely to prevent accidental overdose.
  • Lifestyle:
    • Use protective equipment in occupations involving selenium (e.g., mining, electronics).
    • Avoid excessive use of selenium-containing shampoos to prevent skin or systemic absorption.
  • Supplements:
    • Use selenium supplements only under medical supervision, especially in selenium-adequate regions.
    • Choose products with third-party testing to ensure safety and accurate dosing.

Safety Considerations and Toxicity Risks

Safety Profile

  • Selenium is essential and safe in recommended amounts, but its narrow therapeutic range (deficiency below 40 μg/day, toxicity above 400 μg/day) requires careful management.
  • Populations at higher risk include:
    • Individuals in selenium-deficient regions (e.g., parts of China, New Zealand).
    • Those in selenium-rich areas or consuming excessive supplements/Brazil nuts.
    • Workers in selenium-related industries (e.g., electronics, glass production).

Toxicity Thresholds

  • Chronic Excess: Selenosis may occur at intakes above 400 μg/day over time.
  • Acute Poisoning: Rare but possible at doses above 1,000 μg, causing severe gastrointestinal or neurological symptoms.

Interactions

  • Selenium interacts with other nutrients:
    • Iodine: Selenium supports thyroid function; deficiency in either exacerbates thyroid issues.
    • Vitamin E: Works synergistically with selenium in antioxidant defense.
    • Copper and Zinc: Adequate levels support overall mineral balance.
  • A balanced diet with iodine, vitamin E, and other minerals supports healthy selenium levels.

Fun Fact

Did you know selenium was named after the moon? Discovered in 1817, it was named “selenium” from the Greek word “selene” (moon) due to its silvery-gray sheen, a nod to its celestial beauty!

Empowering Your Health Choices

Selenium is an essential trace mineral vital for antioxidant protection and thyroid health, but balance is critical to avoid deficiency or toxicity. By consuming a varied diet, using supplements cautiously, minimizing occupational exposure, and monitoring intake in selenium-rich areas, you can maintain optimal levels. If you suspect selenium imbalance, consult a healthcare provider for testing and guidance. Stay informed and proactive to keep your health first!