Selenium

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What is selenium?

Selenium is a chemical element with the symbol "Se" and atomic number 34, discovered in 1817 by Jöns Jacob Berzelius.

Selenium is a non-metal with intermediate properties between the elements - over and underneath on the periodic table - sulfur and tellurium; it has some similarities even with arsenic.

In its elementary state or in its pure form it is considered a fairly rare element and is found mainly linked to the minerals of metal sulfide - on an industrial level, it is derived secondarily during refining. Pure selenides or selenate compounds are rather rare.

In traces, quantifiable as a few tens of micrograms (μg), selenium is necessary for the cellular functioning and survival of many organisms, including all animals including humans. It is believed that the selenium content in the human body is between 13-20 mg. However, it must be remembered that considerable amounts of selenium salts have very serious toxic effects.

Selenium is an indispensable component for the formation of antioxidant enzymes glutathione peroxidase (GSH-Px) and thioredoxin disulfide reductase which, in animal and vegetable eukaryotic cells, have the function of obstructing - albeit indirectly - the oxidation of certain molecules on cell membranes. It is also found in biological catalysts of the iodase or deiodinase type, responsible for the conversion of certain thyroid hormones. Note : the selenium requirement in plants changes according to the species.

Due to its importance in body homeostasis and its properties, selenium has become a widespread ingredient in food supplements - multisino and vitamin, etc. - and in dietetic and / or fortified foods - including known potatoes, artificial milk, etc.

Functions and Properties

Functions and properties of selenium

For more information read: Functions and Properties of Selenium

Although it is toxic in large doses, selenium is a trace element and a micronutrient essential for humans and others.

In the human body, selenium is present in organic form (selenocysteine ​​or slenium-cysteine ​​and selenomethionine or selenium-methionine) and inorganic (selenites and selenates).

Organic selenium is present primarily as selenocysteine ​​and is a cofactor of the antioxidant enzymes glutathione peroxidase and thioredoxin disulfide reductase, which protect cell membranes from oxidative stress. Thanks to its ability to protect cell membranes from oxidation, selenium therefore has a protective effect against cardiovascular diseases.

Organic selenium is also used in the metabolism of thyroid hormones, in the form of a cofactor for 3 enzymes deiodasi or deiodinasi. It is therefore necessary for the transformation of thyroxine (T4) into triiodothyronine (T3), and as such plays a leading role in supporting thyroid function. To learn more, consult the article: Selenium and Thyroid.

It also appears to play an antagonistic role against heavy metals, such as mercury, cadmium and silver.

In plants, selenium can have a defense function, resulting toxic in forages for the animals that consume them. Certain plants are considered indicators of selenium in the soil, because without it they could not grow and develop.

Nutrition

Recommended levels of selenium

There is no real recommended daily dose of selenium. For adults, LARN - Recommended Nutrient Intake Levels for the Italian population - and the American RDA - Recommended Dietary Allowance - recommend a selenium intake of 55 μg / day.

The following tables will summarize various parameters concerning selenium; in particular: RDA, PRI, AR and UL.

* RDA : Recommended Dietary Allowance

* PRI : recommended intake for the population, from LARN - Recommended Levels of Nutrient Intake for the Italian population

* AR : average requirement for the Italian population, from LARN - Recommended Nutrient Intake Levels for the Italian population

* UL : maximum tolerable level of intake, from LARN - Recommended Nutrient Intake Levels for the Italian population

Diet

Foods rich in selenium

Dietary selenium is provided mainly by foods of marine origin and offal. Among the plants that contain more selenium we can mention brazil nuts and some cereals; also some mushrooms are rich in selenium.

However, it must be remembered that the level of this mineral in vegetables and mushrooms is generally proportional to its abundance in the soil. The famous selenium potatoes are produced enriching the soil with the mineral during fertilization. Growing in an environment rich in selenium, the potatoes accumulate greater quantities of the mineral; nevertheless, its actual bioavailability and the relative - possible - health benefits are yet to be clarified.

The selenium present in foods in the form of sulfur seleniumamino acids - selenium-cysteine ​​and selenium-methionine - is more absorbable than selenites and selenates normally contained in food supplements.

Selenium acts in synergy with vitamin E, which is why the two nutritional principles are frequently associated in food supplements with an antioxidant action.

Dietary supplements and diet foods or fortified with selenium

As a dietary supplement, selenium is available in many forms, including multivitamins and salts, which generally contain 55 or 70 μg / serving. Selenium specific supplements usually contain 100 or 200 μg / dose.

Only in June 2015 did the US Food and Drug Administration (FDA) set the minimum and maximum levels of selenium in infant formulas.

shortage

Nutritional deficiency of selenium

Selenium deficiency is possible and more likely in: subjects with severely impaired bowel function and adjoining malabsorption, those subjected to total parenteral nutrition and people in advanced age - over 90 years. Furthermore, those who exclusively feed on plant foods from selenium-deficient soils are at high risk. In this regard it is curious to note that, although New Zealand soil contains low levels of selenium, no negative effects on the general population have been detected.

Selenium deficiency, diagnosed by detecting low selenoenzyme activity in brain and endocrine tissues - <60% of normal - occurs only in conjunction with poor nutritional intake and added stress factors, such as high exposure to mercury or increased oxidative stress due to vitamin E deficiency

Selenium interacts with various nutrients, especially iodine and vitamin E. The effect of chronic selenium deficiency on human health remains uncertain, especially in relation to Kashin-Beck disease - see below. Furthermore, selenium interacts with other minerals such as zinc and copper.

Symptoms of selenium deficiency

Chronic selenium deficiency causes a heart disease known as Kashin-Beck disease, prevalent in some areas of China whose soils are particularly poor in selenium. Low levels of selenium are linked to: increased risk of cancer, cardiovascular disorders, inflammatory diseases and other diseases associated with free radical damage, including premature aging and cataract formation.

Selenium and serious diseases

Some epidemiological studies have highlighted the possibility that the nutritional deficiency of selenium - measured by blood levels - may somehow correlate with a certain number of serious and / or chronic diseases. These include: cancer, diabetes mellitus, HIV / AIDS and tuberculosis.

A study on rodents showed that dietary supplementation with selenium could have a chemopreventive effect for some types of cancer.

A study carried out on 118 patients with exocrine pancreatic cancer (EPC) and 399 hospital controls in Eastern Spain found that high concentrations of selenium are inversely associated with the risk of EPC. However, in prospective randomized, blinded, controlled studies in humans, supplementation with selenium failed to decrease the incidence of any disease. Not even a meta-analysis performed on these studies found an overall decrease in mortality.

Deficiency of selenium in agriculture and farming

Some regions - for example in North America - characterized by low selenium soil, give rise to forages and food products equally lacking in the mineral. In this regard it has been shown that some animal species may be affected by this deficiency unless selenium is integrated into feed or given by injection. As if this were not enough, ruminants have a limited capacity to absorb this mineral, especially if fed only with herbaceous fodder - it is possible that the content of cyanogenic glycosides of certain plants such as white clover can further reduce selenium absorption . It is therefore easy to understand that these animals are particularly prone to mineral deficiency and that, consequently, the food products derived and intended for human consumption reflect this characteristic.

Toxicity

Toxicity of selenium

High doses of selenium supplements in pregnant animals can disrupt the Zn: Cu ratio and lead to the reduction of body zinc - which must be kept under control. However, further studies are needed to confirm this interaction.

Although selenium is an essential trace element, taken in excess it becomes toxic to the body. Excessive quantities can cause selenosis, with toxic effects that can be identified as follows: hair loss, nail fragility, nausea, vomiting, abdominal pain, diarrhea, mental confusion, fatigue, irritability, neurological damage and smell of garlic in the breath. Extreme cases of selenosis can result in liver cirrhosis, pulmonary edema and death.

For these reasons it is advisable not to exceed the so-called Tolerable Upper Intake Level; this threshold, defined through a 1986 study and a 1992 follow-up, is almost impossible to achieve with food alone and corresponds to 400 μg / day - Panel on Dietary Antioxidants and Related Compounds, Subcommittees on Upper Reference Levels of Nutrients and Interpretation and Uses of DRIs, Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, Food and Nutrition Board, Institute of Medicine (August 15, 2000). Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Institute of Medicine. pp. 314-315. The second in-depth analysis has actually found that the maximum intake of selenium is around 800 μg / day - therefore 15 μg / day per kilogram of body weight - but still suggested halving it.

In China, some people who consumed corn grown in excessively rich selenium soil showed toxicity syndrome.

Elemental selenium and most metal selenides have relatively low toxicity with regard to low bioavailability. In contrast, selenates and selenites have a mode of action similar to that of arsenic trioxide and are very toxic. The chronic toxic dose of selenite for humans is around 2400 to 3000 μg / day. Hydrogen selenide is an extremely toxic and corrosive gas. Selenium is also found in various organic compounds, such as dimethyl selenide, selenomethionine, selenocysteine ​​and methylselenocysteine, which have a high bioavailability and in large doses are toxic.

On April 19, 2009, 21 polo horses died due to an error in the choice of the selenium-based ingredient used in animal feed. The concentration of selenium in the plasma was up to 10-15 times higher than normal in the blood and 15-20 times higher in the liver.

Agricultural runoff and groundwater contamination can cause selenium poisoning. This process of infiltration of selenates - coming mainly from coal combustion, mines, metal melting, landfills and so on. - is aggravated by the drying up of the groundwater, which exponentially increases the final concentration. High levels of selenium in waterways have caused congenital disorders in oviparous species - birds and fish. High methylmercury levels in the diet can amplify toxicity damage.

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