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Selenium is a trace mineral and powerful antioxidant which is vital for the human body and required for proper function of many organs, tissues and systems.
Following absorption, selenium is incorporated into protein enzymes (called “selenoproteins“). Selenoproteins are types of protein that contain selenium in the form of an amino acid. There are around 25 different selenoproteins, but only half of them have their functions identified. They are very important for human health and their activity is dependent on an adequate supply of selenium from diet or supplements.
In order to be healthy we have to supply our body with selenium on a daily basis because it makes our immune system stronger, defends body cells, DNA and mitochondria against free radical damage, and plays a key role in maintaining thyroid function and a healthy metabolism.
Selenium has also antiviral, antibacterial and antifungal properties, helps reduce the risk of autoimmune diseases, various types of cancer, and thyroid problems, and is essential for male and female fertility and reproduction.
Fatigue, thyroid problems, muscle weakness, whitening of the beds of the fingernails, weak immunity, decreased cognition, hair loss, discolouration of the hair and skin, poor growth, infertility, dandruff, premature ageing.
Soil depletion (leading to low concentration of selenium in foods), poor diet, heated, processed and refined foods, use of stimulants, smoking, stress, age, trauma, burns, gastrointestinal disorders such as Crohn’s disease, stomach surgery.
Many people are deficient in selenium as both plant and animal foods are often too low in this mineral due to severe soil depletion. The content of selenium in soil depends on the location. Studies show that soils in parts of UK, Europe, and Africa are low in selenium and people living there are at higher risk of developing problems associated with selenium deficiency such as weak immunity or thyroid problems.
Selenium levels and average intake in the UK have fallen over recent decades. In 1978 the average intake was 60 mcg per day, while in 1994 it dropped to 34 mcg per day! It was caused by nutritional changes and also because of moving away from using selenium-rich wheat from North America to European flour (lower in selenium) for bread making.
Most Americans consume adequate amounts of selenium as the average daily selenium intake in Americans from foods and supplements is 120 mcg per day.
The best by far food source of selenium are Brazil nuts, mostly due to the quality of the soil. However, the concentration of selenium even in in Brazil nuts, depends on the levels of this mineral in the soils that they are grown in. It has been found that Brazil nuts are much higher in selenium if grown in the selenium rich soils of the Eastern region of Amazon, in comparison to the Western part of Brazil >.
The difference in concentration of selenium in Brazil nuts from different sources can be huge. For example, just one nut from East Amazon may contain over 200 mcg of selenium while one nut from Western Brazil may contain less than 10 mcg! In the most extreme example the selenium in one sample of individual nuts tested by Chang (1995) ranged from 6 mcg to 2560 mcg!!! Read more >
Although the RDA for selenium for adults is 55 mcg per day, the average daily intake of selenium in the US in areas where soil is richer in this mineral is over 100 mcg per day, which leads to a conclusion that we need much more than recommended 55 mcg.
Since recommended by health authorities daily intakes are almost always way to low (RDAs for vitamin D and B12 are the best examples) we need more selenium than suggested 55 mcg for adults (current recommended daily allowance for selenium in UK is 75mcg for men and 60mcg for women).
Considering the recommended daily intake (55-75 mcg) and the upper daily limit of selenium (400 mcg), the consumption of Brazil nuts should be limited to not more than 4 kernels (nuts) daily (depending on the source and selenium content).
If you can associate yourself with the deficiency risk factors, listed above, you may need 200 mcg per day for a longer time.
Consuming 1-2 Brazil nuts every day plus multivitamin containing 50-100 mcg selenium should be enough too.
Without Brazil nuts in the diet supplementing with 150 to 200 mcg selenium a day seems to be a good idea too.
It is best to take selenium supplements in combination with food and vitamin E because they facilitate each other’s absorption. Many supplements combine the two together.
0 – 6 months 50 mcg (< 1 kernel)
7 – 12 months 60 mcg (< 1 kernel)
1-3 years 100 mcg (1 kernel)
4-8 years 150 mcg (1.5 kernel)
9-13 years 300 mcg (3 kernels)
14-18 years 400 mcg (4 kernels)
19 years + 400 mcg (4 kernels)
Do not exceed the tolerable upper limit of 400 mcg of selenium per day. If we assume that one average (weighing about 5g) Brazil nut kernel comes from a selenium rich soil it may contain about 100mcg of selenium, it would meant that consuming just 4 kernels may provide us with 400 mcg of selenium which is the upper daily limit. Of course it is perfectly safe if from time to time we have more than that but I would avoid consuming larger amounts of Brazil nuts on a regular basis for a long period of time due to the risk of overdosing selenium.
But, to be honest, the assumption that too many Brazil nuts eaten on a regular basis may be harmful does not have to be true, as there are many individuals who consume more than 10 Brazil nuts a day without any side effects.
Selenium toxicity is rare but doses as high as 400 mcg per day can be even harmful if used for a very long time, while 700 mcg a day are safe for short-term use.
In 2008 there was an outbreak of selenium toxicity (selenosis). The source was identified as a liquid dietary supplement that contained 200 times the labelled concentration of selenium in the form of sodium selenite. Of 201 cases identified in ten states in the US, one person was hospitalized. The dose of selenium consumed was 41749 μg/day! (source >)
What is more important is rather the quality of the nuts and avoiding the aflatoxin content. Aflatoxin comes from the carcinogenic “excrements” of the fungi that are often accumulating on nuts, when they are not appropriately handled by the manufacturers.
Although selenium can be found in water the best source of this vital trace element should be food and not water because water is not designed to feed our body but to cleanse it. In many cases the minerals such as calcium carbonate found in hard water can be very harmful as they are inorganic.
Selenium is needed to fight oxidative damage and degradation of cells and protect against mutations and DNA damage that can cause cancer and other diseases. Supplementing selenium using high doses have shown that it benefits anti-cancer abilities of the body. According to studies, a daily dose of 200 mcg of selenium is effective at reducing the risk of cancers and cancer-caused mortality especially with regards to the prostate, liver, colorectal and lung cancers. Selenium attaches itself to glutathione and works with it to reduce and repair damage done to DNA, preventing cell mutations and tumour growth. There is evidence that selenium not only reduces cancer risk, but also helps slow down progression of existing cancers. Studies show that in areas where the soil is lowest in selenium, cancer risk is significantly higher.
In one study of 1,300 older people, the prevalence of cancer among those who took 200 mcg of selenium per day for about seven years was reduced by 42 % compared to those on a placebo. Cancer deaths for participants taking the selenium were reduced almost by 50 %. In addition, the men who took selenium had 63 % fewer prostate cancers, 58 % fewer colorectal cancers, 46 % fewer lung cancers and overall 37 % fewer cancers. Selenium was also found to reduce the risk of lung cancer to a greater degree than quitting smoking! It is also important to mention that after a very long time of supplementation no cases of selenium toxicity occurred!
According to the conclusion of one study, “Using two different murine models of cancer, we showed that the Selenium and Beta glucan combination strongly suppressed the growth of cancer, mostly probably via stimulation of immunity. A combination of glucan with Selenium offers superior stimulation of immunity and inhibition of cancer growth.” (>)
Some studies have found an association between selenium deficiency and progression to AIDS as HIV-infected individuals often have low selenium levels.
Selenium boosts immunity and helps defend viral and bacterial infections, autoimmune diseases and allergies. For instance, in people who already contracted HIV, selenium has been shown to also be able to slow down the progression of the disease into AIDS. It was discovered that 200 micrograms of Selenium per day helps prevent viruses from replicating.
Thyroid stores more selenium than anywhere else in the body because selenium plays a big role in metabolic processes. Some studies have linked low levels of selenium with low thyroid function as selenium deficiency may reduce the effectiveness of the thyroid hormones. Selenium regulates thyroid function, helps remove thyroid-harming substances and supports normal thyroid structure. Thyroid controls important body functions, including temperature, appetite, sleep, weight, energy and more. Problems with thyroid function can result in irritability, weight gain or loss, trouble sleeping, muscle weakness, fatigue, etc. Selenium is also important in thyroid T4 to T3 conversion, and therefore can be helpful in coping with low mood (if it is caused by an underactive thyroid). Many individuals claim that the amount of selenium in a few Brazil nuts consumed every day is sufficient to keep low mood at bay. Selenium regulates the production of reactive oxygen within the thyroid and protects it against autoimmune attack caused by antibodies. It is therefore believed that apart from vitamin D and zinc deficiency also lack of selenium contributes to autoimmune thyroid diseases such as Hashimoto (underactive thyroid) or Grave’s disease (overactive thyroid) and in pregnant women to the appearance of anti-TPO antibodies (another autoimmune thyroid condition).
Selenium increases blood flow and is required for proper sperm motility, two key elements involved in conception and treating infertility. Selenium is incorporated in the sperm mitochondria and may affect the health and function of the sperm. It looks like both low and high sperm selenium concentrations may have a negative impact on the number of sperm. It is therefore vital to have enough selenium in the blood but not too much.
Selenium is often praised for its role in antioxidant activity which is thought to be due to different seleno-proteins found in this trace element. Since selenium is an antioxidant it has a potential to protect the body against various forms of cancer, boost immune system to fight off viruses and other pathogens, defend against heart disease, and slow down ageing process. Selenium is an essential component of glutathione (the king of all antioxidants), which activates other antioxidants and protects fats in cell membranes. Selenium is also required to keep glutathione in its active form. Like zinc, selenium is also used in our body to create superoxide dismutase (SOD), another most powerful antioxidant able to not only neutralise deadly free radicals but also convert them into water and oxygen which benefit the body. As an antioxidant selenium also slows down ageing process and increases longevity.
According to a review of 25 studies, a 50% increase in blood selenium levels was associated with a 24% reduced risk of coronary artery disease. In addition, selenium also helps lower inflammation in the body, one of the main risk factors for heart disease. According to another review which included 16 controlled studies with over 433,000 participants with coronary heart disease taking selenium supplements reduced levels of the inflammatory marker CRP. At the same time, selenium also increased levels of glutathione, the most powerful antioxidant found in the body which also protects the heart and brain preventing heart attacks and strokes.
A large number of studies suggest that selenium may be very beneficial in preventing and reducing Alzheimer’s pathology. They have demonstrated that Alzheimer’s patients have lower blood levels of selenium and that selenium from both food and supplements may improve memory in patients with Alzheimer’s disease. According to one study supplementing with just only one selenium-rich Brazil nut per day improved verbal fluency and other mental functions in patients with mild cognitive impairment >.
– It was found that zinc and selenium concentrations were reduced in patients with sepsis (blood infection). Studies point towards zinc and selenium supplementation playing a therapeutic role in preventing and treating sepsis.
– Selenium deficiency may contribute to chronic asthma and hay fever. Studies have demonstrated that patients with chronic hay fever have lower levels of selenium. However, selenium supplementation has not been recommended with drug therapy for hay fever. According to some studies, also people with asthma experienced less symptoms after supplementing selenium.
– It is thought that the antioxidant behaviour of selenium also helps to reduce free-radical damage in the eyes and low blood levels of selenium are often found in those with cataracts.
– Selenium helps the body to properly use proteins, including those that make up the hair. This is probably the reason why low selenium levels are often associated with hair loss. In addition, selenium is also toxic to the scalp fungus that causes dandruff and therefore it is often included in anti-dandruff shampoos.
– Antioxidant ability of selenium to defend against oxidative damage helps to slow the ageing process of the skin. It also helps in the recovery and repair of skin tissue, bringing relief from itching and inflammation associated with skin conditions such as eczema or psoriasis.
– Selenium increases the bioavailability of zinc.
– Selenium deficiency aggravates effects of vitamin E deficiency.
– Vitamin E can prevent selenium toxicity.
– Selenium deficiency aggravates effects of vitamin E deficiency.
– Vitamin E and selenium improve each other’s absorption.
– Vitamin C may increase absorption of sodium selenite and retention of the absorbed selenium.
– Selenium increases the bioavailability of zinc.
Selenium may interact with antacids, chemotherapy drugs, corticosteroids, cholesterol-lowering statin drugs and birth control pills. If you are on any of these, speak with your doctor before supplementing with selenium.
Testing the blood for selenium doesn’t seem to be reliable as blood tests will only show the amount of selenium taken recently. Also, the accuracy of hair tests is not consistent because the mineral is stored differently throughout various parts of the body, organs and systems.
Written by Slawomir Gromadzki, MPH
– Chang JC, Gutenmann WH, Reid CM and Lisk DJ. (1995) Selenium content of Brazil nuts from two geographic locations in Brazil. Chemosphere. 30(4):801-2.
– Kannamkumarath SS, Wrobel K, Wrobel K, Vonderheide A, Caruso JA. (2002) HPLC-ICP-MS determination of selenium distribution and speciation in different types of nut. Analytical and Bioanalytical Chemistry. 373(6):454-60.
– Yang J. (2009) Brazil nuts and associated health benefits: A review. Food Science and Technology. 42(10):1573–1580.
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