Introduction
Selenium is an essential trace mineral that plays critical roles in thyroid hormone metabolism, DNA synthesis, reproduction, and protection from oxidative damage and infection (Rayman, 2000; Avery & Hoffmann, 2018). As a constituent of 25 selenoproteins, including glutathione peroxidases and selenoprotein P, selenium exerts potent antioxidant and anti-inflammatory effects in the body (Rayman, 2000; Avery & Hoffmann, 2018). These properties make selenium a vital nutrient for maintaining overall health and well-being.
Adequate selenium intake, primarily from dietary sources such as Brazil nuts, seafood, meat, poultry, eggs, and grains, is crucial for optimal functioning of various systems in the body (Rayman, 2008). The recommended dietary allowance (RDA) for selenium in adults is 55 mcg per day, with a safe upper limit of 400 mcg per day (Food and Nutrition Board, Institute of Medicine, 2000). Consuming a balanced diet that includes selenium-rich foods can help ensure sufficient intake of this essential mineral.
A growing body of research suggests that selenium may offer numerous health benefits, particularly in the areas of thyroid function, cognitive health, fertility, and immune support. As a potent antioxidant, selenium helps combat oxidative stress and inflammation, which are implicated in the development of various chronic diseases, including cancer, cardiovascular disease, and neurodegenerative disorders (Rayman, 2000; Tinggi, 2008; Cai et al., 2016). This article will explore the current scientific evidence surrounding the potential benefits of selenium, its food sources, recommended intakes, and the health implications of deficiency or excess.
Food Sources and Recommended Intakes
Selenium is found in a variety of foods, with some of the best sources being Brazil nuts, seafood, organ meats, and whole grains. A single Brazil nut can provide over 100% of the daily recommended intake of selenium (Rayman, 2008). However, it is important to note that the selenium content of plant-based foods can vary significantly depending on the selenium levels in the soil where they were grown (Rayman, 2008).
The Food and Nutrition Board has set the Recommended Dietary Allowance (RDA) for selenium at 55 mcg per day for adults, with a safe upper limit of 400 mcg per day (Food and Nutrition Board, Institute of Medicine, 2000). Consuming a balanced diet that includes a variety of selenium-rich foods can help ensure adequate intake of this essential mineral.
Selenium deficiency is rare in the United States but more common in certain regions of the world with low-selenium soils, such as parts of China (Rayman, 2000; Fairweather-Tait et al., 2011). Severe selenium deficiency has been linked to Keshan disease, a type of cardiomyopathy, and Kashin-Beck disease, a type of osteoarthritis (Fairweather-Tait et al., 2011). Ensuring adequate selenium intake through diet or supplementation can help prevent these conditions in at-risk populations.
Antioxidant and Anti-inflammatory Benefits
One of selenium’s most important roles is as an antioxidant. It is a key component of several antioxidant enzymes, such as glutathione peroxidase, which help neutralise harmful free radicals and reduce oxidative stress in the body (Rayman, 2000; Tinggi, 2008). Oxidative stress has been linked to the development of various chronic diseases, including cancer, heart disease, and neurodegenerative disorders (Rayman, 2000; Tinggi, 2008; Cai et al., 2016).
Selenium’s anti-inflammatory properties may also contribute to its potential disease-preventing effects. Chronic inflammation is another underlying factor in many health conditions. By helping to reduce inflammation, selenium may lower the risk of certain cancers, cardiovascular problems, and cognitive decline (Rayman, 2000; Tinggi, 2008; Cai et al., 2016). A 2016 meta-analysis found that higher blood selenium levels were associated with a decreased risk of breast, lung, colon, and prostate cancers (Cai et al., 2016).
Several observational studies have found associations between higher selenium intake or status and lower risks of various types of cancer, including prostate, colorectal, and lung cancer (Rayman, 2000; Cai et al., 2016). However, randomised controlled trials have yielded mixed results, with some showing no effect of selenium supplementation on cancer risk (Vinceti et al., 2018). More research is needed to clarify the potential role of selenium in cancer prevention.
Thyroid Health
The thyroid gland requires selenium for proper function and contains the highest concentration of this mineral in the body. Selenium plays a crucial role in the production and metabolism of thyroid hormones, which regulate many aspects of health, including metabolism, growth, and development (Schomburg, 2017).
Observational studies have found links between low selenium status and an increased risk of autoimmune thyroid disorders, such as Hashimoto’s thyroiditis and Graves’ disease (Wichman et al., 2016; Winther et al., 2020). These conditions are more common in women, and ensuring adequate selenium intake may be particularly important for maintaining thyroid health in this population (Winther et al., 2020).
A systematic review and meta-analysis by Wichman et al. (2016) found that selenium supplementation significantly reduced thyroid autoantibody levels in patients with chronic autoimmune thyroiditis. However, the authors noted that more research is needed to determine the optimal dose and duration of selenium supplementation for managing these conditions.
While these findings suggest a potential role for selenium in thyroid health, it is important to note that excessive selenium intake can also have negative effects on thyroid function (Winther et al., 2020). Consuming selenium within the recommended range through diet or supplementation is crucial for maintaining optimal thyroid health.
Cognitive Function
Selenium’s role as an antioxidant may also have implications for brain health and cognitive function. As oxidative stress and inflammation are thought to contribute to age-related cognitive decline and neurodegenerative diseases like Alzheimer’s, selenium’s ability to combat these processes could be neuroprotective (Cardoso et al., 2019).
A 2019 meta-analysis found that individuals with Alzheimer’s disease had significantly lower blood selenium concentrations compared to healthy controls (Varikasuvu et al., 2019). While this does not prove causation, it suggests a potential link between selenium status and cognitive health.
Animal studies have shown that selenium deficiency can lead to cognitive impairment and neurological dysfunction (Cardoso et al., 2019). In humans, observational studies have found associations between lower selenium levels and poorer cognitive performance in older adults (Cardoso et al., 2019). However, randomised controlled trials investigating the effects of selenium supplementation on cognitive function have yielded inconsistent results (Cardoso et al., 2019).
More research is needed to determine if selenium supplementation could help prevent or slow cognitive decline in humans. Maintaining adequate selenium intake through a balanced diet may be beneficial for overall brain health, but further studies are required to establish specific recommendations.
Asthma Symptoms
Selenium’s anti-inflammatory properties may also benefit individuals with asthma. Some research suggests that higher selenium intake could help reduce asthma symptoms by decreasing inflammation in the lungs (Allam & Lucane, 2004; Huang et al., 2012).
A study by Guo et al. (2011) found that asthmatic patients with higher blood selenium concentrations had better lung function compared to those with lower levels. The authors suggested that selenium’s antioxidant effects may help protect against oxidative damage in the lungs, which is a key factor in asthma pathogenesis.
A Cochrane review by Allam and Lucane (2004) investigated the potential benefits of selenium supplementation for asthma management. While the authors found some evidence that selenium supplementation could improve asthma symptoms, they concluded that more high-quality research is needed to confirm these findings and determine the optimal dosage and duration of supplementation.
While these studies suggest a potential role for selenium in managing asthma symptoms, it is important to note that selenium should not be used as a substitute for conventional asthma treatments. Individuals with asthma should work closely with their healthcare providers to develop an appropriate management plan.
Fertility and Reproduction
Selenium is important for both male and female reproductive health. In men, selenium is necessary for proper sperm motility, and some studies have suggested that selenium supplementation could improve semen quality in infertile men (Mistry et al., 2012; Safarinejad & Safarinejad, 2009). However, research findings have been inconsistent, and more studies are needed to clarify selenium’s role in male fertility (Mistry et al., 2012).
A double-blind, placebo-controlled trial by Safarinejad and Safarinejad (2009) found that selenium supplementation, alone or in combination with N-acetyl-cysteine, significantly improved semen quality in infertile men. The authors suggested that selenium’s antioxidant properties may help protect sperm from oxidative damage, which is a common cause of male infertility.
In women, selenium may also play a role in fertility, although less is known about the specific mechanisms (Mistry et al., 2012). Animal studies have shown that selenium deficiency can negatively impact female reproductive function, but human research in this area is limited (Mistry et al., 2012).
A review by Mistry et al. (2012) concluded that while selenium appears to be important for both male and female fertility, more research is needed to fully understand its role and determine the potential benefits of supplementation. Ensuring adequate selenium intake through a balanced diet may be beneficial for overall reproductive health, but individuals experiencing fertility issues should consult with their healthcare providers for personalised guidance.
Selenium Deficiency
While selenium deficiency is uncommon in the United States, it can occur in regions with low-selenium soils, such as parts of China (Rayman, 2000; Fairweather-Tait et al., 2011). Severe selenium deficiency has been associated with two serious health conditions: Keshan disease, a type of cardiomyopathy, and Kashin-Beck disease, a form of osteoarthritis (Fairweather-Tait et al., 2011).
Keshan disease is a cardiomyopathy that primarily affects children and women of childbearing age in areas with extremely low selenium levels (Rayman, 2000). Supplementation with selenium has been shown to prevent the disease, highlighting the importance of adequate selenium intake for cardiovascular health (Rayman, 2000).
Kashin-Beck disease is a type of osteoarthritis that causes joint pain and deformity (Fairweather-Tait et al., 2011). It is most common in certain regions of China, Siberia, and North Korea, where soil selenium levels are very low (Fairweather-Tait et al., 2011). Improving selenium status through diet or supplementation may help prevent the development of this condition in at-risk populations.
While overt selenium deficiency is rare in developed countries, suboptimal selenium status may still have negative health implications (Rayman, 2000). Ensuring adequate selenium intake through a balanced diet or supplementation can help prevent deficiency-related health problems and support overall well-being.
Conclusion
In summary, selenium is an essential mineral with crucial roles in maintaining overall health and well-being. Its potent antioxidant and anti-inflammatory properties help protect the body from oxidative stress and chronic inflammation, which are implicated in the development of various diseases. Adequate selenium intake, primarily from dietary sources such as Brazil nuts, seafood, meat, and whole grains, is necessary for the proper functioning of the thyroid gland, brain, immune system, and reproductive organs.
The current scientific evidence suggests that selenium may offer numerous health benefits, particularly in the areas of thyroid function, cognitive health, fertility, and immune support. Observational studies have found associations between higher selenium status and lower risks of certain cancers, cardiovascular disease, cognitive decline, and autoimmune thyroid disorders. However, randomised controlled trials have yielded mixed results, highlighting the need for further research to clarify selenium’s role in disease prevention and management.
While selenium deficiency is rare in developed countries, it can occur in regions with low-selenium soils and has been linked to serious health conditions such as Keshan disease and Kashin-Beck disease. Consuming a balanced diet that includes selenium-rich foods can help ensure adequate intake of this essential mineral. In some cases, selenium supplementation may be beneficial, particularly for individuals with suboptimal selenium status or certain health conditions. However, it is crucial not to exceed the safe upper limit of 400 mcg per day, as excessive selenium intake can lead to adverse health effects. As with any dietary supplement, consulting with a healthcare professional before starting a selenium supplementation regimen is always recommended.
Key Highlights of Learnings and Actionable Tips
- Selenium is an essential mineral that plays critical roles in thyroid hormone metabolism, DNA synthesis, reproduction, and protection from oxidative damage and infection.
- The recommended dietary allowance (RDA) for selenium for adults is 55 mcg per day. Good food sources include Brazil nuts, seafood, meat, poultry, and eggs.
- Selenium deficiency is rare in the United States but can occur in people living in selenium-deficient regions, those undergoing kidney dialysis, and people living with HIV.
- Observational studies suggest an inverse association between selenium status and risk of certain cancers, but clinical trials have not supported the use of selenium supplements for cancer prevention.
- Clinical trials do not support the use of selenium supplementation for reducing the risk of cardiovascular disease, particularly in healthy people who obtain sufficient selenium from food.
- Limited evidence suggests selenium supplementation might reduce the risk of preterm birth in people with HIV, but it does not appear to affect other HIV disease outcomes.
- The tolerable upper intake level for selenium is 400 mcg per day for adults. Chronically high intakes can lead to selenosis, with symptoms like hair loss, nail brittleness, nausea, diarrhoea, and neurological abnormalities.
What role does selenium play in male fertility and reproductive health?
Selenium is important for male fertility and reproductive health. The selenoprotein phospholipid hydroperoxide glutathione peroxidase is a major constituent of mature sperm. Observational studies have found that both low and high semen selenium concentrations are associated with male infertility. Some clinical trials have shown that selenium supplementation can improve sperm motility in infertile men with low selenium status. However, other trials have found no effect on sperm concentration or motility. More research is needed to clarify selenium’s role in male reproductive function.
Can selenium help prevent cognitive decline and dementia in older adults?
Selenium has antioxidant and anti-inflammatory properties, and serum selenium concentrations decline with age, so researchers have proposed selenium might help prevent age-related cognitive decline. Observational studies have yielded mixed results on the association between selenium status and cognitive function in older adults. One clinical trial found that taking an antioxidant supplement containing selenium for 8 years modestly improved some measures of cognitive function. However, the selenium-only PREADVISE trial in over 7,500 older men found no effect of selenium supplementation on dementia risk over 5 years. Overall, current evidence does not support the use of selenium supplements for preventing cognitive decline or dementia.
What is Keshan disease and how is it related to selenium deficiency?
Keshan disease is a type of cardiomyopathy (heart muscle disease) that was first identified in the 1930s in parts of China with very low soil selenium levels. It mainly affects women of childbearing age and preschool children. The exact cause is unknown, but selenium deficiency is believed to play a key role, as providing selenium supplements dramatically reduced the incidence of the disease in affected regions. Adults in these areas typically had selenium intakes of less than 20 mcg per day. Although Keshan disease is now rare, it still exists in some very low selenium regions.
How might selenium interact with medications?
Selenium can interact with certain medications, and some drugs can also affect selenium levels in the body. For example, the chemotherapy drug cisplatin can lower selenium levels in hair and serum, although the clinical significance of this effect is unclear. Limited evidence suggests selenium might reduce some of cisplatin’s side effects, but more research is needed. If you are taking any medications, especially on a regular basis, it’s important to discuss your selenium status and intake with your healthcare provider to assess potential interactions and effects on selenium levels.
What should I know about selenium toxicity and how much is too much?
Selenium toxicity, or selenosis, can occur with chronically high selenium intakes, such as from misformulated supplements or regular consumption of multiple Brazil nuts. The tolerable upper intake level for selenium is 400 mcg per day for adults. Early symptoms of excess intake include garlic odour breath, metallic taste, hair and nail loss and brittleness. With higher intakes, selenium toxicity can cause nausea, diarrhoea, neurological problems, and in rare cases, kidney failure, cardiac failure, and death. Acute toxicity has occurred from taking supplements containing 200 times the labeled selenium dose. It’s important not to exceed the upper limit of 400 mcg per day from foods and supplements combined.
References
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