Selenium

What is selenium?

Selenium is an essential trace element. The body needs it only in small amounts, but it is required for several enzymes and proteins known as selenoproteins.

These proteins are involved in antioxidant defense, thyroid-hormone metabolism, immune function and reproduction.

Where does selenium occur naturally?

The selenium content of food depends strongly on the selenium concentration of the soil. Good sources may include:

• Brazil nuts
• Fish and seafood
• Meat and organ meats
• Eggs
• Dairy products
• Whole grains

Brazil nuts can contain very high and highly variable amounts. Regularly eating large quantities may therefore lead to excessive intake.

What functions does selenium have?

Selenium contributes to:

• Normal immune function
• Normal thyroid function
• Protection of cells from oxidative stress
• Normal sperm production
• Maintenance of normal hair and nails

Important selenoproteins include glutathione peroxidases, thioredoxin reductases and iodothyronine deiodinases.

Selenium and the thyroid

The thyroid contains more selenium per gram of tissue than many other organs. Selenium-dependent enzymes help convert the thyroid hormone T4 into the active hormone T3 and help protect thyroid tissue from oxidative stress.

A deficiency may therefore affect thyroid metabolism. However, supplementation does not automatically improve thyroid disease, and excessive selenium may be harmful.

Selenium deficiency

Severe deficiency is uncommon in many European countries but may occur in people with:

• Very low dietary intake
• Malabsorption disorders
• Long-term parenteral nutrition
• Dialysis
• Certain severe illnesses

Symptoms may include impaired immune function, muscle weakness, hair or nail changes and altered thyroid metabolism.

Can too much selenium be harmful?

Yes. Selenium has a relatively narrow range between adequate and excessive intake. Chronic overconsumption can cause selenosis.

Possible signs include:

• Garlic-like breath odor
• Metallic taste
• Nausea or diarrhea
• Hair loss
• Brittle nails
• Skin changes
• Neurological complaints

Using several selenium-containing supplements at the same time can quickly increase total intake.

Selenium and longevity

Selenium is relevant to antioxidant and thyroid pathways that are often discussed in healthy-aging research. Observational studies have linked both low and very high selenium status with unfavorable outcomes.

This suggests that an adequate range may be more important than maximizing intake. Supplementation should ideally be based on diet, laboratory values and professional assessment.

Conclusion

Selenium is essential for thyroid function, antioxidant defense and immune function. Because the safe intake range is relatively narrow, supplementation should be carefully dosed and unnecessary high intakes should be avoided.