Selenium

Selenium is a semi-metal. It is relatively rare 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 is very rarely found in nature in elemental form; it is usually found in compounds with sulphur. Selenium minerals are also very rare. Selenium is therefore mainly extracted from by-products of copper production. Sulphide ores, such as pyrite, iron pyrite, chalcopyrite or zinc blende, usually contain selenium sulphides, but only in small quantities. 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 light meters, but also for storage and amplifier foils in X-ray diagnostics. In glass production, a small proportion 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. It is used as a therapy-supporting drug in the treatment of smallpox 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 are suspected of having a carcinogenic effect. For example, selenium is also one of the carcinogenic ingredients in tobacco smoke.

Functions in the body

Selenium plays an important role in detoxifying the body and is a component of some enzymes. It is covalently, i.e. firmly, bound to the enzyme glutathione peroxidase. This enzyme enhances the conversion of free radicals, especially hydrogen peroxide (H202), into harmless derivatives with the help of glutathione, which is available in fairly high concentrations in animal cells. Free radicals are chemically extremely reactive molecules that can damage DNA and lead to mutations. They are therefore classified as carcinogenic (cancer-causing) substances. As a component of the enzyme thyroxine-5-deiodase, selenium is involved in the activation of thyroid hormones. It is also said to play a role in immune defense and is involved in the detoxification and 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, 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 ensure a varied diet that includes more than just one good source of this important trace element.

Potential 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 does not automatically equate to more health benefits. An overdose of selenium above 400 mcg per day has toxic effects and can lead to serious health problems. Even if 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 by-products 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 fulfill 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 and cancer, as well as premature ageing and an increased risk of stroke (2, 3, 4, 5, 6). Antioxidants such as selenium help to 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 may 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 ability to reduce DNA damage and oxidative stress, improve immune system function and destroy cancer cells (7). A review of 69 studies involving over 350,000 people found that high blood levels of selenium 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 obtained from the natural diet and not in the form of supplements. However, some studies suggest that selenium supplementation 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 suffering from ovarian and cervical cancer (9).

• Summary: Higher blood levels of selenium may protect against certain cancers, while selenium supplementation may help improve the quality of life of 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 reduce levels of markers of inflammation in the body, which is a major risk factor for heart disease. For example, a review of 16 controlled trials with over 433,000 participants with coronary heart disease showed that taking selenium supplements lowered levels of the inflammatory marker CRP. In addition, selenium increased the levels of glutathione peroxidase, which is a powerful antioxidant (11). All of this suggests that selenium may help reduce the risk of heart disease by reducing inflammation and oxidative stress. Oxidative stress and inflammation are associated with atherosclerosis, or a build-up of plaque 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 people suffering from Alzheimer's is constantly 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, multiple sclerosis and Alzheimer's (13). Several studies have also shown that Alzheimer's patients have lower blood selenium levels (14, 15). In addition to this, some studies have shown that selenium in both dietary and supplement form may 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).

• Summary: A selenium-rich diet could help prevent mental decline and improve memory performance in Alzheimer's patients.

Selenium is important for thyroid health

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 to 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 as it regulates metabolism and controls the body's growth and development (21). Selenium deficiency has been linked to thyroid disorders such as Hashimoto's - an underactive thyroid where the immune system attacks the thyroid gland. An observational study of over 6,000 people found that low serum selenium levels were associated with an increased risk of autoimmune thyroid disease (22). In addition, some studies have shown that selenium supplements may have 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 led to improvements in mood and general well-being in Hashimoto's patients (23).

However, further research is needed before selenium supplements can be recommended for people suffering from Hashimoto's.

• Summary: Selenium protects the thyroid gland from oxidative stress and is necessary for thyroid hormone production. Selenium may be beneficial for people suffering from Hashimoto's or other thyroid disorders, but more research is needed.

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 the health of the immune system. 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 a selenium deficiency can lead to a slower immune response (24). Studies have also linked selenium deficiency to an increased risk of death and faster disease progression in HIV patients, while supplementation has been shown to result in fewer hospitalizations and improved symptoms in these patients (25).

In addition, selenium supplements may help strengthen the immune system in patients with influenza, tuberculosis and hepatitis C (26).

• Summary: Selenium is essential for the health and proper function of the immune system. Higher selenium levels may be helpful in improving 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 constricted, leading to symptoms such as wheezing, shortness of breath, chest tightness and coughing (27). Asthma has been linked to increased levels of oxidative stress and inflammation in the body (28).

Due to selenium's ability to reduce inflammation in the body, some studies suggest that this trace mineral may be effective in reducing symptoms associated with asthma.

Scientific research suggests that people who suffer 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 symptoms associated with asthma. 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 evidence in this area is conflicting and more studies are needed to fully understand the role of selenium in the development and treatment of asthma (31).

• Summary: Selenium may be beneficial for asthma patients due to its ability to reduce inflammation in the body. However, further research is needed.

Deficiency symptoms

The consequences of selenium deficiency are not yet fully understood. However, studies suggest a link between high blood pressure and various heart diseases. Studies have also shown links between selenium deficiency and the incidence of cancers such as liver, bowel and lung cancer. Paralysis, liver damage and metabolic disorders have been observed in grazing animals that graze in selenium-deficient areas. There is also evidence that a selenium deficiency can lead to infertility in men. This is because, firstly, the maturation of sperm is impaired by a selenium deficiency and, secondly, their motility is reduced.

Overdose and poisoning

Before it was discovered that selenium is essential for humans, the trace element was considered to be one of the most toxic elements. However, it should be noted that the toxicity of selenium compounds depends on how well the respective compounds are absorbed from the gastrointestinal tract, i.e. how well they enter the blood. The compounds selenite, selenate and selenoamino acids, for example, are particularly well absorbed. Elemental selenium and stable metal selenides, on the other hand, are poorly absorbed. There is insufficient specific information in the literature about the toxic concentrations of the individual selenium compounds; only the quantities of the pure element selenium that are still considered non-toxic are given. With a regular intake of selenium, a quantity of approx. 400 µg (= 10-6 g) is considered non-toxic.

Acute selenium poisoning is rare and is usually due to an 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 lead to symptoms of poisoning in high concentrations, and the different routes of absorption (lungs, skin, gastrointestinal tract), no general symptoms can be described. What they all have in common, however, is that the air breathed by the poisoned person smells of garlic. Other symptoms observed include gastrointestinal disorders, headaches and hair loss. Changes to the fingernails (white spots and stripes) and peripheral neuropathies can also be caused by an excess of selenium.

Selenium is suspected of having a carcinogenic effect in large quantities.

In severe cases, acute selenium poisoning can lead to serious intestinal symptoms, neurological symptoms, heart attacks, kidney failure and death (34).

Selenium for use

Whether an additional intake of selenium can generally be recommended is controversial because knowledge about the trace element is currently still inadequate. However, it is now considered certain that the intake of selenium is recommended for cancer, certain cardiovascular diseases and specific forms of arthritis. However, the amounts of selenium to be taken in each case vary depending on the disease and, in the case of cancer, also depending on the respective treatment status.

Interactions

Tablets containing sodium selenite should not be taken together with reducing agents such as vitamin C, as otherwise elemental selenium is produced which is not absorbed into the blood from the gastrointestinal tract. A minimum interval of one hour should be observed between the intake of sodium selenite and vitamin C. However, this restriction does not apply to sodium selenate.

Requirements

The German Nutrition Society assumes a daily requirement of 30 to 70 µg for people over the age of 16. Pregnancy and breastfeeding, older people, smokers, cancer patients and people with a weakened immune system may have an increased selenium requirement.

Requirements in sport

As the soil in Europe is rich in selenium, our food generally contains sufficient amounts of this trace element. Because of its immune-stimulating and antioxidant effect, my advice is nevertheless to supplement with 100-200mcg selenium daily with meals

Conclusion

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 may 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.

References

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