In the elementary state or in any case in pure form it is considered a fairly rare element and is found mainly linked to metal sulphide minerals - at an industrial level, it is obtained secondarily during refining. On the other hand, pure selenides or selenated compounds are quite rare.
In traces, quantifiable as a few tens of micrograms (μg), selenium is necessary for cellular functioning and the survival of many organisms, including all animals including man. It is believed that the selenium content in the human body is understood between 13-20 mg However, it must be remembered that considerable quantities of selenium salts have toxic effects, even very serious.
Selenium is an essential component for the formation of antioxidant enzymes glutathione peroxidase (GSH-Px) and thioredoxin disulfide reductase which, in animal and plant eukaryotic cells, have the function of hindering - albeit indirectly - the oxidation of certain molecules on cell membranes. It is also found in biological catalysts of the deiodinase or deiodinase type, responsible for the conversion of certain thyroid hormones Note: the need for selenium in plants varies according to the species.
Due to its importance in body homeostasis and its properties, selenium has become a very popular ingredient in food supplements - multisalines and vitamins, etc. - and in dietary and / or fortified foods - including the well-known potatoes, artificial milk. etc.
Although toxic in large doses, selenium is an essential trace element and micronutrient for humans and beyond.
In the human organism, selenium is present in organic (selenocysteine or selenium-cysteine and selenomethionine or selenium-methionine) and inorganic form (selenites and selenates).
Organic selenium is mainly present 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 deiodinase or deiodinase enzymes. 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, see 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, as without it they could not grow and develop.
for the Italian population - and the American RDA - Recommended Dietary Allowance - recommend a selenium intake of 55 μg / day.
In the next tables various parameters concerning selenium will be summarized; in particular: RDA, PRI, AR and UL.
55μg / day
Pregnant and Nourishing
65-75μg / day
Newborns up to 6 months
10μg / day
Children between 6 and 12 months
15μg / day
Children between 1 and 6 years old
20μg / day
Children between 7 and 10 years
30μg / day
Children between 11 and 14 years
40μg / day
* RDA: Recommended Dietary Allowance
* PRI: recommended intake for the population, from LARN - Recommended Nutrient Intake Levels 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
. Among the vegetables that contain more selenium we remember the Brazil nuts and some cereals; some mushrooms are also rich in selenium.However, it must be remembered that the level of this mineral in plants and mushrooms is generally proportional to its abundance in the soil. The famous selenium potatoes are produced by enriching the soil with the mineral during fertilization. Growing in an environment rich in selenium, potatoes accumulate greater quantities of the mineral; nevertheless, its effective bioavailability and the related - possible - health benefits are still to be clarified.
Selenium present in foods in the form of sulfuric selenium amino 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, for this reason the two nutritional principles are frequently associated in food supplements with antioxidant action.
Food Supplements and Dietary Foods or Fortified with Selenium
As a dietary supplement, selenium comes in many forms, including multivitamins and salines, which typically 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 minimum and maximum levels of selenium in infant formulas.
, those undergoing total parenteral nutrition and elderly people - over 90 years old. In addition, those who eat exclusively plant foods from soils lacking in selenium are at high risk. In this regard, it is curious to note that, although the New Zealand soil contains low levels of selenium, no negative effects have been found on the general population.
The actual selenium deficiency, which can be diagnosed by detecting a low activity of selenoenzyme 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. Also, 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 parts of China whose soils are particularly poor in selenium. Low selenium levels 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 in some way be related to a number of serious and / or chronic diseases. These include: cancer, diabetes mellitus, HIV / AIDS and tuberculosis.
A study in rodents has shown that dietary supplementation with selenium could exert a chemopreventive effect for some types of cancer.
A study of 118 exocrine pancreatic cancer (EPC) patients and 399 hospital controls in Eastern Spain found that high selenium concentrations are inversely associated with EPC risk. However, in prospective, randomized, blinded, controlled trials in humans, selenium supplementation failed to decrease the incidence of any disease. Not even a meta-analysis performed on these studies found an overall decrease in mortality.
Selenium deficiency in agriculture and breeding
Some regions - for example in North America - characterized by a soil with a low content of selenium, give rise to fodder and agri-food products equally deficient in the mineral. In this regard, it has been shown that some animal species can suffer from this deficiency unless selenium is integrated into feed or administered by injection. As if that were not enough, ruminants have a limited capacity to absorb this mineral, especially if fed only with herbaceous forage - it is not excluded that the cyanogenic glycoside content of certain plants such as white clover may further reduce the absorption of selenium. . It is therefore easy to understand that these animals are particularly subject to deficiency of the mineral and that, consequently, the food products derived and intended for human consumption reflect this characteristic.
, brittle nails, nausea, vomiting, abdominal pain, diarrhea, mental confusion, fatigue, irritability, neurological damage and the 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 reach 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 study actually found that the maximum selenium intake is about 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 selenium-rich soil showed toxicity syndrome.
Elemental selenium and most metal selenides have relatively low toxicity due to low bioavailability. In contrast, selenates and selenites have a similar mode of action to that of arsenic trioxide and are very toxic. The chronic toxic dose of selenite for humans is approximately 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 are toxic in large doses.
On April 19, 2009, 21 polo horses died from an error in the choice of the selenium-based ingredient used in the 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 than normal. times higher in the liver.
Agricultural runoff and groundwater contamination can lead to selenium poisoning. This process of infiltration of selenates - mainly from coal burning, mining, metal smelting, landfills etc. - is aggravated by the drying up of the aquifers, which exponentially increases the final concentration. High levels of selenium in waterways have caused congenital disorders in oviparous species - birds and fish. High levels of dietary methylmercury can amplify toxicity damage.
See other articles tag Selenium - Mineral salts
