Selenium is an essential trace mineral. The body uses it to make selenoproteins, a group of proteins involved in antioxidant defence, thyroid hormone metabolism, immune function, reproduction, and DNA-related processes.
Selenium is needed only in small amounts. Both too little and too much can cause health problems, so it is not a nutrient where more is automatically better.
Biological Role
Selenium is incorporated into proteins as the amino acid selenocysteine. Important selenoproteins include glutathione peroxidases, thioredoxin reductases, and iodothyronine deiodinases.
These proteins help:
- Limit oxidative damage.
- Support thyroid hormone activation and metabolism.
- Support immune and reproductive function.
- Take part in normal cellular maintenance.
Selenium and iodine are linked through thyroid biology, but selenium supplements do not replace diagnosis or treatment of thyroid disease.
Food Sources
Food selenium content depends partly on soil selenium levels. Sources include:
- Brazil nuts.
- Fish and shellfish.
- Meat and poultry.
- Eggs.
- Some cereals and grains.
Brazil nuts can be very high in selenium, so frequent large servings can push intake too high.
Intake
UK NHS guidance gives adult daily requirements of 75 mcg for men and 60 mcg for women aged 19 to 64. NIH guidance in the United States gives a recommended adult amount of 55 mcg a day, with higher amounts during pregnancy and breastfeeding.
These figures come from different national reference systems, but both show that selenium is required in microgram amounts, not milligram amounts.
Deficiency and Excess
Selenium deficiency is uncommon in people with varied diets in many high-income countries, but it can occur where soil selenium is low or where medical conditions affect intake or absorption.
Severe deficiency has been linked with Keshan disease, a cardiomyopathy described in low-selenium regions, and with impaired immune or thyroid-related function.
Excess selenium can cause selenosis. Features may include garlic-like breath, metallic taste, nausea, diarrhoea, brittle hair or nails, hair loss, skin rash, fatigue, irritability, and nerve symptoms. Excess usually comes from high-dose supplements or very high selenium foods taken in large amounts.
Research
Selenium has been studied for cancer, cardiovascular disease, thyroid disease, fertility, and immune function. The evidence is mixed and depends on baseline selenium status, dose, chemical form, and population. Routine high-dose supplementation is not supported for the general population.
See Also
References
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