Selenium: Benefits, Deficiency, Excess

Selenium is an essential trace element found in dozens of selenium-dependent enzymes and proteins. It plays a role in antioxidant protection, thyroid function, immune function, reproductive system function, and brain function. [1]

Selenium's unique feature is that its "safety corridor" is very narrow. A deficiency increases the risk of specific endemic diseases (Keshan disease, Kashin-Beck disease), impairs immunity, and deteriorates cardiovascular health. Excess leads to selenosis, which affects the skin, nails, hair, and nervous system, and can lead to severe cardiovascular and renal complications. [2]

Selenium levels in soil and water vary greatly across regions. Therefore, in some countries, deficiency remains the primary concern, while in others, excess selenium can occur with active supplementation and consumption of foods high in selenium (such as Brazil nuts) when adequate intake is already present. [3]

Modern reviews emphasize that optimal selenium intake is associated with a lower risk of a number of diseases, but the effect is U-shaped. Both deficiency and excess are associated with increased morbidity, including cancer and cardiovascular disease. [4]

Therefore, the goal is not to “get as much” or “completely eliminate” selenium, but to ensure a moderate but stable intake from food and not to overuse supplements without indications. [5]

Table 1. Selenium in a nutshell: benefits and risks

Parameter	Health benefits
Main functions	Antioxidant protection, thyroid function, immunity, reproduction
In case of shortage	Increased risk of endemic diseases, immune disorders, and worsening prognosis for infections
In excess	Selenosis: brittle nails, hair loss, neurological symptoms, heart and kidney damage
Peculiarity	Very narrow range between deficiency and toxicity
The optimal approach	Ensure adequate selenium intake with food; supplements should be used only as directed.

Biological role of selenium and selenoproteins

Selenium is a component of a special amino acid, selenocysteine, which is embedded in the active sites of many enzymes. These proteins are called selenoproteins. Key among them are glutathione peroxidase, thioredoxin reductase, and iodothyronine deiodinase. They are responsible for antioxidant protection, redox reactions, and thyroid hormone metabolism. [6]

Selenium's antioxidant function is manifested in the ability of glutathione peroxidase to neutralize peroxides and other reactive oxygen species. This protects cell membranes, lipoproteins, and DNA from damage, especially under conditions of stress, inflammation, smoking, and chronic diseases. [7]

Selenium, via deiodinases, is involved in the conversion of thyroxine to the more active triiodothyronine and in the inactivation of excess thyroid hormones. Therefore, adequate selenium status is important for proper thyroid function and may influence the course of autoimmune thyroiditis and some forms of hypothyroidism. [8]

In the immune system, selenoproteins influence the maturation and function of T-lymphocytes, cytokine production, and natural killer cell activity. Several studies show that low selenium status is associated with worse viral infections and a higher risk of adverse outcomes; however, supplementation is not always effective and depends on the underlying deficiency. [9]

Selenium is also important for reproductive function, especially in men: it is involved in sperm maturation, protecting sperm from oxidative stress, and influencing motility and fertility. [10]

Table 2. The main selenium-dependent systems of the body

System	The role of selenium	Possible consequences of deficiency
Antioxidant protection	Glutathione peroxidase and other enzymes neutralize peroxides	Increased oxidative stress, cell damage
Thyroid gland	Deiodinases regulate hormone activity	Metabolic disorders, possible worsening of thyroiditis
Immunity	Support of T-cell and NK cell function	Increased susceptibility to infections
Cardiovascular system	Myocardial protection from oxidative stress	In severe deficiency - risk of Keshan disease
Reproductive system	Sperm quality, protection from damage	Decreased fertility in men

Consumption standards and safety range

For adults, the recommended daily intake of selenium is typically around 55 mcg. For pregnant and breastfeeding women, the requirement is slightly higher (up to approximately 60-70 mcg, depending on national recommendations). [11]

At the same time, upper tolerable intake levels have been established, exceeding which is dangerous over a long period of time. In most countries, the upper limit (TUL) for adults is approximately 400 mcg of selenium per day, taking into account all sources (food, fortified foods, supplements). [12]

It's important to understand that the true "optimum zone" is narrower than simply the range from the minimum requirement to the upper limit. Modern research suggests a U-shaped relationship between selenium intake and the risk of a number of diseases, particularly certain types of cancer: both low and excessively high intakes are associated with increased risk. [13]

One large study estimated the safe range for reducing cancer risk in a given population to be approximately 110–125 mcg per day, with both lower and significantly higher levels associated with increased risk. This highlights the need for caution when administering selenium “shock doses,” especially in areas with already high background levels. [14]

For children and adolescents, the norms are lower and depend on age, but the logic is the same: a small amount per day is necessary, and constantly going beyond the upper limit is undesirable due to the risk of accumulation and toxicity. [15]

Table 3. Estimated daily selenium intake (mcg/day)

Group	Recommended intake	Upper permissible level
Children 1-3 years old	20	90
Children 4-8 years old	30	150
Teenagers 9-13 years old	40	280
Teenagers 14-18 years old	55	400
Adults	55	400
Pregnant and lactating women	60-70	400

Dietary sources of selenium and the influence of geography

The main factor determining selenium content in foods is its amount in the soil. In some regions, soils are deficient in selenium, while in others, they are rich. This affects both grain and animal feed, as well as the final selenium concentration in meat, milk, and plant foods. [16]

Classic sources of selenium include seafood, fish, meat, organ meats, eggs, certain grains, and nuts. Brazil nuts are particularly rich in selenium: sometimes a single nut already covers or even exceeds the daily requirement. Therefore, excessive daily consumption of these nuts can lead to toxicity. [17]

In most regions, a mixed diet of meat, fish, eggs, and grains provides sufficient selenium without the need for supplementation. The exception is areas with very low selenium levels in soil and water, where the risk of endemic deficiency is higher and food fortification or targeted supplementation programs may be used. [18]

For people on plant-based diets, the main sources of selenium are grains, legumes, and nuts, but their selenium content is highly variable. Therefore, some vegetarians and vegans may be at risk for deficiency, especially when consuming foods from regions with selenium-poor soils. [19]

Some countries use selenium fortification of fertilizers to increase its content in grain (for example, Finland), which has improved the status of the population without the mass prescription of additives. [20]

Table 4. Examples of foods rich in selenium (approximately)

Product and portion	Selenium, mcg (approximately)
Brazil nuts (1-2 pieces)	It can be from 50 to >100
Fish (tuna, sardine, salmon, 100 g)	30-90
Seafood (shrimp, mussels, 100 g)	20:45
Meat, poultry (100 g)	15-35
Egg (1 pc.)	10-20
Whole grains, legumes (serving)	Varies greatly, approximately 5-20

Selenium deficiency: when it occurs and how it manifests itself

Clinically significant selenium deficiency is more often described in regions with very selenium-poor soils. There, it is associated with the development of specific endemic diseases: Keshan disease (cardiomyopathy in children and women of reproductive age) and Kashin-Beck disease (osteoarthropathy with joint deformity). [21]

Outside endemic areas, deficiency is often subclinical and accompanies conditions with malnutrition and absorption, such as severe gastrointestinal diseases, long-term selenium-deficient parenteral nutrition, severe forms of alcoholism, and some bariatric surgeries. In such cases, deficiency increases oxidative stress and worsens the prognosis of the underlying disease. [22]

Low selenium status is associated with increased rates of infections, more severe viral illnesses, worse outcomes in HIV, and some critical conditions. However, supplementation provides significant benefit primarily in cases of proven deficiency; with sufficient baseline levels, the effect is limited or absent. [23]

Non-specific deficiency symptoms may include increased fatigue, weakened immunity, hair loss, poor skin and nail health, and reproductive dysfunction in men. These symptoms are non-specific and occur in a variety of conditions, so testing and dietary assessment are necessary to confirm deficiency. [24]

Risk groups include patients with severe chronic diseases, forced diets with very low protein and animal product content, some vegans with poor dietary variation, and people from regions with low selenium content in the soil. [25]

Table 5. Who is at risk for selenium deficiency?

Group	Reason for increased risk
Residents of regions with selenium-poor soils	Low selenium content in all local foods
Patients on long-term parenteral nutrition without selenium	Lack of dietary intake
People with severe bowel diseases after resection	Malabsorption
Vegetarians/vegans on a very limited diet	Low variability of selenium sources, soil influence
Patients with severe alcoholism, cachexia	General severe malnutrition and micronutrient deficiencies

Excess selenium and selenosis: symptoms and causes

Chronic selenium excess leads to a condition called selenosis. Classic symptoms include brittle and falling hair, splitting and brittle nails, an unpleasant garlicky odor from the mouth, a metallic taste, skin rashes, irritability, and neurological impairment. Severe toxicity can also cause respiratory, cardiac, and renal dysfunction. [26]

Historically, severe cases of selenosis have been described in regions with excessively high selenium levels in the soil and grains. People who ate local grains received hundreds and even thousands of micrograms of selenium per day over long periods of time. Today, such situations are rare and typically localized.

A far more pressing problem for city dwellers is the uncontrolled use of supplements and excessive consumption of foods with very high selenium content (for example, regularly eating handfuls of Brazil nuts) when they already receive sufficient amounts from their diet. Chronically exceeding the upper tolerable level gradually increases the risk of toxicity. [28]

In very large single doses (due to dosage errors, accidental ingestion of concentrate), acute poisoning with nausea, vomiting, abdominal pain, diarrhea, dizziness, and respiratory distress is possible. Such situations require immediate medical attention. [29]

People with kidney or liver disease, pregnant or breastfeeding women, and anyone already taking antioxidant supplements should be especially careful with selenium-containing supplements: the total dose of micronutrients in such regimens is often close to the upper limits. [30]

Table 6. Main signs of possible excess selenium

Symptom	Possible explanation
Garlic-like breath, metallic taste	Selenium metabolites are excreted in exhaled air.
Hair breakage and loss	Toxic effects on hair follicles
Brittle and peeling nails	Violation of the keratin structure
Irritability, neurological symptoms	Effect on the nervous system
Nausea, vomiting, abdominal pain	Signs of acute or subacute poisoning

Selenium and Disease: What Modern Research Shows

The relationship between selenium and the risk of cancer, cardiovascular disease, depression, and other conditions is being actively studied. A review of meta-analyses in recent years found that adequate, but not excessive, selenium levels are associated with a lower risk of several malignancies, some cardiovascular diseases, depression, and endemic diseases associated with deficiency. [31]

However, large randomized trials of selenium supplementation in people with sufficient selenium status (for example, the renowned SELECT prostate cancer prevention trial) have shown no reduction in cancer risk with 200 mcg of selenium per day. This means that the association "higher levels within the normal range - lower risk" does not automatically mean that supplementation on top of an adequate diet provides protection. [32]

For the cardiovascular system, the data are also mixed. Low selenium status is associated with an increased risk of certain heart diseases, and Keshan disease has been described in cases of severe deficiency. Meanwhile, randomized trials of selenium supplementation in individuals with sufficient status have not confirmed a significant reduction in cardiovascular events. [33]

Data on autoimmune thyroid diseases, polycystic ovary syndrome, and male infertility are of interest: in some studies, moderate doses of selenium as part of combination therapy improved laboratory parameters and some clinical outcomes. However, the overall level of evidence is still low, and there are no universal recommendations for "everyone." [34]

As a result, current guidelines take a cautious stance: maintaining adequate selenium status is important, but prophylactic use of high doses of supplements to “protect against cancer and heart attack” in the context of a normal diet is not justified and is potentially risky. [35]

Table 7. What is more or less clear about selenium and diseases

State	The role of selenium according to modern data
Endemic diseases (Keshan, Kashin-Bek)	Selenium deficiency is a key risk factor, and supplementation is beneficial for those with a deficiency.
Oncological diseases	Adequate status is associated with lower risk, supplementation in people without deficiency has not been proven to prevent
Cardiovascular diseases	Deficiency is unfavorable, supplementation with sufficient status does not significantly reduce the risk
Autoimmune thyroiditis	There is evidence of potential benefit as part of therapy, but the evidence is limited
Male infertility	Moderate doses may improve sperm counts in deficiency cases

Practical recommendations: how to safely provide the body with selenium

First, rely on diet rather than supplements. A mixed diet that includes fish, seafood, moderate amounts of meat and eggs, whole grains, and nuts provides the daily requirement of selenium in most regions without the risk of toxicity. [36]

Second, be careful with "champion" foods. Brazil nuts are indeed very rich in selenium and can be used as a convenient source, but it's safer to limit yourself to 1-2 nuts per day rather than handfuls, especially when combined with a normal diet. [37]

Third, selenium supplements should only be taken when indicated. These may include laboratory-confirmed deficiency, living in regions with severe deficiency, or certain medical conditions prescribed by a doctor. "Just in case of cancer prevention" is not a scenario supported by current data.

Fourth, if you're already taking multivitamins or antioxidant supplements, it's important to calculate your total selenium intake to avoid exceeding the upper limit when consuming fortified foods and "superfoods." If in doubt, it's wise to discuss the situation with your doctor and, if necessary, check your selenium levels or selenium status markers. [39]

Finally, it's worth remembering that selenium is just one piece of a larger picture. Health and the risk of cancer, heart attack, and depression are influenced more by a combination of factors, not just individual micronutrients: a healthy diet, physical activity, quitting smoking, weight and blood pressure control, and adequate sleep. Selenium plays an important, but not magical, role in this picture. [40]

Table 8. A quick selenium checklist for the reader

Step	Action
1	Assess your diet: do you regularly eat fish, seafood, eggs, cereals, and nuts?
2	Avoid overindulging in Brazil nuts and other "superfoods" high in selenium
3	Check the selenium content of multivitamins and antioxidant complexes
4	Do not take individual selenium supplements without proper medical advice and dose monitoring.
5	If you suspect a deficiency or excess, discuss the need for testing with your doctor.
6	Remember that the optimal selenium level is the "golden mean," not the maximum dose.