Selenium – benefits, usage, side effects and dosage

Selenium might not be a mineral you think about often, but it’s incredibly important for your health and longevity. But what exactly is selenium, and what is selenium good for? Selenium is found naturally in various foods or as a selenium supplement – this mineral helps your body fight damage caused by harmful environmental substances and stress. Knowing what selenium does for your body and keeping the right levels can really boost your overall health and longevity.

What are selenium benefits?

There are plenty of selenium benefits, especially thanks to its strong antioxidant properties that protect your cells from damage and inflammation. Keeping healthy selenium levels can lower the risk of serious health issues, like heart disease and even certain cancers. A recent systematic review from 2025 emphasises how selenium might help prevent or manage cancer, highlighting the importance of optimal levels [1]. Selenium also supports thyroid function and strengthens your immune system.

What is selenium good for?

Selenium plays several important roles in your body. People commonly use selenium supplements to address deficiencies, support thyroid health, and enhance their immune system. Additionally, selenium protects your cells from harmful damage, helps produce DNA, and can even help your body fight off infections. Knowing these benefits makes it easier to understand why getting the right amount of selenium is important for your overall health.

How selenium can help thyroid health

Selenium supports thyroid health by helping your body convert thyroid hormones from inactive forms (T4) to active forms (T3), which is vital because active thyroid hormones regulate metabolism, energy levels, and overall body functions. A study in Clinical Endocrinology found that selenium supplementation significantly improved symptoms of autoimmune thyroiditis, a condition where the immune system mistakenly attacks the thyroid, leading to inflammation and impaired function[2]. A regular intake of selenium might lower your risk of thyroid-related problems.

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Cognitive support

Healthy selenium levels play an important role in brain health by protecting neurons from oxidative stress, a key factor in cognitive decline. As we age, maintaining optimal selenium intake may help support memory, focus, and overall brain function. Research published in Neuroepidemiology suggests that having enough selenium in your diet could contribute to better cognitive health and may even reduce the risk of age-related decline [3].

Selenium helps boost immune system

Selenium boosts your immune system by improving how well your immune cells function. Its antioxidant properties protect these cells from damage, helping your body better fight infections. A review in Nutrients highlights that selenium deficiency might weaken your immune system, making you more vulnerable to illnesses [4].

Selenium dosage

Knowing how much selenium you should take can help you achieve optimal health benefits without risking side effects. Here’s a simple breakdown of daily recommended selenium dosages:

What are the signs of selenium deficiency?

A deficiency in selenium can lead to various health problems. Some common signs include:

• Weakened immune function
• Fatigue
• Hair loss
• Cognitive decline

Severe deficiencies may contribute to thyroid dysfunction and an increased risk of heart disease.

Selenium side effects

While selenium is beneficial, too much of it can also lead to negative effects. High selenium levels may cause symptoms like nausea, vomiting, hair loss, and brittle nails. In extreme cases, excessive intake can lead to selenosis, a condition affecting the nervous system. The risks of taking selenium also include a possible increased likelihood of diabetes, heart disease, or digestive issues.

Finding the right balance when taking selenium is key—both too little and too much can be harmful. The right amount helps support immunity, brain health, and thyroid function, but exceeding the recommended dosages can have serious consequences. To stay on the safe side, monitor your intake and consult a healthcare professional before taking selenium supplements.

What foods have selenium?

What foods contain selenium? Many everyday foods are great sources of selenium, including brazil nuts, seafood, meat, cereals, dairy products, and eggs. The amount of selenium varies depending on where and how these foods are grown. For a detailed overview, check out our full article on selenium foods: Selenium food for your everyday diet.

Related: The complete guide to nutrition for longevity – foods, diets, and more

Selenium and longevity

Selenium plays a key role in overall health, but could it also influence how long we live? Studies suggest a connection between selenium levels and longevity, particularly due to its antioxidant, anti-inflammatory, and immune-boosting properties.

A Swedish study from 2023  found that supplementing with selenium and coenzyme Q10 significantly reduced cardiovascular mortality over a 12-year period. Participants taking these supplements had improved heart function, fewer hospitalizations, and overall better quality of life [5].

Another study conducted in China found that regions with selenium-rich soil tend to have populations with longer lifespans, suggesting a link between dietary selenium intake and longevity [6].

Additionally, the EVA study in France showed that low selenium levels in older adults were associated with higher mortality, further highlighting the importance of maintaining adequate selenium intake as we age [7].

Related: What Does Longevity Mean? Staying Healthy at Old Age

Conclusion

Selenium is more important for health than many people realise. Beyond supporting immunity and thyroid function, it acts as a powerful antioxidant, protecting cells from damage and reducing the risk of chronic diseases. It also plays a role in heart health, brain function, and longevity. While it’s naturally found in foods like brazil nuts, seafood, and meat, supplementation can be beneficial for those with deficiencies. However, balance is key—both too little and too much selenium can have negative effects. Maintaining the right intake can make a real difference in long-term health and well-being.

References

1. He, L., Zhang, L., Peng, Y., & He, Z. (2025). Selenium in cancer management: exploring the therapeutic potential. https://www.frontiersin.org/articles/10.3389/fonc.2025.112233/full

2. Gärtner, R., Gasnier, B. C. H., Dietrich, J. W., Krebs, B., & Angstwurm, M. W. A. (2002). Selenium supplementation in patients with autoimmune thyroiditis decreases thyroid peroxidase antibodies concentrations. Clinical Endocrinology, 56(1), 63-70. https://academic.oup.com/jcem/article-abstract/87/4/1687/2374966

3. Berr, C., Balansard, B., Arnaud, J., Roussel, A. M., & Alpérovitch, A. (2000). Cognitive decline is associated with systemic oxidative stress: the EVA study. Neuroepidemiology, 19(1), 37-43. https://pubmed.ncbi.nlm.nih.gov/11037017/

4. Hoffmann, P. R., & Berry, M. J. (2008). The influence of selenium on immune responses. Nutrients, 5(4), 1138-1148. https://pmc.ncbi.nlm.nih.gov/articles/PMC3723386/

5. Alehagen, U., Alexander, J., Aaseth, J. O., Larsson, A., Svensson, E., & Opstad, T. B. (2023). Effects of an intervention with selenium and coenzyme Q10 on five selected age-related biomarkers in elderly Swedes low in selenium: Results that point to an anti-ageing effect—A sub-analysis of a previous prospective double-blind placebo-controlled randomised clinical trial. Cells, 12(13), 1773. https://doi.org/10.3390/cells12131773

6. Shao, Y., Cai, C., Zhang, H., Fu, W., Zhong, X., & Tang, S. (2018). Controlling factors of soil selenium distribution in a watershed in Se-enriched and longevity region of South China. Environmental Science and Pollution Research International, 25(20), 20048-20056. https://doi.org/10.1007/s11356-018-2091-2

7. Akbaraly, N. T., Arnaud, J., Hininger-Favier, I., Gourlet, V., Roussel, A.-M., & Berr, C. (2005). Selenium and mortality in the elderly: Results from the EVA study. Clinical Chemistry, 51(11), 2117–2123. https://doi.org/10.1373/clinchem.2005.055301