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Selenium belongs to the semimetals. It occurs relatively rarely on earth. The element occurs in a number of different modifications, such as metallic gray selenium or amorphous and crystalline red selenium. Selenium was discovered as an element in 1817 by Jöns Jakob Berzelius (1779-1848). He named selenium after the Greek word "selene" for the moon. This was done in reference to tellurium (tellus = earth). Selenium very rarely occurs in nature in elemental form, mostly it is found in compounds with sulfur. Selenium minerals are also very rare. Selenium is therefore mainly extracted from by-products formed during copper production. Sulfide ores, such as pyrite, iron pyrites, chalcopyrite or zinc blende, usually contain selenium sulfides, but only in small amounts. Due to the special electrical properties of gray metallic selenium, it is mainly used in the semiconductor industry for the production of photodiodes, photocells, phototransistors, solar cells, radar systems or exposure meters, but also for storage and amplifier foils in X-ray diagnostics. In glass production, a small amount of selenium (0.1 - 0.2 %) gives the glass a bright red color. You see it every day in the red glass of traffic lights, for example. In medicine, selenium is considered an essential trace element. In the body, it is found in teeth and bones, as well as in enzymes. Selenium is found in anti-dandruff shampoos and in a number of medications for skin diseases. For example, it is used as a therapy-supporting drug in the treatment of Kleienpilz and psoriasis. Selenous acid (H2SeO3), like white arsenic, used to be taken in small quantities as an oxidizing agent for weight gain. However, it should also be noted that some selenium compounds are highly toxic and suspected carcinogens. For example, selenium is also counted among the carcinogenic ingredients of tobacco smoke.
Functions in the body
Selenium plays an important role in the detoxification of the body and is a component of some enzymes. It is covalently, i.e. tightly, bound to the enzyme glutathione peroxidase. This enzyme enhances the conversion of free radicals, mainly hydrogen peroxide (H202), into harmless derivatives with the help of glutathione, which is available in quite high concentrations in animal cells. Free radicals are chemically extraordinarily reactive molecules that can lead to DNA damage and thus to mutations. They therefore belong to the carcinogenic (cancer-causing) substances. As a component of the enzyme thyroxine-5-deiodase, selenium is involved in the activation of thyroid hormones. Furthermore, it is said to play a role in immune defense and to be involved in the detoxification or elimination of heavy metals. Selenium forms stable metal selenides with some heavy metals, which are not absorbed from the gastrointestinal tract and are therefore excreted.
The best food sources of selenium
Fortunately, there are many healthy foods that are rich in selenium. These include the following excellent sources:
- Brazil nuts: 137% of the daily requirement per nut (5 grams)
- Halibut: 106% of the daily requirement in 160 grams
- Yellowfin tuna: 77% of the daily requirement in 85 grams
- Oysters: 77% of the daily requirement in 85 grams
- Sunflower seeds: 62% of the daily requirement in 56 grams
- Shiitake mushrooms: 51% of the daily requirement in 1 cup (145 grams).
- Chicken: 50% of the daily requirement in 140 grams
- Eggs: 44% of the daily requirement in 2 large eggs (100 grams).
- Sardines: 36% in 4 sardines (48 grams)
The amount of selenium contained in plant-based foods can vary depending on the selenium content of the soil in which the plants were grown. In other words, this means that the selenium content in plants is highly dependent on where they are grown. For example, one study showed that the selenium content of Brazil nuts varied greatly from region to region. While a Brazil nut from one region contained up to 288% of the daily requirement, the selenium content of nuts from another region was only 11% of the daily requirement per nut (32). For this reason, it is important to pay attention to a varied diet that includes more than just one good source of this important trace element.
Possible health benefits of selenium and their scientific basis.
Selenium has a whole range of potential health benefits, which we will look at in more detail in the following sections. In this context, it is important to note that more selenium should not automatically be equated with more health benefits. An overdose of selenium above 400 mcg per day has toxic effects and can cause serious health damage. Although such poisoning is relatively rare, one should always stay close to the recommended daily dosage of 70 mcg for non-athletes and a maximum of 100 to 200 mcg for people with increased selenium requirements, and never exceed 400 mcg per day (33).
Selenium acts as a powerful antioxidant
Antioxidants are compounds in foods that protect cells from damage caused by free radicals. Free radicals are normal byproducts of metabolic processes that occur in the body on a daily basis. Free radicals have a bad reputation, but they are actually essential for health. They perform important functions, including protecting the body from disease. However, things like smoking, alcohol abuse and stress can lead to excessive amounts of free radicals. This leads to oxidative stress, which can damage healthy cells (1). Oxidative stress has been linked to chronic diseases such as heart disease, Alzheimer's disease, and cancer, as well as premature aging and increased risk of stroke (2, 3, 4, 5, 6). Antioxidants such as selenium help reduce oxidative stress by keeping the amount of free radicals under control. They work by neutralizing excess free radicals, protecting cells from damage caused by oxidative stress.
- Summary: Selenium is a powerful antioxidant that fights oxidative stress and helps protect the body from chronic diseases such as heart disease and cancer.
Selenium could reduce the risk of certain cancers
In addition to reducing oxidative stress in the body, selenium may also help reduce the risk of certain cancers in other ways. This has been attributed to selenium's abilities to reduce DNA damage and oxidative stress, improve immune system function, and destroy cancer cells (7). A review of 69 studies that included over 350,000 people found that high selenium blood levels may protect against certain types of cancer including breast cancer, lung cancer, colorectal cancer, and prostate cancer (8). It is important to note that this effect was only associated with selenium supplied through the natural diet and not in the form of supplements. However, some research suggests that supplementation with selenium may reduce the side effects of radiation therapy. For example, one study found that oral selenium supplements improved overall quality of life and reduced radiation-induced diarrhea in women who suffered from ovarian and cervical cancer (9).
- Abstract: Higher selenium blood levels may protect against certain cancers, while selenium supplementation could help improve the quality of life for people undergoing radiation therapy.
Selenium could protect against heart disease
A diet rich in selenium could help keep the heart healthy, as low selenium levels are associated with an increased risk of coronary heart disease. In an analysis of 25 observational studies, a 50% increase in selenium blood levels was associated with a 24% lower risk of coronary heart disease (10). Selenium may also lower levels of markers of inflammation in the body, which is one of the major risk factors for heart disease. For example, a review of 16 controlled trials involving more than 433,000 participants with coronary heart disease showed that taking selenium supplements lowered levels of the inflammatory marker CRP. In addition to this, selenium increased levels of glutathione peroxidase, which is a powerful antioxidant (11). All of this suggests that selenium may help lower the risk of heart disease by reducing inflammation and oxidative stress. Oxidative stress and inflammation are associated with atherosclerosis, or a buildup of plaques in the arteries. Atherosclerosis can lead to dangerous health problems such as stroke and heart attack (12). Incorporating more selenium-rich foods into the diet is an excellent way to minimize levels of inflammation and oxidative stress.
- Summary: Selenium may help keep the heart healthy by controlling oxidative stress and reducing the risk of coronary artery disease.
Selenium can help prevent mental decline
Alzheimer's is a devastating disease that causes memory loss and negatively affects thinking and behavior. This disease is one of the six leading causes of death in the Western world. The number of Alzheimer's patients is continuously growing. For this reason, it is very important to find ways to prevent this degenerative disease. Oxidative stress is believed to be involved in both the development and progression of neurological diseases such as Parkinson's disease, multiple sclerosis, and Alzheimer's disease (13). Several studies have also shown that Alzheimer's patients have lower selenium blood levels (14, 15). In addition to this, some studies have shown that selenium, both in the form of food and supplements, could improve memory in Alzheimer's patients (16). One small study found that as little as one selenium-rich Brazil nut per day could improve verbal fluency and other mental functions in people with mild cognitive impairment (17). In addition, the so-called Mediterranean diet, which is rich in selenium-rich foods such as seafood and nuts, is associated with a lower risk of developing Alzheimer's disease (18, 19).
- Abstract: A selenium-rich diet could help prevent mental decline and improve memory performance in Alzheimer's disease patients.
Selenium is important for the health of the thyroid gland
Selenium is important for proper thyroid function. In fact, the thyroid gland contains higher amounts of selenium than any other organ in the human body (20). This trace element helps protect the thyroid gland from oxidative damage and also plays an important role in the production of thyroid hormones. A healthy thyroid gland is important because it regulates metabolism and controls growth and development of the body (21). Selenium deficiency has been associated with thyroid diseases such as Hashimoto's - hypothyroidism in which the immune system attacks the thyroid gland. An observational study of over 6,000 individuals found that low serum selenium levels were associated with an increased risk of autoimmune thyroid disease (22). In addition to this, some studies have shown that selenium supplements may confer benefits for people diagnosed with Hashimoto's disease. One study review concluded that daily use of a selenium supplement for three months resulted in lower levels of thyroid antibodies. Supplementation also resulted in improvements in mood and general well-being in Hashimoto's patients (23).
However, more research is needed before selenium supplements can be recommended for individuals suffering from Hashimoto's.
- Summary: Selenium protects the thyroid gland from oxidative stress and is necessary for thyroid hormone production. Selenium may be useful for people suffering from Hashimoto's or other thyroid diseases, but more research is needed in this regard.
Selenium can improve the function of the immune system
The immune system keeps the body healthy by recognizing and fighting potential threats. These threats include bacteria, viruses and parasites. Selenium plays an important role in immune system health. As an antioxidant, it helps reduce oxidative stress in the body, which reduces inflammation and improves immune function.
Studies have shown that elevated selenium blood levels can be associated with an enhanced immune response. On the other hand, it has been shown that selenium deficiency can lead to a slower immune response (24). Studies have also linked selenium deficiency in HIV patients to an increased risk of death and faster progression of the disease, while supplementation has been shown to result in fewer hospitalizations and improved symptoms in these patients (25).
In addition to this, selenium supplements could help boost the immune system in patients with influenza, tuberculosis, and hepatitis C (26).
- Abstract: Selenium is essential for health and proper immune system function. Higher selenium levels may be helpful to improve immune system function in patients with HIV, influenza, tuberculosis, and hepatitis C.
Selenium could reduce asthma symptoms
Asthma is a chronic disease that affects the airways. The airways to the lungs can become inflamed and narrowed, leading to symptoms such as wheezing, shortness of breath, a tightness in the chest, and coughing (27). Asthma has been associated with increased levels of oxidative stress and inflammation in the body (28).
Because of selenium's ability to reduce inflammation in the body, some studies suggest that this trace mineral may be effective in reducing asthma-related symptoms.
Scientific research suggests that people suffering from asthma have low levels of selenium in their blood. One study showed that asthma patients with higher blood selenium levels had better lung function than asthma patients with lower selenium levels (29).
Selenium supplements may also be helpful in reducing asthma-related symptoms. For example, one study found that asthma patients who took 200 mcg of selenium per day were able to reduce their corticosteroid medication, which they normally need to control their symptoms (30).
However, the body of studies in this area is conflicting, and larger studies are needed to fully understand the role of selenium in the development and treatment of asthma (31).
- Abstract: Selenium may be beneficial to asthma patients due to its ability to reduce inflammation in the body. However, more research is needed.
The consequences of a selenium deficiency are not yet fully understood. However, studies suggest a link between high blood pressure and various heart diseases. Links between selenium deficiency and the incidence of cancers such as liver, colon and lung cancer have also been outlined in studies. Paralysis, liver damage and metabolic disorders have been observed in grazing animals that grazed in selenium-deficient areas. There is also evidence that selenium deficiency can lead to infertility in males. This occurs firstly because the maturation of sperm is disturbed in the case of a selenium deficiency and secondly because their motility is reduced.
Overdose and poisoning
Before it was discovered that selenium is essential for humans, the trace element was considered one of the most toxic elements. It should be noted, however, that the toxicity of selenium compounds depends on how well the respective compounds are absorbed from the gastrointestinal tract, i.e. enter the blood. For example, the compounds selenite, selenate, and selenoamino acids are particularly well absorbed. Elemental selenium and stable metal selenides, on the other hand, are poorly absorbed. Concrete information about the toxic concentrations of the individual selenium compounds is insufficient in the literature; only the amounts of the pure element selenium that are still considered non-toxic are given. With a regular intake of selenium, the amount of about 400 µg (= 10-6 g) is considered non-toxic.
Acute selenium poisoning is rare and usually due to excessive oral intake of selenium.
Symptoms of overdose or poisoning
Due to the large number of selenium compounds (selenide, selenite, selenate, selenoamino acids, etc.), which in high concentrations lead to symptoms of poisoning, and the different routes of absorption (lungs, skin, gastrointestinal tract), no general symptomatology can be described. What they all have in common, however, is that the breath of the poisoned person smells of garlic. Other symptoms observed are gastrointestinal disturbances, headache and hair loss. Changes in the fingernails (white spots and stripes) and peripheral neuropathies can also result from an excess of selenium.
Selenium is suspected of having a carcinogenic effect in larger quantities.
In severe cases, acute selenium poisoning can lead to serious intestinal symptoms, neurological symptoms, myocardial infarction, renal failure, and death (34).
Whether an additional intake of selenium can be recommended in general is controversial, because knowledge about the trace element is currently still insufficient. However, it is now known that the intake of selenium is recommended for cancer, certain cardiovascular diseases and specific forms of arthritis. The amounts of selenium to be taken in each case vary, however, depending on the disease, and in the case of cancer also depending on the respective treatment status.
Tablets containing sodium selenite should not be taken together with reducing agents such as vitamin C, as otherwise elemental selenium is formed which is not absorbed from the gastrointestinal tract into the blood. A minimum interval of one hour should be maintained between the intake of sodium selenite and vitamin C. However, this restriction does not apply to sodium selenate.
The German Society for Nutrition assumes a daily requirement of 30 to 70 µg for people over the age of 16. There may be an increased selenium requirement during pregnancy and breastfeeding, in older people, smokers, cancer patients and people with a weakened immune system.
Demand in sport
Since the soils in Europe are rich in selenium, our diet generally contains sufficient amounts of this trace element. My advice, because of its immunostimulant and antioxidant effect, nevertheless to supplement with 100-200mcg of selenium daily with meals.
Selenium is a powerful trace element that is essential for the proper functioning of the body. It plays a crucial role in metabolism and thyroid function, and helps protect the body from damage caused by oxidative stress. Selenium is not only essential for health, but could also help improve immune function, slow age-related mental decline, and even reduce the risk of heart disease.
Selenium is found in a wide variety of foods, ranging from oysters to mushrooms. Adding more selenium-rich foods to your diet is an excellent way to maintain good health.