*By Dr. Devan
Introduction
Selenium is a trace mineral—required by the human body in only microgram amounts—yet its role in health is profound. First identified as an essential nutrient in 1957, selenium has since been recognized as a cornerstone of antioxidant defense, immune function, thyroid regulation, and reproductive health. Too little selenium impairs vital processes, while too much can cause toxicity. Thus, selenium epitomizes the delicate balance of micronutrients in human health: small in quantity but mighty in effect.
The Biochemistry of Selenium
Selenium is incorporated into proteins in the form of selenocysteine, often referred to as the “21st amino acid.” These selenoproteins are central to antioxidant protection, redox balance, and hormone metabolism. More than 25 selenoproteins have been identified in humans, including:
Glutathione peroxidases (GPx): Enzymes that neutralize harmful peroxides and protect cells from oxidative damage.
Thioredoxin reductases: Maintain cellular redox state, vital for DNA synthesis and repair.
Iodothyronine deiodinases: Regulate thyroid hormone activation and metabolism.
Without adequate selenium, these critical enzymes cannot function properly, leading to oxidative stress, hormonal imbalances, and immune dysfunction.
Dietary Sources of Selenium
Selenium content in food depends heavily on the selenium content of the soil where crops are grown or animals are raised. Some key dietary sources include:
Brazil nuts – the richest natural source; just 1–2 nuts can meet the daily requirement.
Seafood – tuna, sardines, shrimp, salmon.
Meats – poultry, beef, lamb, pork.
Grains and cereals – especially if grown in selenium-rich soils.
Eggs and dairy – moderate sources.
The recommended daily allowance (RDA) for selenium is about 55 micrograms per day for adults, with slightly higher needs during pregnancy and lactation.
Selenium and Antioxidant Defense
Oxidative stress—caused by an imbalance between free radicals and antioxidants—is a driver of aging and many chronic diseases. Selenium, through glutathione peroxidase and thioredoxin reductase, helps neutralize free radicals and prevent lipid, protein, and DNA damage.
Clinical studies suggest selenium sufficiency may help reduce risks of:
Cardiovascular disease – by limiting oxidative damage to blood vessels.
Cancer – through DNA protection and improved immune surveillance.
Neurodegeneration – reducing oxidative damage in brain cells linked to Alzheimer’s and Parkinson’s diseases.
Selenium and Thyroid Health
The thyroid gland contains more selenium per gram of tissue than any other organ in the body. Selenium-dependent enzymes (iodothyronine deiodinases) regulate the activation of thyroid hormones by converting thyroxine (T4) into the active form triiodothyronine (T3).
Deficiency can lead to:
Hypothyroidism symptoms (fatigue, weight gain, cold intolerance).
Goiter development (enlargement of the thyroid).
Increased susceptibility to autoimmune thyroid diseases such as Hashimoto’s thyroiditis and Graves’ disease.
Supplementing selenium has shown benefits in reducing thyroid peroxidase antibodies in autoimmune thyroiditis, highlighting its therapeutic potential.
Selenium and Immune Function
Adequate selenium strengthens the immune system by:
Enhancing the proliferation of activated T-cells.
Improving antibody production.
Supporting the cytotoxic activity of natural killer (NK) cells.
Deficiency impairs resistance to infections. For example, selenium-deficient individuals are more susceptible to viral infections, and some viruses mutate more rapidly in selenium-deficient hosts, making them more virulent.
Selenium and Reproductive Health
Selenium plays a crucial role in fertility and reproductive function in both men and women:
In men – Selenium is essential for spermatogenesis. Selenoproteins in sperm mitochondria protect against oxidative damage, ensuring motility and viability. Deficiency is associated with low sperm count, reduced motility, and infertility.
In women – Adequate selenium supports ovarian follicle development, implantation, and successful pregnancy outcomes. Deficiency has been linked to recurrent miscarriage and complications such as preeclampsia.
Thus, selenium supplementation may benefit couples struggling with fertility issues, particularly in regions with selenium-poor soils.
Selenium and Cancer
The role of selenium in cancer prevention has been widely studied. Its protective mechanisms include:
Neutralizing carcinogenic free radicals.
Supporting DNA repair.
Enhancing immune detection of abnormal cells.
Epidemiological studies have shown that populations with higher selenium status have lower rates of certain cancers, especially prostate, colorectal, and lung cancers.
However, large-scale trials such as the SELECT study (Selenium and Vitamin E Cancer Prevention Trial) yielded mixed results, showing no clear benefit of supplementation in already selenium-replete populations. This highlights the importance of context: selenium may reduce cancer risk in deficient individuals, but excessive supplementation can be harmful.
Selenium and Cardiovascular Health
By reducing oxidative stress and inflammation, selenium may help prevent atherosclerosis and cardiovascular disease. Some studies suggest that low selenium levels are associated with increased risk of heart disease and cardiomyopathy.
A classic example is Keshan disease, a form of cardiomyopathy in selenium-deficient regions of China, which was largely prevented through selenium supplementation programs.
Selenium and Neurological Health
Selenium contributes to brain health by:
Protecting neurons from oxidative damage.
Supporting neurotransmitter function.
Maintaining redox balance in the central nervous system.
Low selenium has been associated with cognitive decline and neurodegenerative conditions. Supplementation may support brain function in the elderly, though more clinical evidence is needed.
Selenium Deficiency
Selenium deficiency can result from:
Poor soil content (common in certain parts of China, Africa, and Europe).
Malnutrition or restrictive diets.
Conditions like Crohn’s disease or kidney dialysis that impair absorption.
Deficiency symptoms include:
Muscle weakness and fatigue.
Weakened immune system.
Infertility in men and women.
Thyroid dysfunction.
Increased risk of Keshan disease (cardiomyopathy) and Kashin-Beck disease (a bone disorder).
Selenium Toxicity (Selenosis)
While deficiency is harmful, excess selenium can be toxic. Chronic overconsumption (usually >400 µg/day) leads to selenosis, characterized by:
Hair and nail brittleness or loss.
Skin rashes.
Nausea, vomiting, and diarrhea.
Nervous system disturbances.
In extreme cases, organ damage.
Therefore, supplementation should be carefully managed and generally not exceed 200 µg/day unless prescribed.
Selenium in Modern Health Discussions
Selenium has gained attention in several emerging health contexts:
COVID-19: Preliminary studies suggested selenium deficiency might worsen COVID-19 outcomes by weakening immune defense.
Diabetes: Selenium’s role in redox balance may influence insulin sensitivity, though high doses may paradoxically increase diabetes risk.
Longevity: Selenium is studied as a micronutrient of healthy aging due to its antioxidant role.
These insights reinforce selenium’s importance but also highlight that supplementation must be targeted and individualized.
Conclusion
Selenium is a paradoxical element: required in tiny amounts yet vital for life. It strengthens antioxidant defenses, supports thyroid function, enhances immunity, and ensures fertility. Deficiency can lead to serious health consequences, while excess can be toxic.
The lesson is balance. In selenium-poor regions, supplementation or dietary planning is essential. In selenium-rich areas, excessive supplementation offers no benefit and may be harmful.
In a world where oxidative stress, immune challenges, fertility issues, and chronic diseases are increasingly common, selenium reminds us that health often depends not on grand interventions but on the microscopic guardians within our cells. Truly, in the realm of nutrition, the smallest elements can make the biggest difference.
*Dr Devan is a Mangaluru-based ENT specialist and author.
Hindusthan Samachar / Manohar Yadavatti