Some Interesting Facts about Selenium
Selenium is a trace element and a natural anti-oxidant that is naturally present in many foods, added to others, and available as a dietary supplement. Selenium, which is nutritionally essential for humans, is a constituent of more than two dozen selenoproteins that play critical roles in Reproduction, Thyroid hormone metabolism, DNA synthesis, and protection from Oxidative damage and Infection.
Selenium exists in two forms: inorganic (selenate and selenite) and organic (selenomethionine and selenocysteine).Both forms can be good dietary sources of selenium.
Soils contain inorganic selenites and selenates that plants accumulate and convert to organic forms, mostly selenocysteine and selenomethionine and their methylated derivatives. Most selenium is in the form of selenomethionine in animal and human tissues, where it can be incorporated nonspecifically with the amino acid methionine in body proteins. Skeletal muscle is the major site of selenium storage, accounting for approximately 28% to 46% of the total selenium pool.
The most commonly used measures of selenium status are plasma and serum selenium concentrations.
Concentrations in blood and urine reflect recent selenium intake.
Analyses of hair or nail selenium content can be used to monitor longer-term intakes over months or years.
Plasma or serum selenium concentrations of 8 micrograms (mcg)/ dL or higher in healthy people typically meet needs for selenoprotein synthesis.
Seafood and organ meats are the richest food sources of selenium.
Other sources include muscle meats, cereals and other grains and dairy products.
The amount of selenium in drinking water is not nutritionally significant in most geographic regions. The major food sources of selenium in the American diet are breads, grains, meat, poultry, fish, and eggs.
The amount of selenium in a given type of plant-based food depends on the amount of selenium in the soil and several other factors, such as soil pH, amount of organic matter in the soil, and whether the selenium is in a form that is amenable to plant uptake.
As a result, selenium concentrations in plant-based foods vary widely by geographic location.
For example, according to the U.S. Department of Agriculture Food Composition Database, Brazil nuts have 544 mcg selenium/ ounce or 68–91 mcg per nut and could cause selenium toxicity if consumed regularly.
Selenium is available in multivitamin/multimineral supplements and as a stand-alone supplement, often in the forms of selenomethionine or of selenium-enriched yeast (grown in a high-selenium medium) or as sodium selenite or sodium selenate.
The human body absorbs more than 90% of selenomethionine but only about 50% of selenium from selenite.
Selenium deficiency produces biochemical changes that might predispose people who experience additional stresses to develop certain illnesses.
For example, selenium deficiency in combination with a second stress (possibly a viral infection) leads to Keshan disease, a cardiomyopathy that occurred in parts of China prior to a government-sponsored selenium supplementation program that began in the 1970s.
Before the Chinese government supplementation program, adults in the Keshan disease areas had average selenium intakes of no more than 11 mcg/day; intakes of at least 20 mcg/day protect adults from Keshan disease.
Selenium deficiency is also associated with male infertility and might play a role in Kashin-Beck disease, a type of osteoarthritis that occurs in certain low-selenium areas of China, Tibet, and Siberia.
Selenium deficiency could exacerbate iodine deficiency, potentially increasing the risk of cretinism in infants.
The following groups are among those most likely to have inadequate intakes of selenium.
– People living in selenium-deficient regions
– People undergoing kidney dialysis
– People living with HIV
Selenium intakes in North America, even in low-selenium regions, are well above the RDA.
However, people in some other countries whose diet consists primarily of vegetables grown in low-selenium areas are at risk of deficiency.
The lowest selenium intakes in the world are in certain parts of China, where large proportions of the population have a primarily vegetarian diet and soil selenium levels are very low. Average selenium intakes are also low in some European countries, especially among populations consuming vegan diets.
Although intakes in New Zealand were low in the past, they rose after the country increased its importation of high-selenium wheat.
Selenium in HIV
Selenium levels are often low in people living with HIV, possibly because of inadequate intakes (especially in developing countries), excessive losses due to diarrhoea, and malabsorption.
Observational studies have found an association between lower selenium concentrations in people with HIV and an increased risk of cardiomyopathy, death, and, in pregnant women, HIV transmission to offspring and early death of offspring.
Some randomized clinical trials of selenium supplementation in adults with HIV have found that selenium supplementation can reduce the risk of hospitalization and prevent increases of HIV-1 viral load; preventing HIV-1 viral load progression can lead to increases in numbers of CD4 cells, a type of white blood cell that fights infection.
Selenium in Cancer
Because of its effects on DNA repair, apoptosis, and the endocrine and immune systems as well as other mechanisms, including its antioxidant properties, selenium might play a role in the prevention of cancer.
Epidemiological studies have suggested an inverse association between selenium status and the risk of colorectal, prostate, lung, bladder, skin, oesophageal, and gastric cancers.
In a Cochrane review of selenium and cancer prevention studies, compared with the lowest category of selenium intake, the highest intake category had a 31% lower cancer risk and 45% lower cancer mortality risk as well as a 33% lower risk of bladder cancer and, in men, 22% lower risk of prostate cancer.
Selenium acts as a co-factor for several key antioxidant enzymes called selenoproteins that recycle cellular antioxidants such as glutathione. This process reduces oxidative stress, a cause of premature aging and chronic disease.
Selenium in Heart
Selenoproteins help prevent the oxidative modification of lipids, reducing inflammation and preventing platelets from aggregating. It also reduces LDL and raises HDL. For these reasons, experts have suggested that selenium supplements could reduce the risk of cardiovascular disease or deaths associated with cardiovascular disease.
Selenium in Brain
Serum selenium concentrations decline with age. Marginal or deficient selenium concentrations might be associated with age- related declines in brain function, possibly due to decreases in selenium’s antioxidant activity.
The trace mineral selenium has a huge impact on brain function. Nerve cells must have selenium to produce glutathione, one of the brain’s most important antioxidants. The brains of animals, for example, fed a low selenium diet make less glutathione. Such selenium-deprived brains also show disturbances in the activity of prominent neurotransmitters serotonin, dopamine, and adrenaline, signifying potential brain damage and dysfunction, according to recent research. Further blood levels of selenium drop as you age—by 7 percent after age sixty and 24 percent after age seventy-five, according to one study.
Selenium in Thyroid Disease
Selenium concentration is higher in the thyroid gland than in any other organ in the body, and, like iodine, selenium has important functions in thyroid hormone synthesis and metabolism.
Women with thyroid peroxidase antibodies tend to develop hypothyroxinaemia while they are pregnant and thyroid dysfunction and hypothyroidism after giving birth. The authors of a Cochrane review of hypothyroidism interventions during pregnancy concluded, based on a trial that administered supplements containing 200 mcg selenium as selenomethionine daily to 151 pregnant women with thyroid peroxidase antibodies, that selenomethionine supplementation in this population is a promising strategy, especially for reducing postpartum thyroiditis. However, the authors called for large randomized clinical trials to provide high-quality evidence of this effect.
Additionally, selenium is also essential for the conversion of T4 to T3, as deiodinase enzymes (those enzymes that remove iodine atoms from T4 during conversion) are selenium-dependent. As T3 is the active form of thyroid hormone, and low T3 can cause hypothyroid symptoms. A double-blind intervention study found that selenium supplementation in selenium deficient subject’s modulated T4 levels, theoretically by improving peripheral conversion to T3. In cases of severe selenium deficiency, conversion of T4 to T3 may be impaired, leading to hypothyroid symptoms. As T3 conversion is not performed by the thyroid, the dependence on selenoproteins for this conversion demonstrates how significant selenium deficiency could lead to hypothyroid symptoms.
Can you overdose on Selenium?
Chronically high intakes of the organic and inorganic forms of selenium have similar effects. Early indicators of excess intake are a garlic odour in the breath and a metallic taste in the mouth. The most common clinical signs of chronically high selenium intakes, or selenosis, are hair and nail loss or brittleness. Other symptoms include lesions of the skin and nervous system, nausea, diarrhoea, skin rashes, mottled teeth, fatigue, irritability, and nervous system abnormalities.